TB infection and disease are caused by the bacterium, Mycobacterium tuberculosis, (M. tb). M. tb, is released in tiny particles when a person with TB disease of the lungs or larynx coughs, sings, talks, or breathes. These particles, called droplet nuclei, are approximately 1-5 microns in size. (A micron is one millionth of a meter.) If air containing these droplet nuclei is inhaled by another person, TB infection may result.

Ventilation can reduce the overall risk of infection in a room in two ways: dilution and removal. When clean air is supplied to a room, it dilutes the concentration of airborne contaminants in the room. Dilution reduces the likelihood that a person in the room will breathe air that may contain contaminants. In the case of M. tb, this effect means that a person will be less likely to inhale one or more droplet nuclei.

Ventilation helps reduce the risk of M. tb transmission in an isolation room, but significant risk remains. For this reason, a respiratory protection program is required even in state-of-the-art isolation rooms.

Air Change Rates

The amount of ventilation in an isolation room is usually expressed in air changes per hour (ACH). By calculating the air change rate, the room ventilation can be compared to published standards, codes, and recommendations. It can also be used to estimate the length of time required to remove infectious particles.

Diffusers, Grilles, and Registers

A ventilation system introduces and removes air by means of air outlets. In health-care applications, outlets are usually mounted on a ceiling or on a wall. Ceiling supply outlets are called diffusers. Wall supply outlets are called grilles or registers. Exhaust (or return) outlets are also called grilles or registers, regardless of whether they are mounted on the ceiling or the wall.

The neck of the outlet is the point at which the outlet connects to the air duct. The neck size is selected to match the airflow quantity. The pattern or style of an outlet is the physical configuration of its face as seen from the room. For example, outlets can have a louvered pattern or a perforated metal pattern.

Ventilation air supplied to a room by a mechanical system will mix with air already in the room. This air mixture is removed by the exhaust. The effectiveness of dilution and removal depends on the effectiveness of the mixing process: the better the mixing, the better the dilution and removal. Stagnation and short-circuiting need to be avoided.

Proper selection and location of the supply and exhaust outlets will help avoid stagnation and short-circuiting.

The supply diffuser is an active device; it directs the flow of air in the room. For a given amount of air, the size of the diffuser neck determines how far this air will travel. The smaller the neck, the farther the air is directed. However, if the neck is too small, airflow is reduced and the diffuser will be noisy.

The pattern of the diffuser face determines the predominant direction of air movement, similar to how adjustable louvers direct air from a car’s air conditioning system. If the diffuser face pattern consisted of parallel louvers at the same angle, then air would be "thrown" in only one direction. Most diffusers are designed to blow in all directions, but there are special models that blow in just two or three directions.

The supply diffuser should be sized and the discharge pattern selected so that supply air reaches all parts of the room. The best diffuser for directing air is a louvered blade, ceiling-mounted type. However, diffuser style is often selected based on esthetic, rather than engineering control, concerns. In general, perforated face diffusers are considered more pleasing to the eye, but they do not direct air as well as most other patterns.

The exhaust grille, in contrast to the supply diffuser, is a passive device; it simply gathers air that is near. To encourage air mixing, the exhaust grille should be located at a point remote from the supply. The grille should have a neck sufficiently large to easily draw in the required exhaust air quantity.

When the exhaust grille collects room air, dust and lint are deposited on the grille and on the exhaust ductwork. Over time, these deposits can clog up the grille and duct, reducing airflow. To compensate, exhaust grilles, ductwork, and fans should be slightly oversized.

Ventilation can also reduce the local concentration of infectious particles at certain locations in a room. This is achieved by coordinating the location of the ventilation outlets with the probable positions of the people in the room. Simply stated, supply air should be introduced near staff, and exhaust air should be collected near patients.

For example, if an emergency department waiting room includes a reception area, the risk of M. tb transmission to staff can be reduced if clean air is supplied at the reception desk and removed at the patient area. This should result in a general air current away from staff and towards patients.

Negative pressure is designed to contain infectious particles within a room by creating a continuous air current going into the room under the door. Therefore, when the room is used as designed, airborne particles generated in the room cannot escape to the corridor.

Negative pressure is created by setting (or balancing) a ventilation system so that more air is mechanically exhausted from a room than is mechanically supplied. This creates a ventilation imbalance, called an offset. The room makes up the offset by continually drawing in make-up air from outside the room.

A negative pressure room must be as airtight as possible to prevent air from being pulled in through cracks and other gaps. This is called sealing a room. In a sealed room, the direction from which the make-up air enters the room and the speed with which it moves can be controlled. The smaller the make-up air opening, the faster the make-up air will move.

Ideally, the room should be well sealed except for a small (typically half-inch high) gap under the door. This should create a strong current under the door into the room. Whenever the door is open, air movement at the doorway is uncertain. Although more air is being drawn into the room than is leaving because of the offset, the large door opening results in a free exchange of air occurring at the door. Air is coming into the room, but air is also leaving.

If the room has leaks, such as those around windows or around lights, control of the offset is lost. If the leaks allow in a greater amount of air than the negative pressure offset, this excess air will flow out of the room under the door. This can cause a room to operate under positive pressure even though the mechanical system is designed to create negative pressure.

In conclusion, the greater the offset and the tighter the room is sealed, the better.

UVGI, which has been shown to inactivate airborne droplet nuclei containing M. tb, may be used to supplement ventilation as an engineering control measure. Because UVGI can have negative short-term health effects on the skin and eyes, a safety plan should be implemented when it is used. UVGI has two applications: in-duct UVGI and upper room air UVGI.

Upper room air UVGI is a useful engineering control for crowded congregate settings, where susceptible people may have prolonged exposure to an unidentified infectious TB patient. Examples are homeless shelters, emergency department waiting rooms, and prison day rooms.

An isolation room has a different type of transmission risk than a congregate setting. In an isolation room, the infectious source (patient) and the individual at risk (health-care worker) are known. Consequently, the health-care worker wears respiratory protection. The health-care worker is at greatest risk in close proximity to the patient. In general, this "near field" area contains the greatest concentration of infectious particles in the air. Although upper room air UVGI will help dilute the overall room concentration of M. tb, it will have little beneficial effect on this near field infection risk.

If used in an isolation room, UVGI will lower the concentration of infectious particles. However, given that staff in the isolation room wear respirators and the room air is exhausted or HEPA-filtered, the added benefit of upper room air UVGI in an isolation room will probably not be significant.