HEPA containment systems for Hospital Class N isolation rooms

This is an excerpt from a published technical article titled ‘Design Considerations for Hospital Class N isolation Rooms’.

Terminal mounted HEPA containment systems

Terminally mounted exhaust filtration systems allow contaminants to be contained within the isolation room; the HEPA filters are installed within the room exhaust grille housings, most often at low level in the bed bay wall and at high level ceiling in the ensuite.

As the ductwork is protected by the HEPA filter, there is a reduced risk of a potential contagion or contaminant spreading in the ceiling service spaces and therefore the need for fully welded/sealed ductwork can be reviewed. As there are multiple rooms within the isolation room system (bed bay, ensuite and sometimes ante room), multiple terminal mounted HEPAs will be needed, and due consideration should be given to the requirements of pre-filters and suitable access for annual validation testing of the HEPA filters and decontamination procedures.

As space within hospitals is at a premium, there is minimal space for pre-filters to protect the HEPA filter (particularly important for damp ensuite areas). Access to the air-off side of the HEPA filter is also required for annual NATA certification. Subject to the HEPA module type, this can be performed via an access panel from an adjacent space or from a room side access panel. If the access door is located within the space, this panel must be cleanroom grade sealed. Testing is often difficult to arrange as the room may be in regular or short notice use.

Generally, decontamination of the room is undertaken via conventional cleaning and wipe down methods. This process does not facilitate the decontamination of the exhaust HEPAs which would need either specific gaseous decontamination or to be included when the room undergoes decontamination.

If contaminants are unknown, gaseous fumigation of the HEPA filters is recommended prior to filter change-outs to ensure the HEPA filter does not harbor any contagions or contaminants. PPE must be used as per facility guidelines.

In an ideal world, a system would have gas tight or bubble tight isolation dampers on the duct prior to the supply inlets and behind the HEPA filters on each exhaust. This provides a barrier to stop the spread of the decontamination gas during fumigation, and prior to discharge of the decon gases to atmosphere via the exhaust.

In this scenario, decontamination can be undertaken up to and including the face of the HEPA filter.

Advantages

• Contaminants are contained within the isolation room (theoretically, the duct is clean after the HEPA filters).
• Ductwork is protected by the HEPAs, reducing the risk of spread in ceiling spaces and the need for fully sealed duct.
• In the event of positive pressurisation, contaminant leakage is only from the room (not the associated ductwork).
• In theory, when the room is decontaminated, decon can be undertaken up to the face of the HEPA.

Disadvantages

• Multiple HEPAs serving each room within the isolation bed bay, therefore, multiple HEPA test locations, pre-filter change-out locations.
• Minimal space for pre-filters to protect the HEPAs (particularly important for ensuite areas).
• Potential risk of damage to HEPA by staff or occupants.
• Access to the air off side of the HEPA filter is needed for annual validation (either from an adjacent space or a sealed in room panel).
• Access to patient areas is needed:
  • Organising when to schedule testing if area is occupied?
  • Is there sufficient space for testing? (traditionally limited ceiling space in ensuite)
  • If suction is lost when terminal access panels are opened, a positive pressure fan may be needed for testing.
• Unless gaseous fumigation of the HEPA is undertaken, it may still harbor contaminants / contagions after room decon.

It is possible for HEPA modules to be made to include pre filters and an access door to the air-off side of the HEPA filter for testing if prior design consideration is undertaken. These require more space and have their own drawbacks.

Inline containment systems

Inline containment exhaust filtration systems are typically located in plant or roof spaces away from the isolation room. As such, they can be larger in size than terminal HEPA modules, and in turn provide sufficient space for higher grade, high capacity pre-filters and high capacity HEPA filters. A single inline HEPA housing can replace multiple terminal HEPA modules, which is a potential testing saving. A bypass around the inline HEPA housing can also be considered to minimise fan energy use when discharge protection of the negative isolation room is not needed.

Although the exhaust duct is in a negative state, fully welded, 100% sealed duct from the isolation room exhaust intake point to the inline HEPA housing may still be considered to minimise possible risk of leakage to void spaces in a positive state scenario. Cleaning and decontamination of this duct is often difficult, and this needs to be considered. If the plant space is located externally, the HEPA housing and duct construction must also be made weatherproof. If the units are in a plant space that is used as an outside air intake plenum, there is a potential risk of contamination into other systems through their intakes.

Advantages

• Single location for HEPA testing, access is outside of patient areas.
• System size can facilitate a single high capacity HEPA (instead of multiple per room within isolation bed bay).
• High efficiency pre-filters can be located in the inline housing (further protection for HEPA).
• Basic pre-filters can be located at the room exhaust intakes (removal will not affect the HEPA).
• HEPA bypass can be provided when not in a negative isolation mode, providing energy savings.

Disadvantages

• Additional plant space needed.
• Often fully welded 100% sealed duct is required.
• If located externally, duct construction must be made weatherproof.
• Difficulty cleaning / decontaminating ductwork from the intake point to the HEPA filters.
• In the event of positive pressurisation, possible contamination to void spaces from ductwork.

The type of inline containment system can also vary greatly, depending on the potential risk and housing location. An inline system may be as simple as an upstream injection port for introducing challenge aerosol in the duct system and an inline HEPA housing, containing a HEPA filter and an access door for scan testing.

As the potential risks increase, and the system requirements become more complex — laboratory type Bag-In / Bag-Out (BIBO) systems are used. Inline HEPA systems can become BIBO style housings used for designated Class Q (negative pressure quarantine) isolation rooms. This type of system would typically include bubble tight isolation dampers, decontamination / fumigation ports, a remote scan arrangement for testing and BIBO arrangements for filter change out.

For detailed information regarding achieving negative pressure, the importance of grille placement and swirl diffusers, exhaust filtration and HEPA requirements, pandemic ward scenarios, and noteworthy cost and energy saving considerations of combined systems, please refer to https://www.airepure.com.au/design-considerations-hospital-class-n-isolation-rooms/.

Image credit: ©stock.adobe.com/au/science photo