Ventilation of indoor spaces provides air that will assist in the dilution and dispersion of smaller particles in the air (for example, dust, pollen and microorganisms) and improve overall air quality.
Ventilation of a space can be provided either mechanically (for example, via a centralised heating, ventilation and air conditioning (HVAC) system or an individual local air conditioning unit) or naturally with passive airflow. Ventilation can also be augmented by filtration.
Poorly ventilated spaces can increase the risk of infectious respiratory particle transmission. It is strongly recommended to optimise ventilation in indoor settings to protect the most vulnerable.
6.1. Air changes per hour (ACH)
Ventilation air change rate is typically measured in air changes per hour (ACH). ACH refers to the number of times the volume of air in a space is completely replaced per hour. Evidence suggests that air change rates of 4-5 are good, 6 is better and more than 6 is best. Aim for 5 or more ACH to help reduce the number of viral particles in the air. Air change can be achieved through any combination of natural ventilation, mechanical ventilation, or devices that augment existing ventilation systems.
While the 5 ACH target is a rate that is likely to be helpful in reducing infectious particles, an optimum number of ACH remains uncertain. ACH levels higher than 5 may remove infectious aerosols from a space faster, but the potential benefits should be balanced with the additional upfront cost, and the costs of periodic maintenance, comfort level, and energy costs that will be incurred.
In standard hospital rooms, a minimum of 6 ACH is required. In negative-pressure isolation rooms, 12 ACH is required. Large volume spaces with very few occupants (e.g., a warehouse) may not require 5 ACH and spaces with high occupancy or higher-risk occupants may need higher than 5 ACH.
6.2. Types of ventilation
Natural ventilation uses natural wind and thermal air differences to bring fresh air (outdoor air) into a space. Natural ventilation is passive outside air flow via openings such as windows, doors and air vents.
The ventilation rate in naturally ventilated spaces can vary significantly throughout the year due to changing weather patterns and unfavourable conditions (for example, temperature, wind and rain) which may cause occupants to open or close windows and external doors sporadically.
Natural ventilation is significantly better than no ventilation, in most circumstances. However, if mechanical or augmented ventilation is available, these may be preferred due to the following risks associated with natural ventilation:
- airflow depends on wind currents, so the rate of ventilation cannot be controlled
- air is not filtered, so there is a risk of exposure to poor quality air, dust and other contaminated airborne particles
- temperature is not controlled, so external environmental conditions such as rain, humidity and heat can reduce patient and staff comfort
- unfiltered air and uncontrolled temperature and humidity may breach specified conditions for storing sterile stock
- when used in combination with mechanical systems such as split systems, natural ventilation may reduce the system’s temperature control and depending on outdoor conditions, may increase condensation levels. This will increase system running costs.
Mechanical ventilation replaces or dilutes indoor air with outside air using mechanical equipment (for example, HVAC systems). Conventional HVAC ventilation systems dilute indoor, potentially contaminated air through the introduction of fresh air from the outside while maintaining indoor air quality (IAQ) and thermal comfort.
Ventilation systems are not designed to prevent COVID infection transmission but to meet requirements to control the accumulation of harmful contamination and provide occupant comfort. A well-maintained and appropriately designed HVAC system contributes to the diffusion and dilution of infectious aerosols, reducing the risk of infection transmission.
Centralised heating, ventilation, and air conditioning (HVAC) systems dilute indoor, potentially contaminated air through the introduction of fresh air from the outside while maintaining indoor air quality (IAQ) and thermal comfort.
A well-designed, located and maintained HVAC system can reduce respiratory particle transmission by:
- supplying clean air to areas where susceptible occupants are located
- containing contaminated air and/or exhausting it to the outdoor environment
- diluting the air in a space with cleaner air from outdoors and/or by filtering the air of aerosols.
Most ventilation systems are designed for thermal comfort, not for preventing the transmission of infection. However, a well-maintained and appropriately designed HVAC system will contribute to the diffusion and dilution of infective respiratory particles, reducing the risk of infection transmission. The system should be well-maintained and serviced as per regular schedules.
Improving ventilation and airflow in indoor settings using an HVAC system should be considered as part of a suite of infection control measures to mitigate COVID-19 transmission risk.
The settings on an HVAC system can be adjusted to maximise ventilation. This needs to be done by a HVAC specialist or an occupational hygienist.
Split system air conditioners
Split system air conditioners (split systems) usually consist of two mechanical units:
- an indoor unit that provides conditioned air into a space (containing heat exchange coils, filters, fan)
- an outdoor unit that transfers refrigerant to and from the indoor unit (contains the compressor, propeller fan, circuit board and heat exchange coils).
Multi-split systems have more than one indoor unit connected to a single outdoor unit. They are used to heat or cool different spaces or rooms.
Ducted split systems have a single outdoor unit connected to a concealed indoor unit, which is then ducted to a single or multiple rooms.
Split systems recirculate air and promote air movement, but usually do not bring fresh air into a space unless specified (for example, unless they are designed to include outdoor air provision). Split systems should be used in conjunction with mechanical or natural ventilation to promote air movement and to minimise pockets of stagnant air. They are not a replacement for natural or mechanical ventilation.
When in use, evaporative cooling systems bring in large quantities of air from outside achieving very good air change rates. Some windows or doors need to be left open for these systems to circulate fresh air effectively.
Run evaporative coolers or ducted systems once or twice a day in ‘fan-only’ mode to flush rooms with fresh outside air.
Evaporative cooling systems need to be serviced regularly to make sure they are running effectively, and the filter changed regularly according to the manufacturer’s instructions.
It is important to note that evaporative cooling systems will only be useful during the warmer periods of the year and are unlikely to be used during the cooler months.
This refers to supplementary devices to improve existing natural or mechanical ventilation.
These can assist in:
- reducing the concentration of aerosolised viral particles
- improving air circulation and distribution in a space to reduce dead spots.
Augmented ventilation does not provide outdoor air to a space and is not considered an appropriate substitute for natural or mechanical ventilation strategies.
Augmented ventilation should aim to contribute to 5 or more ACH of clean air in the space to help reduce the risk of airborne infection transmission.
6.3. Air cleaning
Filters are designed to remove particles from air streams as they pass through. The Minimum Efficiency Reporting Value (MERV) is a scale for the measurement of an air filter’s ability to capture particles between 0.3 to 10 microns (µm) as air passes through. MERV filters range from 1 to 16, the higher the rating, the smaller the particles they capture (the more efficient they are). The minimum recommended filter is MERV ≥ 13; these filters are cost efficient compared to higher MERV rated filters, and are efficient at capturing airborne viruses.
Upgrading filters to those with ratings of MERV 13 or higher will reduce the transport of airborne particles while systems are operating, which may help reduce airborne infectious disease transmission within rooms and between rooms.
H13 grade high-efficiency particulate air (HEPA) filters are efficient at filtering larger than 0.3 µm diameter particles in standard tests. Particles larger than 0.3 microns include pollen, pet dander, dust, mould spores, smoke and bacteria as well as aerosols.
Ultra-violet (UV) light can be used as a supplemental treatment to inactivate airborne viruses such as SARS-CoV-2 in some indoor environments. UV disinfection has been used in some high-risk healthcare settings to prevent and control respiratory disease transmission. It supplements other ventilation strategies as it can reduce airborne virus concentrations in indoor spaces. However, it does not increase air exchange rates or remove particles from the air. For more information, see section 6.5. Air treatment with germicidal ultraviolet (GUV).
Portable air cleaners
An air cleaning device is a portable air circulator which draws air through a series of filters to remove particles before releasing cleaned air. Air cleaning devices can recirculate air back into a room or be ducted to exhaust air to the outside. Air cleaners can be used to increase the air exchange in a space and increase clean air change rate when used appropriately. The use of air cleaning devices can be considered in:
- workplaces with low ventilation rates despite implementation of natural and mechanical ventilation; or
- settings with elevated risk of COVID-19 infection transmission.
Position portable air cleaning devices so that air intakes are clear of obstructions. Most air cleaning devices draw air in from the front so that you can position them near a wall or in a corner, to promote good air movement. Portable air cleaning devices should be positioned with a small amount of space around the sides and the back.
Position air cleaning devices:
- away from open doors and windows
- in areas of low movement (‘dead spots’); often in corners or the points furthest away from any door and window openings
- in corners or dead spots to aid air circulation
- near supply grilles, where possible, to aid circulation of the filtered air
- to ensure that they do not create trip hazards, such as from loose cables
- to ensure that they do not obstruct entry and exit paths, such as fire exits.
Portable air cleaning devices should not be positioned near open windows or underneath extract grilles. Do not place objects on top of air cleaning devices.
Like split systems, electrical fans (including portable pedestal, box and fixed ceiling fan types) can circulate air in a room and promote air movement in a space, but do not provide fresh air. Air currents and movement provided by fans can encourage dilution and even distribution of particles (including viral particles).
If there are existing mechanical and natural ventilation strategies in place, fans may be used to encourage even air distribution. Pedestal or portable fans should ideally be placed in dead spots or areas with poor airflow, avoiding a potential build-up of viral particles in this area. Windows and doors should remain open where possible. A fan can be placed in front of an open window (facing to the outside) to increase air flow by pushing indoor air outside.
Fans should not be used if someone in the space has respiratory symptoms that are consistent with COVID-19 or is suspected or confirmed to have COVID-19. Once the person has left the space, fans may resume operation. Fans should not be directed to blow air from one person directly onto another person.
You should also:
- avoid the use of the high-speed settings
- use ceiling fans at low velocity
- direct the fan discharge towards an unoccupied corner and wall spaces or up above the occupied zone
- position portable or pedestal fans:
- in areas of low movement (‘dead spots’), often in corners or the points furthest away from any door and window openings
- in corners or dead spots to aid air circulation
- to ensure that they do not create trip hazards, such as from loose cables
- to ensure that they do not obstruct entry and exit paths, such as fire exits.
6.5. Air treatment with germicidal ultraviolet (GUV)
GUV can be used as a supplemental treatment to inactivate airborne viruses, such as SARS-CoV-2. GUV can be effective in many spaces, but it can be especially useful as an additional layer of protection to reduce infectious particles in indoor spaces that host large gatherings or where the risk of disease transmission is high. Historically, UV aerosol disinfection has been used in high-risk healthcare settings to prevent and control respiratory disease transmission.
As UV is ionising radiation, installation of UV disinfection devices requires careful consideration and extensive professional consultation for a range of factors, such as occupational health and safety, material durability and design of space.
Upper-room (or upper-air) GUV uses specially designed GUV fixtures mounted on walls or ceilings to create a treatment zone of ultraviolet (UV) energy that is focused up and away from people. These fixtures treat air as it circulates from mechanical ventilation, ceiling fans, or natural air movement. The advantage of upper-room GUV is that it treats the air closer to and above people who are in the room.
In-duct GUV systems are installed within a heating, ventilation, and air conditioning (HVAC) system.
- coil treatment GUV to keep HVAC coils, drain pans, and wetted surfaces free of microbial growth. These devices produce low levels of UV energy. This energy is continually delivered 24 hours a day, which is why they are effective.
- air treatment GUV systems which apply intense UV energy to inactivate airborne pathogens as they flow within the HVAC duct; however, air speed must be slow enough to allow adequate exposure to UV.
HVAC air treatment GUV systems require more powerful UV lamps or a greater number of lamps, or both, to provide the necessary GUV required to inactivate pathogens in a short period of time.
Air treatment systems are often placed immediately downstream of the HVAC coils. This location keeps the coil, drain pan, and wetted surfaces free of microbial growth and treats the moving air.
Standard GUV systems are designed to avoid exposing people to injury caused by direct exposure to UV energy.
6.6. Ventilation indicator devices (CO2 monitors)
Carbon dioxide (CO2) monitoring can provide information on ventilation in a space. Changes in CO2 concentration can indicate a change in room occupancy and the amount of outdoor air delivered can be adjusted.
However, there is not a direct link between CO2 concentration and the risk of COVID-19 transmission. CO2 concentration cannot predict if one or more room occupants has COVID-19 infection, the amount of airborne viral particles produced by infected people, or whether the HVAC system is effective at diluting and removing viral concentrations near their point of generation.
Nonetheless, a CO2 concentration below 900 parts per million (ppm) can be used as an indicator of good ventilation. It is important to note, however, that this concentration may not be appropriate for assessing the ventilation of all types of spaces and occupancies.
There are other methods of monitoring and remote sensing including monitors of pollution, volatile organic compounds (VOCs), carbon monoxide, temperature and other air contaminants. There are also options which allow information to be viewed in real time on a dashboard.
For more information on indoor CO2 recommendations refer to:
- Australian Building Codes Board,
- Australian Standards,
- Centres for Disease Control and Prevention (CDC) May 2023,
6.7. Activities and devices that move air
The following devices and activities can create air currents or turbulence which may disperse aerosols.
Hand and hair dryers
These are safe to use and unlikely to contribute to the spread of COVID-19.
Vehicle air conditioning
When in a shared vehicle, the heating and air conditioning system should be turned to fresh air mode (not recirculated air) to bring fresh outdoor air into the car. Windows should be kept open when practicable.
Singing and music in group settings, such as choirs and wind instruments
During singing and playing of wind instruments, droplets and aerosols are emitted and can follow ambient airflow patterns in a space. If a person is infectious, they may transmit COVID-19.
Measures that may reduce the risk of infection transmission include:
- singing or playing outside or in a well-ventilated room
- physical distancing between singers or players
- collecting and disposing of condensation and saliva hygienically
- performing hand hygiene after playing musical instruments
- cleaning surfaces between each use.
E-cigarettes and vaping devices
It is recommended that people maintain a 2 metre distance from a person who is vaping or smoking.
The frequent hand-to-mouth action and sharing devices with others may increase the risk of infection. Hand hygiene should be performed before and after using the device.
6.8. Ventilation strategies for acute healthcare settings
Ventilation cannot be considered as a sole infection control measure but should be used in conjunction with other infection control strategies. All acute healthcare services must undertake a risk assessment and planning for a range of ventilation and air-cleaning strategies to prevent COVID-19 transmission.
Ventilation and air cleaning strategies include:
- barrier air flow – the air flow from a COVID-19 patient’s room (hospital isolation room) or from a COVID-19 patient ward/zone (single rooms, or shared isolation areas used for the care of suspected and confirmed COVID-19 patients) should be actively ducted to the atmosphere outside of buildings and maintained at negative pressure (for example, clean air flows into the room passively and contaminated air is extracted out).
- filtration – to remove any suspended respiratory particles that may return to the air handling unit. F8 or F9 (ideal) filters are examples.
Isolation rooms and air changes per hour
Patients with confirmed or suspected COVID-19 should be cared for in a negative-pressure isolation room (preferable) or a standard-pressure single room.
Isolation rooms should include self-closing doors, an ensuite bathroom, high quality sealing of the room, an anteroom, independent supply air and exhaust, and exhaust ducts under negative pressure within the building.
In standard hospital rooms, a minimum of 6 ACH is required. In negative-pressure isolation rooms, 12 ACH is required.
Toilet and bathroom ventilation in isolation rooms
Toilet and bathroom ventilation systems should be kept at negative pressure and continuously run, 24 hours, 7 days a week if possible. Shower steam and toilet flushing may potentially cause aerosolisation of existing viral particles in the bathroom environment. Toilet windows are not recommended to be opened. This ensures correct ventilation direction and maintain negative pressure.
Natural ventilation should only be used if mechanical ventilation is not possible or available. This is because the natural airflow rate depends on window size and variable weather and may not quickly remove airborne particles. However, in healthcare facilities with no mechanical HVAC systems, natural ventilation is superior to no ventilation at all.
HVAC system maintenance and engineering support
All maintenance staff should wear appropriate PPE. Facility engineers should monitor and record the HVAC system metrics and the negative-pressure function of rooms in control zones daily. Bedside nurses should also monitor these and check whether alarms are working as part of their handover.
Scheduled maintenance should be reported to the infection control team quarterly (every 3 months). Air movement may also be mapped on an ad hoc basis, for example, using a smoke stick to determine air flow direction.
Hospital engineers should play a key role in reducing disease transmission in healthcare facilities. Their roles may include but are not limited to:
- ensuring, whenever possible, that all facilities are up-to-date with the latest practice standards
- creating new healthcare facility designs, including key points of entry such as emergency, admission and waiting rooms, incorporating the appropriate infrastructure including HVAC systems that separate high-risk areas; enough physical space and HVAC system capacity to upgrade filtration; the ability to increase ventilation to 100% outdoor air; and the ability to humidify air
- providing the capacity for quick installation of improved HVAC filtration
- providing the capacity for rapid, temporary increase in the outdoor ventilation rate in the event of an infectious disease outbreak.
- actively managing competing priorities to improve environmental outcomes through appropriate mitigation strategies.
Table 8: Ventilation strategies
|Strategy||Key actions checklist|
|Air exchanges per hour (ACH)|
Standard pressure hospital rooms should have a minimum of 6 ACH.
Negative pressure and quarantine isolation hospital rooms (with airborne respiratory virus infections) should have at least 12 ACH.
|Negative pressure isolation rooms|
Air flow from a COVID-19 patient’s room should be actively ducted to outside the building.
Maintain independent supply air and exhaust to prevent recirculation and ensure clean air entry into the room.
Exhaust ducts should be under negative pressure within the building.
Should have: 12-15 ACH or 145L per second; doors that remain closed, high-quality sealing; an anteroom; differential pressure gauges outside the room; local alarm systems to monitor fan status; low level exhaust; and clinical handbasin.
Install a minimum F8 or F9 (ideal) multi-pocket or V bank filter in the AHU as main filter. Minimum filtration grade MERV 13-16 (F8 or F9) is recommended.
Consider HEPA filtration where existing system performance can be maintained.
|Air circulation||Switch off or minimise air recirculation.|
|Barrier air flow (controlled volumetric air movement)|
Change balance of supply air to extract air at the air handling unit (AHU) within a zone either by:
- adjusting the fan speed via a variable speed drive
- adjusting the volume control damper positions at the AHU
Increase supply air into adjacent zones to force air towards designated COVID red zones.
Maintain low differential pressure between control zone and adjacent space (in the order of -2 to -5 pa).
Total air movement should be 150-200 l/s/double door into the zone.
|Dilution ventilation||Maximise the use of outdoor air as reasonably possible (100% outdoor air is preferable).|
|Temperature and humidity||Maintain relative humidity and temperature limitations and controls set by the healthcare facility.|
Not considered an adequate method to mitigate airborne transmission within healthcare facilities.
Should not be considered unless that is the only viable ventilation option.
Label any HVAC systems set to pandemic mode.
Monitor and record HVAC system metrics daily.
Minimum maintenance requirement.
Appropriate PPE should be worn by all staff and operators during maintenance work.
|Onsite support provided by hospital engineers|
At the building level, hospital engineers may identify:
- vulnerabilities with air intake, wind direction, shielding etc.
- building systems and safety zones in the general building environment.
- approaches to interrupting air supply to designated ‘shelter-in-place’ locations in general building environments.
- cohorting possibilities for pandemic situations so that whole areas of a hospital may be placed under isolation and negative pressure.
|Other important considerations|
Ensure all doors are always closed where appropriate.
Utilise negative pressure rooms for confirmed COVID-19 patients and AGPs.
Avoid non-essential emission sources such as burning incense.
Be aware of corridor HVAC returns: wards with corridor return ducts move air from patient rooms through the corridor towards the return duct.
6.9. Ventilation strategies for residential care settings
Whenever possible, these strategies should be implemented in consultation with an occupational physician or ventilation professional.
The supply of outdoor air to HVAC systems should be increased as much as reasonably possible in all areas of the facility.
Where an HVAC system recirculates air between different rooms, this recirculation should be turned off to rooms housing people with confirmed or suspected COVID-19.
Installation of standalone air conditioning units or placement of air cleaning devices (air scrubbers, air filters and air purifiers) with HEPA filtration should be considered to improve ventilation in areas housing people with suspected COVID-19. However, consultation with an occupational physician or ventilation professional should be undertaken and a risk assessment or needs analysis should also be considered.
If the ventilation rate cannot be increased mechanically, or if the recirculation mode cannot be improved or changed, natural ventilation strategies can be adopted. These include:
- opening windows if it is safe to do so (this should only be considered if outdoor temperatures are comfortable or if the room is vacant)
- creating new openings by modifying doors or windows
- installing air extractors or whirlybirds to enhance the effects of other ventilation strategies.
The usual indoor temperature and humidity set points should be maintained. Resident and staff thermal comfort and safety should be prioritised. If an HVAC system has humidity control, the relative humidity should be kept at 40 to 50%.
Facilities should consider the comfort of residents and HCWs when in PPE for prolonged periods.
Where possible, residents should be cared for in single rooms with their own ensuite bathrooms.
Ventilation or exhaust fans in bathrooms should operate at all times in bathrooms of people who are confirmed or suspected to have COVID-19.
In amber and red zones, portable fans are discouraged because air currents may accelerate airborne transmission of aerosolised viral particles. When fans are unavoidable, they should be placed in locations where fan air flow will not be directed from one person directly towards another and where possible in front of an open window (facing to the outside) to increase air flow and push indoor air outside.
In blue and green zones, portable fans are safe to use.
In amber and red zones where there are no other viable alternatives to maintain ventilation within the space, ceiling fans may be used with caution, they should operate at the lowest setting with the door closed if possible.
Reviewed 22 June 2023