Technical Reference

A. Introduction

B. Key Terms Used in this Guideline

C. Air

1. Modes of Transmission of Airborne Diseases

2. Airborne Infectious Diseases in Healthcare Facilities

a. Aspergillosis and Other Fungal Diseases

b. Tuberculosis and Other Bacterial Diseases

c. Airborne Viral Diseases

3. Heating, Ventilation, and Air Conditioning Systems in Healthcare Facilities

a. Basic Components and Operations

b. Filtration

c. Ultraviolet Germicidal Irradiation (UVGI)

d. Conditioned Air in Occupied Spaces

e. Infection Control Impact of HVAC System Maintenance and Repair

4. Construction, Renovation, Remediation, Repair, and Demolition

a. General Information

b. Preliminary Considerations

c. Infection Control Risk Assessment

d. Air Sampling

e. External Demolition and Construction

f. Internal Demolition, Construction, Renovations, and Repairs

5. Environmental Infection Control Measures for Special Healthcare Settings

a. Protective Environments (PE)

b. Airborne Infection Isolation Areas (AII)

c. Operating Rooms

6. Other Aerosol Hazards in Healthcare Facilities

D. Water (Refer to CDC 2001 Water Document)

E. Environmental Services (Refer to CDC 2001 - Environmental)

The Guideline for Environmental Infection Control in Healthcare Facilities, 2001 is a compilation of recommendations for the prevention and control of infectious diseases that are linked to healthcare environments. This document: 1) updates and revises several sections (i.e., cleaning and disinfection of environmental surfaces, environmental sampling, laundry and bedding, and regulated medical waste) from the previous editions of the Centers for Disease Control and Prevention [CDC] document entitled Guideline for Handwashing and Hospital Environmental Control;1, 2 2) incorporates discussions of air and water environmental issues from the Guideline for the Prevention of Nosocomial Pneumonia;3 3) consolidates relevant environmental infection control measures from several other CDC guidelines;4 - 9 and 4) discusses two topics not addressed in previous CDC guidelines -- infection control issues related to the presence of animals in healthcare facilities, and water quality in hemodialysis settings.

Part II, "Recommendations for Environmental Infection Control in Healthcare Facilities," presents control measures for preventing infections associated with air, water, or other environmental concerns within healthcare facilities as appropriate. These recommendations represent the consensus of the Healthcare Infection Control Practices Advisory Committee (HICPAC), a 12-member committee that advises CDC on issues related to the surveillance, prevention, and control of healthcare-associated infections primarily in United States healthcare facilities.10 As of January 1999, HICPAC expanded its infection control focus from acute-care hospitals to all venues where healthcare is provided (e.g., outpatient surgical centers, urgent care centers, clinics, outpatient dialysis centers, physicians’ offices, skilled nursing facilities). The topics addressed in this guideline are generally applicable to a variety of healthcare venues throughout the United States. This document is intended for use primarily by infection control practitioners, epidemiologists, employee health and safety personnel, engineers, informational system specialists, administrators, environmental service and housekeeping professionals, and architects for these facilities.

The healthcare environment contains a diverse population of microorganisms, but only a select few are significant pathogens for susceptible humans. Microorganisms are present in great numbers in moist, organic environments, but some can also persist under dry conditions. Although pathogenic microorganisms can be detected in air and water and on fomites, it is difficult to assess their role in causing infection and disease.11

There are few reports which clearly delineate a "cause and effect" with respect to the environment, in particular for the housekeeping surfaces. Seven levels of proof are used to evaluate the strength of evidence for an environmental source or means of transmission of infectious agents.11 In the order of their rigor, these are: 1) the organism can survive after inoculation onto the fomite; 2) the organism can be cultured from in-use fomites; 3) the organism can proliferate in or on the fomite; 4) some measure of acquisition of infection cannot be explained by other recognized modes of transmission; 5) retrospective case-control studies show an association between exposure to the fomite and infection; 6) prospective observational studies may be possible when more than one similar type of fomite is in.6 use; and 7) prospective studies allocating exposure to the fomite to a subset of patients show an association between exposure and infection. An additional level of proof is that decontamination of the fomite results in the elimination of disease transmission.12

Applying these proofs to disease investigations allows scientists to assess the contribution of the environment to disease transmission. The identification of a pathogen (e.g., vancomycin-resistant enterococci [VRE]) on an environmental surface during an outbreak serves as an illustration of this point. The presence of the pathogen does not automatically establish its causal role; its transmission from source to host could be through indirect means, such as via hand transferral.11 The surface, therefore, would be considered one of a number of potential reservoirs for the pathogen, but not the "de facto" source of exposure.

An understanding of how infection occurs after exposure, based on the principles of the "Chain of Infection," is also important in evaluating the contribution of the environment to healthcare-associated disease.13 All of the components of the "Chain" must be operational for infection to occur. That is, infection requires: 1) an adequate number of pathogenic organisms [dosage]; 2) pathogenic organisms of sufficient virulence; 3) a susceptible host; 4) an appropriate mode of transmission or transferral of the organism in sufficient number from a source to the host; and 5) the correct portal of entry into the host. The presence of the susceptible host has focused recent attention on the importance of the healthcare environment and opportunistic pathogens in air and water and on fomites. As a result of advances in medical technology and therapies (e.g., intensification of cytotoxic chemotherapy; progress of transplantation medicine), a greater number of patients are becoming increasingly immunocompromised in the course of treatment and are therefore at increased risk of acquiring healthcare-associated opportunistic infections.

Trends in healthcare delivery are also changing the distribution of patient populations and increasing the number of immunocompromised persons in healthcare settings other than acute-care hospitals, especially in light of early discharge of patients from care. According to the American Hospital Association (AHA), the number of hospitals in the United States in 1998 totaled 6,021, with 1,013,000 beds.14 This represents a 5.5% decrease in the number of acute-care facilities and a 10.2% decrease in the number of beds over the 5-year period 1994-1998. 14 The total average daily census in U.S. acute-care hospitals in 1998 was 662,000 (65.4%) -- 36.5% less than the average daily census of 1,042,000 in 1978. 14 As the number of acute-care hospitals declines, the length of stay in these facilities is concurrently decreasing, primarily for immunocompetent patients. Those patients remaining in acute-care facilities are likely be those who require extensive medical interventions and are therefore at high risk for opportunistic infection.

The growing population of severely immunocompromised patients is at odds with demands on the healthcare industry to remain viable in the marketplace, to incorporate modern equipment, new diagnostic procedures, treatments, and to construct new facilities. Increasing numbers of healthcare facilities are likely to be faced with some construction in the near future as hospitals consolidate to reduce costs, defer care to ambulatory centers and satellite clinics, and try to create more "home-like" acute-care settings. In 1998, approximately 75% of the healthcare construction projects were for renovation or building outpatient facilities;15 the number of outpatient projects rose by 17% between 1998 and 1999. 16 An aging population is also creating increasing demand for assisted-living facilities and skilled nursing centers. Construction of assisted-living facilities in 1998 rose by 49%, with 138 projects completed at a cost of $703 million.16 Overall, from 1998 to 1999, healthcare construction costs increased by 28.5%, from $11.56 billion to $14.86 billion.16

Examples of outbreaks which could have been prevented had this partnership been in place include: 1) transmission of infections due to Mycobacterium tuberculosis, varicella-zoster virus [VZV], and measles [rubeola] virus apparently facilitated by inappropriate air-handling systems in healthcare facilities;6 2) disease outbreaks due to Aspergillus spp.,17 – 19

Mucoraceae,20 and Penicillium spp. associated with the absence of environmental controls during periods of construction;21 3) infections and/or colonizations of patients and staff with vancomycin-resistant Enterococcus faecium [VRE] and Clostridium difficile, presumably acquired in an indirect manner from contact with organisms present on environmental surfaces in healthcare facilities;22 - 25 and 4) outbreaks and pseudoepidemics of legionellae,26, 27 Pseudomonas aeruginosa,28 - 30 and the nontuberculous mycobacteria [NTM]31, 32 linked to water and aqueous solutions in healthcare facilities. The purpose of this guideline is to provide useful information for healthcare professionals and engineers alike in efforts to provide quality healthcare to their patients. The recommendations herein provide guidance to minimize and/or prevent transmission of pathogens in the indoor environment.

A variety of airborne infections in susceptible hosts can result from exposures to clinically significant microorganisms released into the air when environmental reservoirs (i.e., soil, water, dust, and decaying organic matter) are disturbed. Once these materials are brought indoors into a healthcare facility by any of a number of vehicles (e.g., people, air currents, water, construction materials, equipment), the attendant microorganisms can proliferate in a variety of indoor ecological niches and, if subsequently disbursed into the air, serve as a source for airborne healthcare-associated infections. Aerosolized oral and nasal secretions from patients represent another important source of pathogens that can be dispersed into the air.33

Respiratory infections can be acquired from exposure to pathogens contained either in droplets or droplet nuclei. Exposure to microorganisms in droplets constitutes a form of direct contact transmission. When droplets are produced during a sneeze or cough, a cloud of infectious particles >5 µm in size is expelled, resulting in the potential exposure of susceptible persons within 3 feet of the source person.6 Examples of pathogens spread in this manner are influenza virus, rhinoviruses, adenoviruses, and respiratory syncytial virus (RSV). Since the transmission of these agents is largely direct and the droplets tend to fall out of the air quickly, measures to control air flow in a healthcare facility (e.g., use of negative pressure rooms) are not generally indicated for preventing the spread of diseases due to these agents. Strategies to control the spread of these diseases are outlined in another guideline.3

The spread of airborne infectious diseases via droplet nuclei is a form of indirect transmission.34 Droplet nuclei are the residuals of droplets that, when suspended in air, subsequently dry and produce particles ranging in size from 1 µm - 5 µm. These particles can: 1) contain potentially viable microorganisms; 2) be protected by a coat of dry secretions; 3) remain suspended indefinitely in air; and 4) be transported over long distances. The persistence of microorganisms in droplet nuclei is favored by dry, cool atmospheric conditions with little or no direct exposure to sunlight or other sources of radiation. Pathogenic microorganisms that can be spread via droplet nuclei include M. tuberculosis, VZV, and measles virus (rubeola).6

Several airborne pathogens have life-cycle forms that are similar in size to droplet nuclei and may exhibit similar behavior in the air. The spores of Aspergillus fumigatus have a diameter of 2 µm - 3.5 µm, with a settling velocity estimated at 0.03 cm/sec, or about 1 meter/hour, in still air. With this enhanced buoyancy, the spores, which resist desiccation, can remain airborne indefinitely in air currents and travel far from their source.35

Aspergillosis is caused by molds belonging to the genus Aspergillus. Aspergillus spp. are prototype healthcare-acquired pathogens associated with dusty or moist environmental conditions. Clinical and epidemiologic aspects of aspergillosis, summarized in Table 1, are discussed extensively in another guideline.3.

Table 1. Clinical and Epidemiologic Characteristics of Aspergillosis

Aspergillus spp. are ubiquitous aerobic fungi that occur in soil, water, and decaying vegetation; the organism also survives well in air, dust, and moisture present in healthcare facilities.91 - 93 The presence of aspergilli in the healthcare facility environment is the most important extrinsic risk factor for opportunistic invasive aspergillosis.69, 94 Site renovation and construction can disturb Aspergillus-contaminated dust and produce bursts of airborne fungal spores. Increased levels of atmospheric dust and fungal spores have been associated with clusters of healthcare-acquired infections in immunocompromised patients.17, 20, 44, 47, 49, 50, 95 - 98

Absorbent building materials (e.g., wallboard) serve as an ideal substrate for the proliferation of this organism if they become and remain wet, thereby increasing the numbers of fungal spores in the area. Patient-care items, devices, and equipment can become contaminated with Aspergillus spp. spores and serve as sources of infection if stored in such areas.57

Most cases of aspergillosis are caused by Aspergillus fumigatus, a thermotolerant/thermophilic fungus capable of growing over a temperature range from 12°C - 53°C (53.6°F - 127.4°F); optimal growth occurs at approximately 40°C (104°F), a temperature inhibitory to most other saprophytic fungi.99 It can use cellulose or sugars as carbon sources; because its respiratory process requires an ample supply of carbon, decomposing organic matter is an ideal substrate.

Other opportunistic fungi that have been occasionally linked with healthcare-associated infections are members of the order Mucorales (e.g., Rhizopus spp.) and miscellaneous moniliaceous molds (e.g., Fusarium spp., Penicillium spp.). Many of these fungi can proliferate in moist environments, such as water-damaged wood and building materials. Some fungi (e.g., Fusarium spp., Pseudoallescheria spp.) can be airborne pathogens as well.100 Some of these agents and their sources in the healthcare environment are listed in Table 2. As with aspergillosis, a major risk factor for disease caused by any of these pathogens is the host’s severe immunosuppression from either underlying disease or immunosuppressive therapy.101, 102

b. American Institute of Architects [AIA] standards stipulate that: 1) exhaust outlets are to be placed >25 feet from air intake systems; 2) the bottom of outdoor air intakes for HVAC systems be 6 feet above ground or 3 feet above roof level; and 3) exhaust outlets from contaminated areas are situated above the roof level and arranged to minimize the recirculation of exhausted air back into the building.120

Infections due to Cryptococcus neoformans, Histoplasma capsulatum, or Coccidioides immitis can potentially occur in healthcare settings if nearby ground is disturbed and a malfunction of the facility’s air-intake components allows these pathogens to enter the ventilation system. C. neoformans is a yeast <2µm in diameter found in soil contaminated with bird droppings, particularly from pigeons.98, 103, 104, 121 H. capsulatum, with particle diameters ranging from 2µm - 5 µm, is endemic in the soil of the central river valleys of the United States. Large numbers of these infectious particles are found associated with chicken coops and the roosts of blackbirds.98, 103, 104, 122

Several outbreaks of histoplasmosis have been associated with disruption of the environment; construction activities in an endemic area may be a potential risk factor for healthcare-acquired airborne infection.123, 124 C. immitis, with an infectious particle of 3µm - 5 µm diameter, has similar potential, especially in the endemic southwestern United States and during seasons of drought followed by heavy rainfall. After the 1994 earthquake centered near Northridge, California, the incidence of coccidioidomycosis in the surrounding area exceeded the historical norm.125

Emerging evidence suggests that Pneumocystis carinii, now classified as a fungus, may be spread via airborne person-to-person transmission.126 Controlled studies in animals first demonstrated that P. carinii could be spread through the air.127 More recent studies in healthcare settings have detected nucleic acids of P. carinii in air samples from areas frequented or occupied by P. carinii-infected patients but not in control areas with no infected patients.128, 129 Despite the earlier assumption that P. carinii pneumonia (PCP) was not spread from person-to-person, clusters of cases have been identified among immunocompromised patients who had contact with a source patient and with each other. Recent studies have examined the presence of P. carinii DNA in oropharyngeal washings and the nares of infected patients, their direct contacts, and persons with no direct contact.130, 131 Molecular analysis of the DNA by polymerase chain reaction (PCR) supports air spread of P. carinii from infected patients to direct contacts, but immunocompetent contacts tend to become transiently colonized rather than infected.131

The role of colonized persons in the spread of PCP remains to be determined. At present, specific modifications to ventilation systems to control spread of PCP in a healthcare facility are not indicated. Current recommendations outline isolation procedures to minimize or eliminate contact of immunocompromised patients not on PCP prophylaxis with PCP-infected patients.6, 132

The prototype bacterium associated with airborne transmission is Mycobacterium tuberculosis. A comprehensive review of the microbiology and epidemiology of M. tuberculosis and guidelines for tuberculosis (TB) infection control have been published.4, 133, 134 Table 3 summarizes clinical and epidemiologic information from these materials. M. tuberculosis is carried by droplet nuclei generated when persons, primarily adults and adolescents, who have pulmonary or laryngeal TB sneeze, cough, speak, or sing;135 normal air currents can keep these particles airborne for prolonged periods and spread them throughout a room or building.136.

Table 3. Clinical and Epidemiologic Characteristics of Tuberculosis 4, 133 - 135, 137 - 140

Gram-positive cocci (i.e., Staphylococcus aureus, group A beta-hemolytic streptococci), also important healthcare-associated pathogens, are resistant to inactivation by drying and can persist in the environment and on environmental surfaces for extended periods. These organisms can be shed from heavily colonized persons and discharged into the air. Airborne dispersal of S. aureus is directly related to the concentration of the bacterium in the anterior nares.141

Approximately 10% of healthy carriers will disseminate S. aureus into the air, and some persons become more effective disseminators of S. aureus than others.142 - 146 The dispersal of S. aureus into air can be exacerbated by concurrent viral upper respiratory infection, thereby turning a carrier into a "cloud shedder."147 Outbreaks of surgical site infections (SSIs) caused by group A beta-hemolytic streptococci have been traced to airborne transmission from colonized operating room personnel to patients.148 - 151 In these situations, the strain causing the outbreak was recovered from the air in the operating room 148, 149, 152 or on settle plates in a room in which the carrier exercised.149 - 151 S. aureus and group A streptococci have not been linked to airborne transmission outside of operating rooms, burn units, and neonatal nurseries.153, 154 Transmission of these agents also occurs via contact (S. aureus) and droplets (group A beta-hemolytic streptococci).

Other gram-positive bacteria linked to airborne transmission include Bacillus spp. which are capable of sporulation as environmental conditions become less favorable to support their growth. Outbreaks and pseudo-outbreaks have been attributed to B. cereus in maternity, pediatric, intensive care, and bronchoscopy units; many of these episodes were secondary to environmental contamination.155 - 58.

Gram-negative bacteria are rarely associated with episodes of airborne transmission because they generally require moist environments for persistence and growth. The main exception is Acinetobacter spp. which can withstand the inactivating effects of drying. In one epidemiologic investigation of bloodstream infections among pediatric patients, identical Acinetobacter spp. were cultured from the patients, air, and room air conditioners in a nursery.159 Aerosols generated from showers and faucets may potentially contain legionellae and other gram-negative waterborne bacteria (e.g., Pseudomonas aeruginosa). Exposure to these organisms is through direct inhalation. However, since water is the source of the organisms and exposure occurs in the vicinity of the aerosol, the discussion of the diseases associated with such aerosols and the prevention measures used to curtail their spread is deferred to the Water portion of Part I.

Some human viruses are transmitted from person to person via droplet aerosols, but very few viruses are consistently airborne in transmission (i.e., routinely suspended in an infective state in air and capable of spreading great distances), and healthcare-associated outbreaks of airborne viral disease are limited to a few agents. Consequently, infection control measures used to prevent spread of these viral diseases in healthcare facilities primarily involve patient isolation, vaccination of susceptible persons, and antiviral therapy as appropriate rather than measures to control air flow or quality.6

Infections due to VZV are frequently described in healthcare facilities. Healthcare-associated airborne outbreaks of VZV infections from patients with primary infection and disseminated zoster have been documented; patients with localized zoster have, on rare occasions, also served as source patients for outbreaks in healthcare facilities.160 - 164 VZV infection can be prevented by vaccination, although patients who develop a rash within 6 weeks of receiving varicella vaccine or who develop breakthrough varicella following exposure should be considered contagious.165

In a limited number of instances, viruses whose major mode of transmission is via droplet contact have been shown to cause clusters of infections in group settings through airborne routes. The factors facilitating airborne distribution of these viruses in an infective state are unknown, but a presumed requirement is a source patient in the early stage of infection who is shedding large numbers of viral particles into the air. Airborne transmission of measles has been documented in healthcare facilities.166 - 169

Institutional outbreaks of influenza virus infections have occurred predominantly in nursing homes,170 - 174 and less frequently in medical and neonatal intensive care units, chronic care areas, HSCT units, and pediatric wards.175 - 178 There is some evidence supporting airborne transmission of influenza viruses by droplet nuclei,179 - 180 and case clusters in pediatric wards suggest that droplet nuclei may play a role in transmitting respiratory pathogens such as adenoviruses and RSV.175, 181, 182 Some suggestive evidence also supports airborne transmission of enteric viruses. A large outbreak of a Norwalk-like virus infection involving more than 600 staff personnel over a 3 week period was investigated in a Toronto, Ontario hospital in 1985. Common sources such as food or water were ruled out during the investigation, leaving airborne spread as the most likely candidate for transmission.183

Smallpox virus, a potential agent of bioterrorism, is spread predominantly via direct contact with infectious droplets, but can be associated with airborne transmission.184, 185 A German hospital study from 1970 documented the ability of this virus to spread over considerable distances and cause infection at low doses in a well-vaccinated population; factors potentially facilitating transmission in this situation included a patient with cough and an extensive rash, indoor air with low relative humidity, and faulty ventilation patterns due to hospital design.186 Smallpox patients with extensive rash are more likely to have lesions present on mucous membranes and therefore have greater potential to disseminate virus into the air.186 Two cases of laboratory-acquired smallpox virus infection in the United Kingdom in 1978 were also thought to be due to airborne transmission.187

Airborne transmission may play a role in the natural spread of hantaviruses and certain hemorrhagic fever viruses (e.g., Ebola, Marburg, Lassa), but evidence for airborne spread of these agents in healthcare facilities is inconclusive.188 Although hantaviruses can be transmitted when aerosolized from rodent excreta,189, 190 person-to-person spread of hantavirus infection from source patients has not occurred in healthcare facilities.191 - 193 Nevertheless, healthcare workers are advised to contain potentially infectious aerosols and wear NIOSH-approved respiratory protection when working with this agent in laboratories or autopsy suites.194 Lassa virus transmission via aerosols has been demonstrated in the laboratory and incriminated in healthcare-associated infections in Africa,195 - 197 but airborne spread of this agent in hospitals of developed nations appears to be inefficient.198, 199 Viral hemorrhagic diseases primarily occur after direct exposure to infected blood and body fluids, and the use of standard and droplet precautions is sufficient to prevent transmission early in the course of these illnesses.200 However, it is unclear whether these viruses can persist in droplet nuclei that might remain after droplet production from coughs or vomiting in the latter stages of illness.201

Although the use of a negative-pressure room is not required during the early stages of illness, its use might be prudent at the time of hospitalization to avoid the need for subsequent patient transfer. CDC guidelines recommend negative-pressure rooms with anterooms for patients with hemorrhagic fever and use of HEPA respirators by persons entering these rooms when the patient has prominent cough, vomiting, diarrhea, or hemorrhage.6, 200

Heating, ventilation, and air conditioning (HVAC) systems in healthcare facilities are designed to: 1) maintain the indoor air temperature and humidity at comfortable levels for staff, patients, and visitors; 2) control odors; 3) remove contaminated air; 4) facilitate air-handling requirements to protect susceptible staff and patients from airborne healthcare-associated pathogens; and 5) minimize the risk of transmission of airborne pathogens from infected patients.35, 120 An HVAC system includes an air inlet or intake; filters; humidity modification mechanisms (i.e., humidity control in summer, humidification in winter); heating and cooling equipment; fans; ducts; air exhaust or outtakes; and registers, diffusers, or grilles for proper distribution of the air (Figure 1).209, 210 Decreased performance of healthcare facility HVAC systems, filter inefficiencies, improper installation, and poor maintenance can contribute to the spread of healthcare-associated airborne infections.

Figure 1. Diagram of a Ventilation System a

The American Institute of Architects (AIA) has published guidelines for the design, construction, and renovation of healthcare facilities that include indoor air-quality standards (e.g., ventilation rates, temperature levels, humidity levels, pressure relationships, minimum air changes per hour [ACH]) specific to each zone or area in healthcare facilities (e.g., operating rooms, laboratories, diagnostic areas, patient-care areas, support departments).120 These guidelines represent a consensus document among authorities having jurisdiction (AHJ), governmental regulatory agencies (i.e., Department of Health and Human Services [DHHS]; Department of Labor, Occupational Safety and Health Administration [OSHA]), healthcare professionals, professional organizations (e.g., American Society of Heating, Refrigeration, and Air-conditioning Engineers [ASHRAE], American Society of Healthcare Engineers [ASHE]), and accrediting organizations (i.e., Joint Commission on Accreditation of Healthcare Organizations [JCAHO]). Many state or local agencies that license healthcare facilities have either incorporated or adopted by reference these guidelines into their state standards. The JCAHO, through its surveys, assures that facilities are in compliance with the space and square footage requirements of this standard for new construction.

Recommendations for engineering controls to contain or prevent the spread of airborne contaminants center on: 1) local exhaust ventilation [i.e., source control]; 2) general ventilation; and 3) air cleaning.4 General ventilation encompasses: 1) dilution and removal of contaminants via filtration and air changes per hour [ACH]; 2) airflow patterns in rooms or areas; 3) airflow direction in facilities; and 4) pressure differentials for special-care areas.

A centralized HVAC system operates as follows. Outdoor air enters the system, where low- efficiency or "roughing" filters remove large particulate matter and many microorganisms. The air enters the distribution system for conditioning to appropriate temperature and humidity levels, passes through an additional bank of filters for further cleaning, and is delivered to each zone of the building. After the conditioned air is distributed to the designated space, it is withdrawn through a return duct system and delivered back to the HVAC unit. A portion of this "return air" is exhausted to the outside while the remainder is mixed with outdoor air and filtered for dilution and removal of contaminants.211 Air from toilet rooms or other soiled areas is usually exhausted directly to the atmosphere through a separate duct exhaust system. Air from rooms housing tuberculosis patients is exhausted to the outside if possible, or passed through a HEPA filter before recirculation. UVGI can be used as an adjunct air-cleaning measure but cannot replace HEPA filtration.

Filtration is the primary means of cleaning the air. There are five methods of filtration (Table 5). During filtration, outdoor air passes through two filter beds or banks, with efficiencies of 20% - 40% and the second >90%, respectively, for a combined efficiency of nearly 100% in removing particles 1µm - 5 µm in diameter.35 The low-to-medium efficiency filters in the first bank have low resistance to airflow, but this feature tends to allow some small particulates to pass onto heating and air conditioning coils and into the indoor environment.35 Incoming air is mixed with recirculated air and reconditioned for temperature and humidity before being filtered by the second bank of filters. The performance of filters with <90% efficiency is measured using either the dust spot test or the weight-arrestance test.35

The second filter bank usually consists of high-efficiency filters. High-efficiency air filtration systems can provide air that is almost particle free. This filtration system is adequate for most patient-care areas in ambulatory care facilities and hospitals, including the operating room environment and areas providing central services.120 Nursing facilities may use 90% dust-spot efficient filters as the second bank of filters,120 while a HEPA filter bank may be indicated for special-care areas of hospitals. HEPA filters are at least 99.97% efficient for removing particles >0.3 µm in diameter. (As a reference, Aspergillus spores are 2.5 - 3 µm in diameter.) Examples of care areas where HEPA filters are used include rooms housing severely neutropenic patients and those operating rooms designated for orthopedic implant procedures.35

Maintenance costs associated with HEPA filters are high when compared to other types of filters, but use of in-line disposable prefilters can increase the life of a HEPA filter by approximately 25%. Alternatively, if a disposable prefilter is followed by a 90%-efficient filter, the life of the HEPA filter can be extended up to 900%. This concept, called "progressive filtration," allows HEPA filters in special care areas to be used for 10 years or more.209 HEPA filter efficiency is monitored with the dioctylphthalate (DOP) particle test using particles that are 0.3 µm in diameter.

HEPA filters are usually fixed into the HVAC system, but portable, industrial grade HEPA units are available which filter air at the rate of 300-800 ft 3 /min are also available. Portable HEPA filter are used to: 1) temporarily recirculate air in rooms with no general ventilation; 2) augment systems that cannot provide adequate airflow; or 3) provide increased effectiveness in airflow.4 Portable HEPA units are useful engineering controls when the central HVAC system is undergoing repairs,213 but these units do not satisfy fresh air requirements.210 The effectiveness of the portable unit for particle removal is dependent on: 1) the configuration of the room; 2) the furniture and persons in the room; 3) the placement of the units relative to the contents and layout of the room; and 4) the location of the supply and exhaust registers or grilles. If portable, industrial-grade units are used, they should be capable of recirculating all or nearly all of the room air through the HEPA filter, and the unit should be designed to achieve the equivalent of >12 air changes per hour (ACH).4 (An average room has approximately 1600 ft 3 of airspace).

Efficiency of the filtration system is dependent on the density of the filters that may create a pressure drop unless compensated by stronger and more efficient fans so that flow of air is maintained. For optimal performance, filters require monitoring and replacement in accordance with the manufacturer’s recommendations and standard preventive maintenance practices.214

Excess accumulation of dust and particulates increases filter efficiency, requiring more pressure to push the air through. The pressure differential across filters is measured by use of manometers or other gauges. A pressure reading that exceeds specifications indicates the need to change the filter. Filters also require regular inspection for other potential causes of decreased performance. Gaps in and around filter banks and heavy soil and debris upstream of poorly-maintained filters have been implicated in healthcare-associated outbreaks of aspergillosis, especially during times of nearby construction.17, 18, 106, 215

As a supplemental air-cleaning measure, UVGI is effective in reducing the transmission of airborne bacterial and viral infections in hospitals, military housing, and classrooms, but it has only a minimal inactivating effect on fungal spores.216 - 221 UVGI is also used in air handling units to prevent or limit the growth of vegetative bacteria and fungi. Most commercially available UV lamps used for germicidal purposes are low-pressure mercury vapor lamps that emit radiant energy predominantly at a wave-length of 253.7 nm.222, 223 Two systems of UVGI have been used in healthcare settings -- duct irradiation and upper-room air irradiation. In duct irradiation systems, UV lamps are placed inside ducts that remove air from rooms to disinfect the air before it is recirculated. When properly designed, installed, and maintained, high levels of UVGI can be attained in the ducts with little or no exposure of persons in the rooms.224, 225 In upper-room air irradiation, UV lamps are either suspended from the ceiling or mounted on the wall.4

Upper air UVGI units have two basic designs: 1) a "pan" fixture with UVGI unshielded above the unit so to direct the irradiation upward; or 2) a fixture with a series of parallel plates to columize the irradiation outward while preventing the light from getting to the eyes of the room’s occupants. The germicidal effect is dependent on air mixing via convection between the room’s irradiated upper zone and the lower patient-care zones.226, 227

Bacterial inactivation studies using BCG mycobacteria and Serratia marcescens have estimated the effect of UVGI as equivalent to 10 ACH - 39 ACH.228, 229 Another study, however, suggests that UVGI may result in fewer equivalent ACH in the patient-care zone, especially if the mixing of air between zones is insufficient.227 The use of fans or HVAC systems to generate air movement may increase the effectiveness of UVGI if airborne microorganisms are exposed to the light energy for a sufficient length of time.226, 228, 230 - 232 The optimal relationship between ventilation and UVGI is not known.

Because the clinical effectiveness of UV systems may vary, UVGI is not recommended for air managment prior to air recirculation from airborne isolation rooms. It is also not recommended as a substitute for HEPA filtration, local exhaust of air to the outside, or negative pressure.4 The use of UV lamps and HEPA filtration in a single unit offers little or no infection control benefits over those provided by the use of a HEPA filter alone.233 Duct systems with UVGI are not recommended as a substitute for HEPA filters if the air from isolation rooms must be recirculated to other areas of the facility.4

Regular maintenance of UVGI systems is crucial and usually consists of keeping the bulbs free of dust and replacing old bulbs as necessary. Safety issues associated with the use of UVGI systems are described in other guidelines.4.

HVAC systems in healthcare facilities have either single-duct or dual-duct systems.35, 234 A single-duct system distributes cooled air (12.8°C [55°F]) throughout the building, and uses thermostatically controlled reheat boxes located in the terminal ductwork to warm the air for individual or multiple rooms. The more common dual-duct system consists of parallel ducts, one with a cold air stream and the other providing a hot air stream. A mixing box in each room or group of rooms mixes the two air streams to achieve the desired temperature. Temperature standards are given as either a single temperature or a range, depending on the specific healthcare zone. Cool temperature standards (20°C -23° C [68°F - 73°F]) are usually associated with operating rooms, clean workrooms, and endoscopy suites.120

A warmer temperature (24°C [75°F]) is needed in areas requiring greater degrees of patient comfort. Most other zones use a temperature range of 21°C - 24°C (70°F - 75°F).120 Temperatures outside of these ranges may be needed on limited occasions in limited areas depending on individual circumstances during patient care (e.g., cooler temperatures in operating rooms during specialized operations).

Four measures of humidity are used to quantify different physical properties of the mixture of water vapor and air. The most common of these is "relative humidity," which is the ratio of the amount of water vapor in the air to the amount of water vapor air can hold at that temperature.235 The other measures of humidity are specific humidity, dew point, and vapor pressure.235

Relative humidity measures the percentage of saturation. At 100% relative humidity, the air is saturated. For most areas within healthcare facilities, the designated comfort range is 30% - 60% relative humidity.120, 210 Relative humidity levels >60%, in addition to being perceived as uncomfortable, promote fungal growth.236 Humidity levels can be manipulated by either of two mechanisms.237 In a water-wash unit, water is sprayed and drops are taken up by the filtered air; additional heating or cooling of this air sets the humidity levels. The second mechanism is by means of water vapor created from steam and added to filtered air in humidifying boxes.

Ventilation supply rates are historically based on the need to control odors and carbon dioxide levels.238 Ventilation rates are voluntary unless a state or local government specifies a standard in healthcare licensing or health department requirements. These standards typically apply to only the design of a facility, rather than its operation.214, 239 Based on the scientific knowledge and professional judgment reflected in the AIA guidelines, ASHRAE has developed ventilation standards designed primarily to satisfy the odor criterion.238 ASHRAE and the American National Standards Institute (ANSI) have produced design recommendations for ventilation and pressure relationships for various patient-care areas.210 Healthcare facilities without specific ventilation standards should follow ANSI/ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality.210, 234

Ventilation guidelines are defined in terms of air volume per minute per occupant, and are based on the assumption that occupants and their activities are responsible for most of the contaminants in the conditioned space.211 Most ventilation rates for healthcare facilities are expressed as room air changes of filtered air per hour (ACH). Peak efficiency for particle removal in the air space occurs between 12 - 15 ACH.35, 240 Ventilation rates vary among the different patient-care areas of a healthcare facility (Appendix B).120

Healthcare facilities generally use recirculated air.35, 120, 234, 241, 242 Fans create sufficient positive pressure to force air through the building duct work and adequate negative pressure to evacuate air from the conditioned space into the return duct work and/or exhaust, thereby completing the circuit in a sealed system (Figure 1). However, because gaseous contaminants tend to accumulate as the air recirculates, a percentage of the recirculated air is exhausted to the outside and replaced by fresh outdoor air (usually a 60/40 mix of outdoor air/recirculated air).

In hospitals, filtered air is typically distributed from the ceiling, with return air collected from the ceiling on the other side of the room. In special situations in which the direction of air movement needs to be controlled (e.g., operating rooms, delivery rooms, catheterization laboratories, angiography rooms, HEPA-filtered rooms for immunosuppressed patients), the air is introduced from ceiling registers or diffusers near the center of the room, flows down to the patient-care zone, and is returned or exhausted through registers located at least 6 inches above the floor. Filtered air is introduced into negative-pressure, airborne infection isolation rooms (AII) above and near the doorway so that it passes through the breathing zone of workers and visitors before passing over the patient and being exhausted near the head of the bed, preferably from the side wall.4, 35

Older hospitals with areas not served by central HVAC systems often use induction units (e.g., fan-coil units, heat-pump units) as the sole source of room ventilation. AIA guidelines for newly-installed systems stipulate that induction units shall be equipped with permanent (cleanable) or replaceable filters with a minimum efficiency of 68% weight arrestance.120 These units may be used only as recirculating units; all outdoor air requirements must be met by a separate central air handling system with proper filtration, with a minimum of two outside air changes in general patient rooms.120, 243 If a patient room is equipped with an individual "through the wall" induction unit, the room should not be used as either AII or as PE.120 These requirements, although directed to new installations, are also appropriate for existing settings. Induction units are prone to problems associated with excess condensation accumulating in drip pans and improper filter maintenance; healthcare facilities should clean or replace the filters in these units on a regular basis while the patient is out of the room.

Laminar airflow ventilation systems are designed to move air in a single pass, usually through a bank of HEPA filters either along a wall or in the ceiling, in a one-way direction through a clean zone with parallel streamlines. Laminar airflow can directed vertically or horizontally; the unidirectional system optimizes airflow and minimizes air turbulence.63, 234

Delivery of air at a rate of 0.5 meters per second (90 + 20 ft/min) helps to minimize opportunities for microorganism proliferation.63, 244, 245 Laminar airflow systems have been used in PE to help reduce the risk for healthcare-associated airborne infections such as aspergillosis in high-risk patients.63, 93, 246, 247 However, data that demonstrate a bona fide survival benefit for patients in PE with laminar airflow are lacking. Given the high cost of installation and apparent lack of benefit, the value of laminar airflow in this setting is questionable.9, 37 Few data support the use of laminar airflow systems elsewhere in a hospital.248

Positive and negative pressures refer to a pressure differential between two adjacent air spaces (e.g., rooms and hallways). Air flows away from areas or rooms with positive pressure, while air flows into areas with negative pressure. AII rooms are set at negative pressure to prevent airborne microorganisms in the room from entering hallways and corridors. PE rooms housing severely neutropenic patients are set at positive pressure to keep airborne pathogens in adjacent spaces or corridors from coming into and contaminating the airspace occupied by such high-risk patients. Self-closing doors are mandatory for both of these areas to help maintain the correct pressure differential.4, 6, 120

Older healthcare facilities may have variable pressure rooms (i.e., rooms in which the ventilation can be manually switched between positive- and negative pressure). These rooms are no longer permitted in the construction of new facilities,120 and their use in existing facilities is discouraged because of difficulties in assuring the proper pressure differential, especially for the negative pressure setting, and the potential for error associated with switching the pressure differentials for the room. Engineering specifications for positive- or negative pressure rooms are given in Table 6.

Healthcare facilities must perform a risk assessment to determine the appropriate number of AII rooms (negative pressure) and/or PE rooms (positive pressure) to serve its patient population. The AIA guidelines require a certain number of AII rooms as a minimum.120

In large healthcare facilities with central HVAC systems, sealed windows help to ensure the efficient operation of the system, especially with respect to creating and maintaining pressure differentials. Sealing the windows in PE areas will help to minimize the risk of airborne contamination from the outside. One outbreak of aspergillosis among immunosuppressed patients in a hospital was attributed in part to an open window in the unit during a time when both construction and a fire happened nearby; sealing the window prevented further entry of fungal spores into the unit from the outside air.111 Additionally, all emergency exits (e.g., fire escapes, emergency doors) in PE wards should be kept closed (except during emergencies) and equipped with alarms.

AIA guidelines prohibit United States hospitals and surgical centers from shutting down their HVAC systems for purposes other than required maintenance, filter changes, and construction.120 Airflow can be reduced, but sufficient supply, return, and exhaust must be provided to maintain required pressure relationships when the space is not occupied. This can be accomplished with special drives on the air-handling units (i.e., a Variable Air Ventilation [VAV] system). Microorganisms proliferate in environments wherever air, dust, and water are present, and air-handling systems can be ideal environments for microbial growth.35 Properly engineered HVAC systems require routine maintenance and monitoring in order to provide acceptable indoor air quality efficiently and to minimize conditions that favor the proliferation of healthcare-associated pathogens.35, 241 Performance monitoring of the system includes determining pressure differentials across filters, regular inspection of system filters, DOP testing of HEPA filters, testing of low- or medium efficiency filters, and manometer tests for positive- and negative-pressure areas in accordance with nationally recognized standards, guidelines, and manufacturers’ recommendations. The use of hand-held calibrated equipment that can provide a numerical reading on a daily basis is preferred for engineering purposes.249, 250 Several methods that provide a visual, qualitative measure of pressure differentials include smoke-tube tests, or placing flutter strips, ping-pong balls, or tissue in the air stream.

Preventive filter and duct maintenance (e.g., cleaning ductwork vents, replacing filters as needed, properly disposing spent filters into plastic bags immediately upon removal) is important to prevent potential exposures of patients and staff during HVAC system shut-down. Additionally, a malfunction of the air-intake system can overburden the filtering system and permit aerosolization of fungal pathogens. Keeping the intakes free from bird droppings, especially those from pigeons, helps to minimize the concentration of fungal spores entering from the outside.98

Accumulation of dust and moisture within HVAC systems increases the risk of spread of healthcare-associated environmental fungi and bacteria. Clusters of infections due to Aspergillus spp., Pseudomonas aeruginosa, Staphylococcus aureus, and Acinetobacter spp. have been linked to poorly maintained and/or malfunctioning air conditioning systems.68, 159, 251, 252 Efforts to limit excess humidity and moisture in the infrastructure and on air stream surfaces in the HVAC system can minimize the proliferation and dispersion of fungal spores and waterborne bacteria throughout the indoor air.253 - 255

Within the HVAC system, water is present in water-wash units, humidifying boxes, or cooling units. The dual-duct system may also create conditions of high humidity and excess moisture that favor fungal growth in drain pans as well as in fibrous insulation material which becomes damp as a result of the humid air passing over the hot stream and condensing. All intake air should be dehumidified to avoid condensation when the air is mixed. If moisture is present in the HVAC system, it is important to avoid periods of stagnation, such as would occur if the system is temporarily shut down. Bursts of organisms tend to be released upon system start-up which may increase the risk of airborne infection.202 Proper engineering of the HVAC system is critical to preventing dispersal of airborne organisms. Endophthalmitis due to Acremonium kiliense infection following cataract extraction in an ambulatory surgical center was traced to aerosols derived from the humidifier water in the ventilation system.202 The organism proliferated because the ventilation system was turned off routinely when the center was not in operation and the air was filtered before humidification, but not afterwards.

Duct cleaning in healthcare facilities has benefits in terms of system performance, but its usefulness for infection control has not been conclusively determined. Duct cleaning typically involves using specialized tools to dislodge dirt and a high-powered vacuum cleaner to clean out debris. Some duct-cleaning services also apply chemical biocides or sealants to the inside surfaces of ducts to minimize fungal growth and prevent the release of particulate matter. Although infrequent cleaning of the exhaust ducts in AII areas has been documented to be a cause of diminishing negative pressure and a decrease in the air exchange rates,210 there are no data to indicate that duct cleaning, beyond what is recommended for optimal performance, improves indoor air quality or reduces the risk of infection.

Exhaust return systems should be cleaned as part of routine system maintenance. Duct cleaning has not been shown to prevent any health problems,256 and U.S. Environmental Protection Agency (EPA) studies indicate that airborne particulate levels do not increase as a result of dirty air ducts, nor do they diminish after cleaning, presumably because much of the dirt inside air ducts adheres to duct surfaces and does not enter the conditioned space.256 Additional research is needed to determine if air duct contamination can significantly increase the airborne infection risk in general areas of healthcare facilities.

Table 7. Ventilation Hazards in Healthcare Facilities That May Be Associated with Increased Potential of Airborne Disease Transmission 35

Construction, renovation, repair, and demolition activities in healthcare facilities require substantial planning and coordination to minimize the risk of airborne infection both during projects and after their completion. Several organizations and experts have endorsed a multi-disciplinary team approach to coordinate the various stages of construction activities (e.g., project inception, project implementation, final walk-through, and completion).120, 241, 242, 263 –266

Education of maintenance and construction workers, healthcare staff charged with the care of with high-risk patients, and persons responsible for controlling indoor air quality can help to minimize dust and moisture intrusion from construction sites into high-risk patient care areas.120, 242, 263, 265 - 268 Visual and printed educational materials should be provided as appropriate in the language of the workers. Staff and construction workers also need to be aware of the potentially catastrophic consequences of dust and moisture intrusion when an HVAC system or water system fails during construction or repair; action plans to deal quickly with these emergencies should be developed in advance and kept on file. Incorporation of specific standards into construction contracts may help to prevent departures from recommended practices as projects progress. Establishing specific lines of communication is important to address problems (e.g., dust control, indoor air quality, noise levels, vibrations), resolve complaints, and keep projects moving toward completion.

Three major topics to consider before initiating any construction or repair activity are: 1) design and function of the new structure or area; 2) assessment of environmental risks for airborne disease and opportunities for prevention; and 3) measures to contain dust and moisture during construction or repairs. Table 9 provides a checklist of design and function considerations to ensure that a planned structure or area can be easily serviced and maintained for environmental infection control.17, 241, 263, 265 - 267 Specifications for the construction, renovation, remodeling, and maintenance of healthcare facilities are outlined in the AIA document, Guidelines for Design and Construction of Hospitals and Health Care Facilities.120, 265

Table 9. Construction Design and Function Considerations for Environmental Infection Control

1. Although bacteria and fungi can be recovered in great numbers from carpet, its use has not been shown to be a consistent risk for healthcare-associated infections in immunocompromised patient-care areas. Use of carpet cleaning methods (e.g., "bonneting") that disperse microorganisms into the air of these special patient care areas, however, may increase the risk of airborne infection.111

Proactive strategies can help prevent environmentally-mediated airborne infections in healthcare facilities during demolition, construction, and renovation. The potential presence of dust and moisture and their contribution to healthcare-associated infections must be critically evaluated early in the planning of any demolition, construction, renovation, and repairs.120, 241, 242, 263, 264, 266 - 269 Consideration must extend beyond dust generated by major projects to include dust that can become airborne if disturbed during routine maintenance and minor renovation activities (e.g., exposure of ceiling spaces for inspection; installation of conduits, cable, or sprinkler systems; rewiring; structural repairs or replacement).263, 266, 267 Other projects that can compromise indoor air quality include construction and repair jobs that inadvertently allow substantial amounts of raw, unfiltered outdoor air to enter the facility (e.g., repair of elevators and elevator shafts) and activities that dampen any structure, area, or item made of porous materials or characterized by cracks and crevices (e.g., sink cabinets in need of repair, carpets, ceilings, floors, walls, vinyl wall coverings, upholstery, drapes, and countertops).18, 263, 267 Molds grow and proliferate on these surfaces should these materials become and remain wet.21, 120, 242, 260, 263, 270 Scrubable materials are preferred for use in patient-care areas.

Knowledge of the airflow patterns and pressure differentials will help to minimize or eliminate the inadvertent dispersion of dust that could contaminate air space, patient-care items, and surfaces.57, 271, 272 During long-term projects, providing temporary essential services (e.g., toilet facilities, vending machines) to construction workers within the site will help to minimize traffic in and out of the area. The type of barrier systems necessary for the scope of the project must be defined.12, 120, 242, 269, 273

Depending on the location and extent of the construction, patients may need to be relocated to other areas in the facility not affected by construction dust.51, 274 This is especially important when construction takes place within units housing immunocompromised patients, severely neutropenic patients, or patients on corticosteroid therapy. Advance assessment of high-risk locations and planning for the possible transport of patients to other departments can minimize delays and waiting time in hallways.51 Hospitals may provide immunocompromised patients with respiratory protection devices for use outside their rooms, although this has not been evaluated for preventing exposure to fungal spores. Protective respirators (i.e., N95) appeared to be well tolerated by patients in one recent study.272

Surveillance activities should augment preventive strategies during construction projects.3, 4, 20, 110, 275, 276 By determining baseline levels of healthcare-acquired airborne and waterborne infections, infection control staff can monitor changes in infection rates and patterns during and immediately after construction, renovations, or repairs.3

Air sampling in healthcare facilities may be used both during periods of construction and on a periodic basis to determine indoor air quality, efficacy of dust control measures, or air-handling system performance via parametric monitoring. Parametric monitoring consists of measuring the physical performance of the HVAC system in accordance with the system manufacturer’s specifications. A periodic assessment of the system can give assurance of proper ventilation, especially for special-care areas and operating rooms (e.g., airflow direction and pressure, ACH, filter efficiency).277

Particle counts in a given air space within the healthcare facility should be evaluated against counts obtained in a comparison area. Particle counts indoors are commonly compared with the particulate levels of the outdoor air. This approach determines the "rank order" air quality from "dirty" (i.e., the outdoor air) to "clean" (i.e., air filtered through high-efficiency filters [90% - 95% filtration]) to "cleanest" (i.e., HEPA-filtered air).277 Comparisons from one indoor area to another may also provide useful information about the magnitude of an indoor air quality problem. Presently, rank-order comparisons among clean, highly-filtered areas and dirty areas and/or outdoors has been suggested as one way to interpret sampling results in the absence of air quality and action level standards.35, 278

In addition to verifying filter performance, particle counts can help determine if barriers and efforts to control dust dispersion from construction are effective. This type of monitoring is helpful when performed at various times and barrier perimeter locations during the project. Gaps or breaks in the barriers’ joints or seals can then be identified and repaired. With respect to occupational health, the American Conference of Governmental Industrial Hygienists (ACGIH) has set a threshold limit value-time weighted average (TLV®-TWA) of 10 mg/m 3 for nuisance dust that contains no asbestos and <1% crystalline silica.279 Alternatively, OSHA has set permissible exposure limits (PELs) for inert or nuisance dust as follows: respirable fraction at 5 mg/m 3 and total dust at 15 mg/m 3 . 280

Although these standards are not measures of a bioaerosol, they are used for indoor air quality assessment in occupational settings and may be useful criteria in construction areas. Application of ACGIH guidance to healthcare settings has not been standardized, but particulate counts in healthcare facilities are likely to be well below this threshold value and approaching clean-room standards in care areas such as operating rooms.100

The most significant technical limitation of air sampling for airborne fungal agents is the lack of standards linking fungal spore levels with infection rates. Despite this limitation, several heathcare institutions have opted to use microbiologic sampling when construction projects are anticipated and/or underway in efforts to assess the safety of the environment for the immunocompromised patients.35, 278

From a practical standpoint, microbiologic air sampling should be limited to assays for airborne fungi; of those, the thermotolerant fungi (i.e., those capable of growing at 35°C - 37°C [95°F -98.6° F]) are of particular concern because of their pathogenicity in immunocompromised hosts.35 Use of selective media (e.g., Sabourauds, inhibitory mold agar) helps with the initial identification of recovered organisms. Microbiologic sampling for fungal spores performed as part of various airborne disease outbreak investigations has also been problematic.18, 49, 106, 111, 112, 278 The precise source of a fungus is often difficult to trace with certainty, and sampling conducted after exposure may neither reflect the circumstances that were linked to infection nor distinguish between healthcare-acquired and community-acquired infections. Because fungal strains may fluctuate rapidly in the environment, healthcare-acquired Aspergillus spp. infection cannot be confirmed or excluded if the infecting strain is not found in the healthcare setting.276 The use of sensitive molecular typing methods (e.g., randomly amplified polymorphic DNA (RAPD) techniques or more recently a DNA fingerprinting technique that detects restriction fragment length polymorphisms in fungal genomic DNA) to identify strain differences among Aspergillus spp., however, is increasing in importance in epidemiologic investigations of healthcare-acquired fungal infection.68, 110, 250, 275, 276, 281 – 285

Table 10. Unresolved Issues Associated with Microbiologic Air Sampling 35, 100, 215, 278, 286

Settle plates, because they rely on gravity during sampling, tend to select for larger particles and lack sensitivity for respirable particles (e.g., individual fungal spores), especially in highly-filtered environments, and thus are considered impractical for general use.35, 278, 287 - 290 Settle plates, however, may detect fungi aerosolized during medical procedures (e.g., during wound dressing changes), as described in a recent outbreak of aspergillosis among liver transplant patients.291

The use of slit or sieve impactor samplers capable of collecting large volumes of air in short periods of time are needed to detect low numbers of fungal spores in highly-filtered areas.35, 278 In some outbreaks, aspergillosis cases have occurred when fungal spore concentrations in PE ambient air ranged as low as 0.9 - 2.2 colony-forming units per cubic meter (CFU/m 3 ) of air.18, 94 Based on the expected spore counts in the ambient air and the performance parameters of various types of volumetric air samplers, investigators of a recent aspergillosis outbreak have suggested that an air volume of at least 1000 L (1 m 3 ) should be considered when sampling highly filtered areas.272

Investigators have also suggested limits of 15 CFU/m 3 for gross colony counts of fungal organisms and <0.1 CFU/m 3 for Aspergillus fumigatus and other potentially opportunistic fungi in heavily filtered areas (>12 ACH and filtration of >99.97% efficiency).120 There has been no reported correlation of these values with the incidence of healthcare-associated fungal infection rates.

Air sampling in healthcare facilities, whether used to monitor air quality during construction, to verify filter efficiency, or to commission new space prior to occupancy, depends on careful notation of the circumstances of sampling. Most air sampling is performed under undisturbed conditions. However, when the air is sampled during or after human activity (e.g., walking, vacuuming), a higher number of airborne microorganisms is detected.286 The contribution of human activity to the significance of air sampling and the impact on healthcare-associated infection rates remain to be defined.

Comparing microbiologic sampling results from a target area (e.g., an area of construction) to those from an unaffected location in the facility can provide information about distribution and concentration of potential airborne pathogens. A comparison of microbial species densities of outdoor air to those obtained from indoor air has been used to help pinpoint fungal spore bursts. Fungal spore densities in outdoor air are variable, although the degree of variation with the seasons appears to be more dramatic in the United States than in Europe.92, 276, 292

Particulate and microbiologic air sampling have been used when commissioning new HVAC system installations, but this is particularly important for newly constructed or renovated PE or operating rooms. Particulate sampling is used as part of a battery of tests to determine if a new HVAC system is performing to specifications for filtration and the proper number of ACH.258, 277, 293 Microbiologic air sampling, however, remains controversial in this application, as there are no standards for comparison purposes. If performed, it should be limited to determining the density of fungal spores per unit volume of air space. High numbers of spores may indicate contamination of air handling system components prior to installation, or a system deficiency when culture results are compared to known filter efficiencies and rates of air exchange.

In one study, peak concentrations in outdoor air at grade level and HVAC intakes during site excavation averaged 20,000 CFU/m 3 for all fungi and 500 CFU/m 3 for Aspergillus fumigatus, compared with 19 CFU/m 3 and 4 CFU/m 3 , respectively, in the absence of construction.270 Important issues to review prior to demolition include:120, 241, 242, 263, 266, 267, 270, 294 1) proximity of the air intake system to the work site; 2) adequacy of window seals and door seals; 3) proximity of areas frequented by immunocompromised patients; and 4) location of the underground water pipes. Strategies for minimizing the intrusion of dust and moisture are summarized in Table 11.

a. Contamination of water pipes during demolition activities has been associated with healthcare-associated transmission of Legionella. 294

Selected air handlers, especially those located close to excavation sites, may have to be shut off temporarily to keep from overloading the system with dust and debris. Care is needed to avoid significant facility-wide reductions in pressure differentials that may cause the building to become negatively pressured relative to the outside. To prevent excessive particulate overload and subsequent reductions in effectiveness of intake air systems that cannot be shut off temporarily, air filters must be inspected frequently for proper installation and function. Excessive dust penetration can be avoided if recirculated air is maximally utilized while outdoor air intakes are shut down. Scheduling demolition and excavation during the winter, when Aspergillus spp. spores may be present in lower numbers, can help, although seasonal variations in spore density differ around the world.92, 276, 292

Dust control can be managed by misting the dirt and debris during heavy dust-generating activities. To decrease the amount of aerosols from excavation and demolition projects, nearby windows, especially in areas with immunocompromised patients, can be sealed and window and door frames caulked or weather-stripped to prevent dust intrusion.50, 290, 295 Monitoring for adherence to these control measures throughout demolition or excavation is crucial. Diverting pedestrian traffic away from the construction sites decreases the amount of dust tracked back into the healthcare facility and minimizes exposure of high-risk patients to environmental pathogens.

Physical barriers to contain smoke and dust will confine disbursed fungal spores to the construction zone.269, 273, 296, 297 The type of barrier required depends on the project’s scope and duration and on local fire codes. Short-term projects that result in minimal dust dispersion (e.g., installation of new cables or wiring above ceiling tiles) require only portable plastic enclosures with negative pressure and HEPA filtration of the exhaust air from the enclosed work area. The placement of a portable industrial-grade HEPA filter device (300 - 800 ft 3 /min.) adjacent to the work area will help to remove fungal spores, but its efficacy is dependent on the supplied ACH and size of the area.

If the project is more extensive than a repair job but still is considered a short-term undertaking, then dust-abatement, fire-resistant plastic curtains (e.g., Visqueen®) may be adequate. These should be completely airtight and sealed from ceiling to floor with overlapping curtains;266, 267, 298 holes, tears, or other perforations should be repaired promptly with tape. A portable industrial-grade HEPA filter unit on continuous operation may be needed within the contained area, with the filtered air exhausted to the outside of the work zone. Patients should not remain in the room when dust-generating activities are performed.

Table 12. Construction/Repair Projects That Require Barrier Structures 120, 242, 263, 266, 267