19 September 2016

Design Considerations for Hospital Class-N Isolation Rooms

HEPA filter

There will always be a requirement for isolation rooms within hospitals.  Factors such as the rapid urbanisation of our cities, increased volume and frequency of overseas travel and the emergence of new and drug resistant organisms have forced these facilities to prepare for patients with infectious afflictions or unidentifiable illnesses.

The fundamental function of an isolation room is to reduce the transmission of airborne infectious pathogens and contaminated droplets from patients to health care workers and to patients outside of the room.  While this aim is clearly desirable, and simple enough to describe, the pathway to achieving this aim requires the complex interplay of air flow, air filtration and treatment systems, temperature and humidity control, state and federal regulations, building sealing, room design and layout, and several other factors.

This article will focus on Class N Isolation rooms, serving to highlight some important design considerations of the current state-of-understanding as it pertains to the Australian environment, and give some perspectives on overseas trends that would be worthy of consideration here.

Class N (Negative Pressure) Isolation Rooms

Class N- negatively pressurised rooms are typically used for patients with known or unknown but suspected infectious diseases that spread via airborne droplet nuclei (such as chicken pox, disseminated shingles, measles or pulmonary tuberculosis) and require airborne droplet nuclei isolation.

The main operation method of negative pressure isolation rooms is to utilise the exhaust air system to contain the infectious contamination within the isolation room and prevent transmission to corridors and attached rooms. Many design aspects such as total effective air-change rates, access doors, sealing walls and floors, and surface finishes all play a part in the overall room function.

Most hospital guidelines nominate a ratio number of isolation rooms to standard bed bays, and address the basic requirements needed for the isolations rooms.

Typical requirements for a Class N negative pressure isolation room include:

  • Ante room / Bed Bay / Ensuite
  • -10 to -15Pa between rooms
  • Architectural construction to be consummate with the sealing and possible pressure extremes (including possible pressure events)
  • System redundancy (the need for duty / fan standby equipment and equipment failure mode alarms)
  • Exhaust air systems that are independent of common exhaust air systems
  • Vertical stack discharges
  • Where contamination or re-entrainment is possible from the discharge, HEPA filters on the exhaust systems
  • Pressure monitoring and alarms (visual and audible)

Achieving Negative Pressure

Pressure balancing is the generally accepted way to set up a Class N negative pressure isolation room. This involves the provision of a set of pressure regimes, whereby each area leaks to the adjacent lower pressure areas, allowing a net migration of air into the isolation room and then out via the exhaust system.  Rooms within a system are typically set up with a -10 to -15Pa differential, between each zone.  The supply air quantity is defined by the greater of, the required air change rate (ACH), AS1668 minimum ventilation air, or the air required to condition the space.  The ACH is the usual driving factor – set by hospital design guidelines and good practice.  The room pressures are then obtained by balancing the exhaust airflows and adjusting the total exhaust system fan discharge volume.

Factors that can and will affect the pressure balance during operation include the loading of filters and the wear and tear of the general facility door, wall and floor seals etc.  The facility Building Management System (BMS) would typically adjust the exhaust system fan discharge volume to maintain the required negative pressure.  Pressure control systems are a fine balancing act to provide adequate pressurisation and system stability, with door interlocks to prevent alarms and control adjustment when they are opened and closed – response timing is critical. One of the most difficult items to achieve is the effective changeover between duty and standby systems, particularly in failure mode scenarios. Often direct electrical interlocks work faster and more effectively than the BMS due to slow response times.

Figure 1: Schematic of isolation room with terminally mounted HEPA’s (pressure balanced)

figure-1-schematic-of-isolation-room-with-terminally-mounted-hepas-pressure-balancedAirflow balancing is not as commonly used, and places less importance on the pressure and more on the direction of air movement. Set airflows are used to provide a net negative inward airflow into the isolation room system, which is then exhausted, creating a negative, but not “pressure controlled” system.  Airflow is typically controlled by constant airflow regulators on each branch.

Once again the supply air is based on calculations undertaken to meet the greater value of room conditioning requirements, ventilation rates (AS1668) and hospital ACH design guidelines. The exhaust air flow rates are then set based on expected flow differentials to achieve the desired goal (quantity of flow and approx. pressure differential). Generally established air leakage rates through door seals, door louvres etc. will be used, to set an exhaust value that achieves the desired (but not critical) nominal pressure regime.

Confidence is provided by the knowledge that there is a” net inward flow” of air into the isolation room system, which is then exhausted. The use of constant airflow regulators means that as door seals wear down, doors are opened and closed, the net airflow into the system does not vary – the pressure will vary, but should always remain negative.

Figure 2: Schematic of isolation room with terminally mounted fixed flow HEPA’s (airflow balanced)


Grille Placement and Swirl Diffusers

It is very important to ensure that there are no stagnant areas of air within the isolation room. Supply and exhaust grille placement is critical to ensure effective operation of the isolation room, while still considering patient comfort and temperature control.  The general idea is to sweep each room with air, drawing contaminants away from entry points.

Bed Bays

High level supply grilles are most often placed near the entry to a room with the air directed (dragged) away from the entry, across the patient, toward the low level exhaust grille at the far side of the room or near the bed head. This provides a clean sweeping motion and minimizes airflow turbulence.

Ante Rooms

Ante rooms may or may not need supply and exhaust outlets and are often set up to utilise the flow of air leakage through the door seals / door grilles (or in wall pressure stabilizing dampers) to create the flow or pressure regime.


The ensuite usually only has an exhaust grille located at high level due to the wet services within the room. Make up air is provided from door leakage, (via a door undercut, a door grille or a pressure stabilising damper) from the bed bay. It is worth considering pre-filtration here and if terminal mounted HEPAs are used, oversizing the HEPA size – the pressure drop across the HEPA is affected by steam / moisture in the air from the shower, which in turn can affect room pressures.

Swirl Diffusers

We are often asked if swirl diffusers can be used. This is an option that the system designer may consider within general hospital spaces; however within an isolation room, contaminant control is the primary directive and we do not recommend them.  The general intent of the existing standards and guidelines is to create a low velocity non turbulent (and predictable) sweeping motion across the bed bay to control contaminants.  Swirl diffusers provide good mixing of the supply air, and as such provide good comfort conditions for the patient.  This mixing provides dilution of contaminants within the space, but does not necessarily facilitate their removal; as such, contaminants can end up randomly distributed throughout the room and may pose a risk to staff / attendees.

Exhaust Filtration and HEPA Requirements

High Efficiency Particle Arrestance (HEPA) filters are designed and constructed to meet extremely specific particle capture requirements, and are rated at H13 (99.97% efficiency @ 0.3 micron), H14 (99.995% efficiency @ 0.3 micron) or U15 (99.9995% efficiency @ 0.3 micron). The “value”, 0.3 micron particle size is used as the test point as it is generally regarded as the Most Penetrating Particle Size (MPPS) – whereby particles above and below this size – are generally easier to capture – and these are the most “elusive” particles to capture.

Image 1: Mini-pleat HEPA filter with H14 efficiency rating to EN 1822:2009

airepanelRegardless of the method used to achieve negative pressure within the isolation room, a risk analysis is required to evaluate if there is a need for exhaust HEPA filtration for the discharges. This should be designed to identify the potential risks and the potential spread, both within the facility and externally.

A typical risk analysis is dependent on the following factors:

  • Location of the facility
  • Admittance potential for possible contagions or contaminants
  • Possible types of high level contagions or contaminants
  • Isolation room location within the facility
  • Isolation room discharge location
  • Chance of re-entrainment and spread to neighbouring systems

To be operated in a cost effective manner, pre-filtration should be considered to protect and extend the life of the HEPA filters. HEPA filters require adequate protection from general dust, lint and contamination by inexpensive, disposable filters to stop premature loading, replacement and as such, retesting. The higher the rating of the pre-filters, the longer the HEPA will last.

To confirm the performance of the HEPA filters, they must be tested on installation and be retested and validated each year by a NATA accredited testing agent. During certification testing, the HEPA media and housing is exposed to a challenge agent, with the downstream side of the filter and housing scanned for leakage.  To perform testing, adequate access needs to be available to physically view and scan the HEPA face.  Access to, or a connection point upstream of the filter is needed to introduce the challenge aerosol.

Whether the HEPA filters are located in terminal housings or inline within the duct; access and testing need to be considered.

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 HEPA’s 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.

Image 2: Pre filtration for terminally mounted exhaust filtration system – 20mm disposable flat panel filter with G4 efficiency rating to EN779:2012

2_-20mm-disposable-flat-panel-filter-with-g4-efficiency-ratingAs 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 HEPA’s 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.  (Refer to optional dampers in Figure 1).

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

Advantages Disadvantages
  • Contaminants are contained within the isolation room (theoretically, the duct is clean after the HEPA filters)
  • Ductwork is protected by the HEPA’s, 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)
  • Multiple HEPA’s 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 HEPA’s (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
  • In theory, when the room is decontaminated, decon can be undertaken up to the face of the HEPA
  • 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.

Image 3: Channel seal terminal HEPA module with access door to air off side of the HEPA filter (fascia removed).


 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 minimize fan energy use when discharge protection of the negative isolation room is not needed.

Image 4: Pre-filtration for inline containment exhaust filtration system – 100mm disposable pleated panel filter with G4 – F7 efficiency rating to EN779:2012

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 minimize 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 Disadvantages
  • 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
  • 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.

Image 5: Inline HEPA module with access door for scan testing (filter access door removed – HEPA filter and pre-filter shown).

5_inline-hepa-module-with-access-door-for-scan-testingAs 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.

Image 6: Bag-In Bag-Out (BIBO) inline housing with bubble tight isolation dampers, decontamination / fumigation ports, pre filter and HEPA filter access and remote scan arrangement (access doors removed).

containment housing unit
Bag-In / Bag-Out
is the term used where sealed gloved bags are installed over service points and access openings to provide an additional protective barrier to operators when changing out or testing filters. When BIBO functionality is required or simply “requested” it introduces a whole new facet and layer of complexity to the design of an inline containment housing; as all operations need to be undertaken from outside of the housing or through a sealed bag.

Bubble tight isolation dampers when closed provide near zero leakage and are used to isolate sections of the systems for emergency contamination control or decontamination processes.

Decontamination / fumigation ports are valved and capped connection ports that allow fumigation gases to be introduced to provide housing and filter decontamination of microbial life forms.

Pandemic Ward Scenarios

Isolation rooms provide the ability to house individual patients; however, some agencies and health boards are looking to build new facilities or have sought the ability to adapt existing facilities to house larger numbers of potentially contagious admittances (such as patients with tuberculosis or influenza) when required.

Possible scenarios include modifying existing wards to operate as isolation “Pandemic” wards with a single pass scenario – supply consisting of 100% outside air and the return / exhaust all being discharged to atmosphere. This can be done from a safely located discharge stack or if needed via HEPA filtered housing systems.

Common plant and duct requirements can provide potential savings in capital cost, maintenance and testing. Further operational advantages can also be gained through staged fan operation, system redundancy and heat / energy reclaim.  A number of factors should be carefully considered (see summary table below).

Advantages Disadvantages
  • Common plant location – reduced number of HEPA’s and fan systems (potential saving in capital cost, maintenance and testing)
  • Common / reduced duct requirements (If isolation rooms are in close proximity)
  • If spare system capacity is provided, adding additional isolation bays is relatively simple
  • Staged fan operation for operational flexibility and optimal performance (areas can be set into unoccupied mode with low flow)
  • Duty / standby (lead lag) requirement for filter change-out
  • Each branch (supply and exhaust) requires a constant airflow regulator (quality cost)
  • When combined, duct systems can be more difficult to fit within the facility
  • If isolation rooms are not in close proximity, long duct runs are required
  • Unless spare system capacity is provided, adding additional bed bays could be costly
  • System redundancy is possible
  • If system redundancy is not designed, installed and setup correctly, the whole system could deviate out of the negative state
  • Heat / energy reclaim is possible, practical and beneficial (when temperature differences are significant)
  • When temperature differences are minimal, additional static is added to the system (no heat reclaim benefit)

Figure 3: Schematic of existing wards (black) with overlaid modifications to become pandemic ward (pink)

figure-3-schematic-of-existing-wards-with-overlaid-modifications-to-become-pandemic-ward Cost and Energy Saving Considerations of Combined Systems

Current Australian guidelines and standards specify that negative pressure isolation rooms must have dedicated duty / standby exhaust fan systems which should not be combined with any other discharge systems. In short, each type N room must have its own dedicated duty / standby exhaust system.

A progressively more common approach overseas is to combine multiple negative bed bay exhaust duct streams via manifolding (similar to how fume cupboards can now be treated with the changes to AS 2243.3). The combining of exhaust streams for isolation rooms is a potential capital cost saving; allowing multiple bed bay systems to have common systems and redundancy.  There is also the ability to common the exhaust containment filtration systems. The large duct systems provide volume stability and control, and potential fan optimization for energy savings.

Figure 4: Schematic of manifolded pandemic ward with heat reclaim

figure-4-schematic-of-manifolded-pandemic-ward-with-heat-reclaimFrom a HVAC and energy point of view, the manifolding of an entire “pandemic ward” system would seem attractive to the end user. Single pass air-conditioning systems are very costly to operate, and small individual systems (350-500 L/s each) can make the implementation of heat reclaim cost prohibitive.  Combining the supply and exhaust system allows a scale at which the use of heat reclaim is possible and economically viable.  The use of run around coil type systems can be undertaken with minimal outlay to provide peak energy savings on high load days, while providing supply and exhaust system separation.

Run around coil systems are preferred as they still provide complete separation of the exhaust and supply air systems without the possibility of cross contamination. Heat wheels are not recommended at all due to the risk of cross contamination, and air to air heat exchanges (AIR/AIR HX) only have a single layer of separation against cross contamination.  If the HX is located after the HEPA filtration system, cross contamination is highly unlikely, but still possible in the event of HEPA failure.

A manifolded type approach to exhaust ducting uses constant airflow regulators to maintain airflow to individual branches, and as such they require flow, pressure containment.

Final Thoughts

Whilst there are many factors affecting the design, implementation and maintenance of Hospital Class N Isolation rooms; there are guidelines and sound recommendations available to assist with your compliance to relevant standards and individual hospital guidelines, as well as your specific objectives for safety, performance and cost.

Key considerations for exhaust filtration include:

  • Suitable access to testing and validation by a NATA accredited testing agent
  • Pre-filtration for the protection of HEPA filters (especially in the ensuite)
  • If terminal mounted HEPA containment are used, oversizing the HEPA filter located in the ensuite to compensate for pressure drop affected by steam / moisture
  • If deemed necessary by the risk assessment, gas tight / bubble tight dampers for use during decon / gaseous fumigation

Published in The Australian Hospital Engineer Journal, Volume 39, No 3, September 2016.  Written by Kristian Kirwin (B.ENG Mechanical) and Shannon Roger (B.Ed) for Airepure Australia 2016.

Airepure Australia offer a range of products, services and consulting expertise that can assist you with your compliance to ACHS, DHS VIC Guidelines (and equivalent for QLD, WA and NSW), ISO/IEC 17025:2005 Requirements, AS 1668.2, AS/NZS 2243.3:2010 and AS/NZS 2243.8:2014. Airepure is a national air filtration company providing unique, powerful and integrated air filtration solutions, ranging from basic HVAC filtration and odour control right through to high end HEPA/ULPA filtration and airborne containment technologies. Airepure recommends ELTA and Fantech Fans.


AS 1668.2: The use of ventilation and air conditioning in buildings. Part 2: Mechanical ventilation in buildings: SAI Global Limited 2012

Victorian Advisory Committee on Infection Control: Guidelines for the classification and design of isolation rooms in health care facilities, Victorian Government DHS, 2007

Infection Control Management of Infectious Diseases: Summary Table, Department of Health and Aging, Government of South Australia, 2015

International Health Facility Guidelines: Part D Isolation Rooms, Version 4, 2015


About the Author

Kristian Kirwin
(B.Eng Mechanical)
National Engineering Manager
Airepure Australia
1300 886 353

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