This section covers methods of improving the ventilation characteristics of an existing room to make it more effective for negative pressure isolation. Previous sections have outlined recommendations for a new state-of-the-art negative pressure isolation room and shown how to assess an existing room to see how it compares with these recommendations. This section describes how to correct deficiencies found during the assessment. The methods outlined below could also be used to convert an existing patient room into an isolation room.

The first step is to ensure that air from the room is not inadequately filtered and recirculated to other areas. The air removed from the room must either be exhausted outdoors to a safe location or HEPA-filtered. If room exhaust is currently connected to a recirculating air system that does not include a HEPA filter, it should be disconnected from this system.

Theoretically, another safe option for correcting a recirculating system is to replace the existing filter with a HEPA filter. However, ICS does not recommend this. A HEPA filter is a specialized piece of equipment that should only be used in a ventilation system specifically designed to accommodate it. HEPA filters are physically larger than most filters and require larger fans to overcome increased resistance to airflow.

As explained previously, the negative pressure value will depend on two factors: how much more air is exhausted than supplied (i.e., the offset); and how well the room is sealed. In general, when converting or upgrading a room, the negative pressure value will not be as high as that attainable for new construction because there is less control over the architectural elements.

ICS recommends that the negative pressure value should be at least minus 0.006" W.C. for upgraded or converted negative pressure rooms. This is more stringent than the CDC Guidelines, which recommend minus 0.001" W.C. as a minimum negative pressure value. In our experience, a pressure difference of 0.001" W.C. will not be consistently maintained if there are other external factors, such as drafts created by elevators and doors that open to the outside.

If the room is not currently connected to an exhaust system, it should be either connected to an existing exhaust system or a new system should be installed. Consult with the building facilities department staff, who will probably hire a mechanical engineering consultant to design this work and oversee the construction.

If there is an accessible exhaust air system nearby, such as a toilet exhaust system, with sufficient capacity, it may be possible to make a new exhaust connection to the existing return register. Otherwise, a new exhaust air fan and ductwork system should be installed. See Section 4.F for new isolation room exhaust recommendations. New exhaust ducts, and new or existing exhaust fans serving isolation rooms, should have the same warning labels used for new isolation rooms. See Section 4.F.

To increase room airflow and/or create, or increase, negative pressure, the existing ventilation system needs to be adjusted to exhaust more air. The supply air quantity may also need to be increased. Airflow is varied using dampers.

Dampers are devices that control the flow of air in ducts, similar to the way valves control the flow of fluids in pipes. Dampers, usually located above the ceiling, should only be adjusted by a facility engineer or certified air balance contractor. To increase airflow, the dampers in the ducts serving the room should be opened wider. It usually takes an air balancer two or three iterations to obtain the desired airflow.

The exhaust airflow rate should be at least 12 ACH. For existing rooms, this recommendation is more restrictive than the CDC Guidelines, which accept an air change rate of 6 ACH. However, 6 ACH will not satisfy some local regulatory agencies, including Cal/OSHA and the Office of Statewide Health Planning and Development (OSHPD) in California. Twelve ACH, which meets all local requirements known to ICS, is readily achievable using HEPA filter units.

The supply should be approximately 100 CFM less than exhaust. Depending on how well the room is sealed, more air may need to be exhausted in order to achieve a larger pressure differential. Most rooms do not have a dedicated ventilation system. They are connected to a fan system that serves other rooms in the building. Before and after adjusting the isolation room airflow, the air balancer should measure the airflow in some of these other spaces to make sure that the isolation room adjustments do not have an adverse effect on ventilation elsewhere.

HEPA filter units are available in a number of different physical configurations, including wall and ceiling mounted types. The most popular configuration is the floor-standing, portable type.

Wall or ceiling-mounted units are less obtrusive and do not take up floor space. They are also less likely to be tampered with by staff and patients. However, floor mounted units are more portable and are easier to service. Regulatory bodies, such as OSHPD in California, may require that a structural engineer oversee the design and construction of the support system for a wall-mounted or ceiling mounted HEPA filter unit.

If negative pressure in the isolation room is satisfactory, but the ventilation rate is low, a HEPA filter can be used to supplement the room airflow rate. The effective ventilation rate of the room is the sum of the central system airflow and the HEPA filter unit airflow.

The size of the unit selected should be based on the additional airflow (in CFM) required to achieve the desired air change rate (in ACH) in your room. To determine the additional airflow:

If a sufficient portion of the discharge from a HEPA filter unit is ducted somewhere outside of the room, then the HEPA filter unit can create negative pressure and replace the need for any extra exhaust. A HEPA filter unit supplements ventilation as follows:

1. The effective exhaust air quantity is increased by an amount equal to the airflow of the HEPA filter unit (because this air is now being removed and droplet nuclei are removed by the filter).

2. The effective supply is increased by an amount equal to the returned air quantity (HEPA unit airflow minus the amount discharged outside the room).
3. The effective negative pressure offset is increased by an amount equal to the HEPA unit airflow discharged outside the room.

Theoretically, the technique described above could also be used to create negative pressure in a room that had no ventilation system. However, this is not recommended because the room would then have no outside air at all, only recirculated, HEPA-filtered air. Building codes mandate that fresh outdoor air be supplied to all occupied spaces that do not have an operable window.

Although Janet, the clinic manager, wanted to bring the isolation room into compliance with CDC engineering control recommendations, she thought her budget was too limited to accomplish this.

Cynthia, the engineer, suggested a portable HEPA filter unit as an affordable upgrade option. A HEPA filter unit would provide additional airflow. If a portion of the discharge were ducted outside, it would also create negative pressure. The first step was to calculate the additional airflow required:

A HEPA filter unit that produced at least this much airflow was required. Cynthia contacted a mechanical equipment supplier. Two units were available: a small $2000 unit rated for 150 to 300 CFM; and a large $2800 unit rated for 250 to 750 CFM. Each unit had a variable speed switch and an optional connection that could be used to duct some of the discharge air outdoors.

Janet suggested buying the small unit to save money. If run at high speed, it would provide more than enough airflow. However, Cynthia explained that most people turn down the fan speed switch because the units can be noisy. The units may also produce less airflow than the catalog claims. She suggested adding a 25% safety factor, then buying a unit listed for this airflow at low or medium speed.

Additional airflow plus safety factor = 250 CFM + 25% = approximately 310 CFM Based on this, the larger unit was selected and placed in the room. Cynthia replaced a window pane with a sheet metal panel. She connected a flexible duct from the HEPA unit discharge to a hole in the sheet metal panel, set the unit to about 300 CFM, and diverted about a third of the discharge air to the outdoors.

The room was now clearly at negative pressure, the airflow was improved, and the noise from the HEPA filter unit was acceptable.

Cynthia’s final measurements showed that the HEPA filter was returning approximately 250 CFM, with 80 CFM of this discharged outside and the remaining 170 CFM recirculating in the room.

Effective supply: 130 CFM + 170 CFM = 300 CFM

Effective exhaust: 150 CFM + 250 CFM = 400 CFM

Effective offset: 400 CFM - 300 CFM = 100 CFM

Once the isolation room upgrade has been completed, procedures to monitor the engineering controls must be implemented. This is essential to ensure that staff will be alerted if the controls fail. The two items that need to be monitored are the airflow rates and the room pressurization.

Room pressurization should be continuously monitored to ensure that the room remains under negative pressure.

The CDC Guidelines recommend that room pressurization be confirmed daily while the room is occupied by a known or suspected infectious TB patient, and at least once a month at other times.

These tests can be done with smoke or a telltale device, such as a tissue. However, ICS recommends that each isolation room be equipped with a permanent room pressure monitor. See Section 4.G.

Records should be kept of all isolation room engineering control tests and measurements. Local regulatory agencies may require that these tests be kept for a number of years. For example, Cal/OSHA requires that records be kept for a minimum of five years.