Negative Air Systems

What is a Negative Air System?

Negative air systems are mechanical ventilation systems required to sustain negative pressure rooms, also called isolation rooms. Individuals with highly contagious airborne diseases are kept in such isolation rooms to prevent the spread of airborne microorganisms. Conversely, sometimes the pressure in a negative pressure room is calibrated to ensure that airborne microorganisms in surrounding areas cannot spread into the isolation room and threaten an immune-compromised individual.

A negative air system ensures that the air pressure inside the room is either lower the air pressure outside the room (in adjacent rooms or corridors). Air naturally flows from areas of high pressure to areas of low pressure. Artificially creating and sustaining a difference in air pressure allows a facility to control whether microorganisms are trapped within an isolation room or pushed away from such a room. Because the air pressure measurement in one area is lower than the air pressure measurement in the adjoining areas, the air pressure is referred to as “negative”.

To create negative air, exhaust systems remove contaminated air and clean, non-contaminated filtered air is supplied at a rate that creates higher air pressure outside of the contaminated area. This ensures contaminated air does not leave the contaminated area and is replaced with an adequate supply of clean air.

Classes

There are three main types of isolation rooms:

  • Airborne Isolation Room (AIR) or Airborne Infection Isolation Room (AIIR) – These rooms are designed and ventilated to limit the spread or microorganisms from the room to surrounding areas. The rooms are kept at a lower air pressure (i.e. “negative”) than the air pressure in surrounding areas. Air naturally remains in the room when a doorway is opened. AIR treatment rooms also require an attached anteroom, which allows an airlock room providing a safe area for healthcare professionals to change into or out of protective clothing, PPE, etc.
  • Protective Environment Room (PER) – These rooms are designed and ventilated to limit the introduction of microorganisms from surrounding areas. The rooms are kept at a higher air pressure (i.e. “positive”) than the air pressure in surrounding areas. Air naturally remains in the surrounding areas rather than flowing into the room. These rooms typically do not require adjacent anterooms.
  • Combination Airborne Isolation and Protective Environment Room (AIR/PER) – These rooms are designed to both limit the spread of microorganisms from the room as well as prevent the introduction of micro-organisms from surrounding areas. One example of this class of room includes operating rooms for infectious individuals. In this case, the adjacent anteroom contains the negative pressure relative to both the OR and the corridor. Air in the anteroom is strictly controlled and filters, ensuring potentially harmful air from either corridor or operating room cannot mix.

General Considerations

To meet the required standard for negative air, the air separation between the rooms must be a minimum of 7.5 pascals (Pa) of either negative or positive air pressure.

Negative air isolation rooms must ensure a minimum of 12 air changes per hour and directional airflow.

Additionally, these rooms must also exhibit a higher level of airtightness to maintain pressure.

Negative air isolation rooms are designated a Type I space by CSA standards, requiring uninterrupted airflow, pressurization and temperature. AIR rooms also require uninterrupted exhaust systems; while PER rooms require uninterrupted supply air systems. As a result, the air-handling units servicing such rooms must have redundancy with parallel, inter-connected systems including automated controls and emergency power.

AIR rooms require supply air with two-stage filtration. PER rooms or combination AIR/PER rooms require supply air with three stage filtration, including HEPA filters located downstream of MERV 8 and MERV 14 filters. Filters require accessibility for testing and safe bag-in-bag-out replacement.

AIR and AIR/PER exhaust air is treated as contaminated exhaust and must comply with CSA filtration requirements. Modern design standards also often include bag-in-bag-out HEPA filtration on the exhaust systems in order to ensure factors such as outdoor wind or nearby buildings do not inhibit safe dispersal of contaminated air.

AIR anterooms must have negative pressure related to the corridor and positive pressure related to the isolation room. Dedicated exhaust air is required for both the anteroom and the isolation room, as well as any adjacent washroom.

Regular testing of isolation rooms by qualified persons is required. Maintenance of isolation rooms includes safe replacement of filters; mandatory annual inspection and cleaning of ductwork; as well as monitoring of doors, frames, walls, ceiling penetrations, etc. which tend to deteriorate and reduce the airtightness of a room over time.

Limitations

The main limitation to a negative air system is the complexity and expense of creating such a controlled environment within a health care facility. Accordingly, not all rooms can be rendered secure via negative pressure and staff must be aware of which rooms offer this control and which type of negative air protection is available.

Creating a negative air system within an already existing facility is a significant undertaking which requires precise coordination between contractors, specialists and operational staff to ensure that no contamination of spaces during construction.

Staff working within isolation rooms need to understand the importance of preserving the environmental conditions of the negative air system. Training on how to utilize the room and the anteroom is essential. Common operational mistakes may include:

  • Opening a window (this changes air pressure and airflow);
  • Using a portable air conditioner, fan, air scrubber or any other device which changes airflow; and
  • Leaving the door of the room or the anteroom open.

How do you know if a Negative Air System is working as designed?

Qualified persons should evaluate negative air system performance using a recognized method.

Negative air pressure should be monitored constantly, with notifications or alarms sent to building operators of any abnormality.

Regular filter maintenance is essential as if the mandatory third party inspections and cleaning required to ensure correct operation and protect the health and safety of workers. Records should be kept of all inspections, maintenance and cleaning, including the individual who performed the duties. Records should be readily available to Occupational Health Committees.

References

  • CSA Z8000 Canadian health care facilities
  • CSA Z317.2 Special requirements for HVAC Systems in healthcare facilities
  • CSA Z317.1 Plumbing
  • CSA Z317.13 Infection control
  • Occupational Health and Safety Regulations, 2020 6-2
  • Occupational Health and Safety Regulations, 2020 6-3