Air Systems for TB Isolation Rooms

September 2000

There are no specifications or standards for TB Isolation Rooms applicable in New Zealand. It is therefore up to the hospital to specify requirements for such rooms. This paper lists and describes various performance requirements for TB Isolation Rooms, which may be specified prior to design and construction or modification of such rooms. Reasons and literature references are presented for each performance requirement.

This paper deals primarily with air systems for negative pressure isolation rooms for infectious patients. Positive pressure isolation rooms for immuno-compromised patients have different requirements.

Relevant Literature

1. Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Facilities, 1994 by U.S. Department of Health and Human Services – Centers for Disease Control and Prevention (CDC) (Available on-line via http://www.cdc.gov)
   This reference provides recommendations on general ventilation, the TB isolation rooms in particular and a supplement, which discusses the purpose and types of air systems used. The use of such engineering controls is considered as one of 13 measures to reduce the risk for transmission of M. tuberculosis.
2. Preventing Transmission of Tuberculosis in Health Care Facilities: An Engineering Approach, D.T. Hitchings, P.E., C.I.H. Member ASHRAE. (Available on-line via http://www.safelab.com)
   This paper elaborates on the CDC Guidelines above and provides detailed examples of Isolation Room designs.
3. Guideline for Isolation Precautions in Hospitals, 1997 by U.S. Department of Health and Human Services – Centers for Disease Control and Prevention (CDC) (Available on-line via http://www.cdc.gov)
   This guideline is more general in nature. It provides the more fundamental air system requirements and refers to the Guidelines above for applications involving TB.
4. Tuberculosis Isolation Room Design using CDC Guidelines, Kenneth E. Gill, HPAC Heating/Piping/AirConditioning, Sept 1997.
   This paper is a case study of the conversion of medical observation cells to respiratory isolation cells in a Texas jail.
5. AS/NZS 2243.3:1995 Australian/New Zealand Standard, Safety in Laboratories Part 3: Microbiology.
   This standard, while not strictly applicable to TB Patient Rooms, does include a set of requirements for Physical Containment Level 3 (PC3) laboratories where there is a risk of serious infection.
6. Health and Safety Issues in New Zealand Mortuaries, OHS, 1998

Safety requirements for mortuaries, including air system controls, are listed. The high-risk mortuary requirements are relevant to TB isolation rooms.

Performance Requirement

1. Dilution and Removal

The air change rate shall be not less than 12 ACH.

The purpose is to reduce the concentration of contaminants in the air by removing contaminated air and replacing it with contamination-free air.

The amount of contamination-free air is an air-change rate expressed in ACH (air changes per hour.)

CDC Guidelines(1) require 6 ACH minimum and recommend 12+ ACH. Higher air change rates are considered preferable.

In the case study presented by Gill(4) the air change rate was 15 ACH.

The requirement for high-risk mortuaries(2) is at least 12 ACH.

2. Airflow Patterns

Air shall generally flow from the supply to the Health Care Worker to the patient (or other infectious source) to the exhaust.

The objectives are to avoid short-circuiting of fresh air from supply to exhaust, to avoid stagnation of air and consequent build-up of contaminant concentration, and to avoid the Health Care Worker being positioned between the infectious source and the exhaust.

This is a recommendation in CDC Guidelines(1), which describe some supply/exhaust configurations.

D.T. Hitchings, in his paper(6), describes practical systems for good room air distribution in more detail and recommends relatively low velocity, non-aspirating type diffusers.

3. Room Pressurisation

Room pressurisation shall be negative for Infectious Isolation Rooms. Provision shall be made for daily monitoring of the pressurisation.

(Optional) An audible alarm to indicate loss of room pressure control shall be provided.

(Note: Pressurisation should be positive for Protective Isolation rooms. Refer Hitchings paper(2))

Air flows from areas of higher pressure to areas of lower pressure. Thus negative pressurisation of isolation rooms is desired so that air flows into the room from adjacent rooms and not from the potentially contaminated isolation room into adjacent rooms.

CDC Guidelines(1,3) specify negative, monitored pressure, but do not specify well what the pressure should be. A minimum of 0.25 Pa is stated as the requirement for control of airflow direction. However such low pressures are very difficult to monitor conveniently.

In the case study presented by Gill(4) the negative pressurisation was 12.5 Pa. Magnehelic pressure gauges were used to indicate pressurisation. Also pressure switches and alarms were added at the nurses station with a 30 sec time-out to allow for door opening and access to the room without setting off the alarm.

For Physical Containment level 3 microbiology laboratories(5), the requirement is 50 Pa. A magnehelic type differential pressure gauge and an audible alarm is also recommended for monitoring of pressurisation.

4. Treatment of Exhaust Air

Either: Exhaust Air shall be recirculated or discharged via HEPA filters.

Or: Exhaust Air shall be exhausted directly to outside via negatively pressurised ducting, away from air-intake vents, persons and animals or recirculated or discharged via HEPA filters.

Provision shall be made for monitoring (pressure differential), testing (integrity) and safe change of HEPA filters. Provision shall be made to automatically or manually adjust airflow to compensate for HEPA filter loading.

The purpose of HEPA filtration is to remove contaminants from the air. HEPA filters remove at least 99.97% of all particles greater than 0.3 ?m in diameter. CDC Guidelines(1) state that M. tuberculosis droplet nuclei probably range from 1?m to 5 ?m in diameter, therefore HEPA filters can be expected to remove infectious droplet nuclei from contaminated air.

CDC Guidelines(1) allow for air to be exhausted directly to outside, away from air intakes, people or animals without HEPA filtration. However, there are good reasons for using HEPA filtration for all exhaust air from isolation rooms:
1. It is difficult to guarantee that exhaust air will not be re-entrained or come in contact with people or animals.
2. It may be desirable to recirculate air for energy saving reasons if the area is air-conditioned.

In the case study presented by Gill(4) the exhaust air handler was fitted with pre-filters, HEPA filters and an ultraviolet germicidal irradiation lamp (UVGI) section downstream of the HEPA filters.

For Physical Containment level 3 microbiology laboratories(5), an exhaust filter is specified which shall be a HEPA filter.

CDC Guidelines(1) stress the importance of proper installation and meticulous maintenance of HEPA filtration systems. A HEPA filter integrity test which checks for leaks through or around the installed HEPA filter is recommended after installation of a HEPA filter and 6-monthly thereafter. Also a manometer or other pressure sensing device installed to indicate filter loading is recommended. Filter maintenance should only be carried out by qualified persons wearing appropriate respiratory protection.

Hitchings(2) recommends use of “bag-in, bag-out” housings for HEPA filters to minimise the risk of exposure of maintenance personnel to potentially infectious materials. This is certainly the preferred method when HEPA filters are located in the ducting. However, they are not required when the HEPA filter is mounted at the room exhaust – i.e. a terminally mounted HEPA filter.
Hitchings(2) also draws attention to compensation for HEPA filter loading. He recommends an automatic active volume control system. In practice, a manually adjustable system may be sufficient and will certainly be less expensive.

5. Anterooms

(Optional) Normal entrance to the isolation room shall be via an airlock. Doors shall open outwards, be self-closing and shall contain glass panels so occupants of the airlock can be seen. With both doors closed, the airlock shall be at negative pressure to the corridor and positive pressure to the isolation room.

An airlock is not required by CDC Guidelines(1) but the Guidelines explain that an anteroom may increase the effectiveness of the isolation room by minimising the potential escape of droplet nuclei into the corridor when the door is opened.

For Physical Containment level 3 microbiology laboratories(5), an airlock is mandatory as it is for cleanrooms (Refer AS 1386.1-1989, Cleanrooms and clean workstations, Part 1: Principles of clean space control.)

On the basis of airflow testing, Total Air care can confirm that with a door open, the airflow becomes essentially un-controlled.

However, in some cases, it may not be possible to fit an airlock. This was the case in Gill’s case study(4).

The pressurisation of the airlock relative to the isolation room and the corridor is recommended by Hitchings(2).

6. Other Requirements

Windows shall be sealed shut.

If windows are opened, pressurisation will be lost and contaminated air may flow out the open window.

CDC Guidelines(1) recommend that openings including windows and electrical and plumbing entries shall be sealed as much as possible.

Windows in a PC3 laboratory must be closed and sealed(5).

(Optional) Water supplied to isolation rooms shall be fitted with back flow prevention.

This is to prevent cross-contamination via the water supply.