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Ventilation is the process of bringing fresh outdoor air inside and letting indoor air out. Ventilation dilutes and disperses particles in the air (for example, dust, pollen, and microorganisms) and improves overall air quality. Poorly ventilated spaces, where contaminated aerosols can remain suspended in the air for prolonged periods of time, can increase the risk of infectious aerosol transmission. It is strongly recommended to optimise ventilation in indoor settings to protect the most vulnerable.
Further information is available at Australian Commission on Safety and Quality in healthcare - Optimising ventilation for infection prevention and control in healthcare .
Ventilation is measured in air changes per hour (ACH). A base rate of 4-6 ACH is recommended. This can be achieved through either mechanical or natural ventilation.
6.1. Types of ventilation
Mechanical ventilation replaces or dilutes indoor air with outside air using mechanical equipment.
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 aerosol 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.
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 virus aerosols, 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 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 have a unit, usually wall-mounted, that recirculates cooled or heated air in a room. The unit is connected to an outdoor refrigerant or heat-pump mechanism. More than one indoor unit can be connected to a single outdoor mechanism in a multi-split system. Although the indoor and outdoor units are connected, the system does not bring in any outdoor air, so they do not themselves improve ventilation.
Split systems are not a form of ventilation. They are useful for regulating temperature and humidity and promoting air movement, but because they do not bring fresh air into a space, a source of ventilation is also needed. Split systems should be used in conjunction with mechanical or natural ventilation to bring in outdoor air.
Note: Some less common split systems have specific outdoor air provision designs and bring in outdoor air.
This is a passive form of ventilation where windows, doors and air vents are opened to allow wind to bring outdoor air into a space.
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 air changes per hour (ACH) cannot be controlled
- air is not filtered, so there is a risk of exposure to poor quality air, dust and less commonly, respiratory particles containing infectious agents
- 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 systems' air movement and temperature control and increase condensation levels. This will increase system running costs.
This refers to supplementary devices, such as portable air cleaners, that are used to improve existing natural or mechanical ventilation. These devices can:
- increase the clean air exchange rate and reduce the concentration of aerosolised viral particles
- improve air circulation and distribution to reduce dead spots.
Augmented ventilation does not provide outdoor air to a space and is not a substitute for natural or mechanical ventilation strategies.
6.2. Air cleaning
Air filters remove particles from airstreams. Filters can be incorporated into mechanical ventilation devices such as HVAC systems, or into augmented ventilation devices such as portable air cleaners.
The Minimum Efficiency Reporting Value (MERV) is a scale of 1 to 16 that measures an air filter’s ability to capture particles between 0.3 to 10 microns (µm) as air passes through. The higher the rating, the higher the efficiency. HEPA filters and other MERV ≥13 filters remove 99.97% of aerosolised particles.
Portable air cleaners
Portable air cleaners with HEPA filtration can supplement HVAC, split systems, or natural ventilation. They can be used in:
- rooms with open windows, to assist the natural ventilation
- rooms with split system air conditioning units, to help remove aerosols from any areas of low air movement (dead spots).
Air cleaners are best used in spaces that have limited ventilation from other sources and in the rooms of persons who have or who are suspected to have COVID-19.
Note: Air cleaners are not a substitute for ventilation or a reason to reduce ventilation within a facility.
Positioning air cleaners
Place the air cleaner:
- in an area of low movement (‘dead spot’). This is often in a corner or the point furthest away from any door and window openings
- away from open doors and windows
- near HVAC supply grilles, so that the filtered air is circulated
- away from extract (return) vents or grilles in the ceilings or walls
- so that the unit and its cables do not create trip hazards
- so that the unit does not obstruct entry and exit paths or fire exits
- so that nothing blocks the air intake (most air cleaners draw air in from the front so that you can position them near a wall or in a corner, although they should have a small amount of space around the sides and back to promote good air movement).
Air cleaners should run continuously on the highest setting that can be tolerated (not the automatic setting) while the room is occupied. If occupants find the air cleaner too noisy, it should be used at a lower setting rather than turned off.
Do not place objects on top of air cleaners.
For information on purchasing air cleaners, see 'Purchasing an air cleaner' in References under section 8.5.
Maintenance and cleaning
These parts of an air cleaning device need regular cleaning or replacement:
- surface – clean regularly in line with the manufacturer’s instructions
- pre-filters – clean as per the manufacturer’s instructions.
- HEPA filters – replace as per manufacturer’s instructions (usually every 6 to 12 months). Used HEPA filters should be placed in a sealed bag and then disposed of in general waste.
When cleaning or maintaining an air cleaner:
- work in a well-ventilated space
- wear single-use surgical masks and gloves when cleaning pre-filters or replacing HEPA filters
- perform hand hygiene after cleaning and maintenance.
Ultraviolet (UV) aerosol disinfection
Ultraviolet (UV) radiation can inactivate viruses, bacteria, and other microorganisms in the air or on surfaces. UV exists naturally as sunlight and can also be produced artificially. There are 3 sub-types of UV that differ by energy levels:
- UV-A (400-315 nm)
- UV-B (315-280 nm)
- UV-C (280-100 nm).
These all pose a risk to human health, although UV-C poses a lesser risk and is recommended over the use of UV-A and UV-B.
UV viral inactivation is dependent on irradiance, exposure time and relative humidity. Although UV exposure can inactivate viruses, these will remain in the air unless removed through filtration. UV aerosol disinfection is available as in-duct UV radiation for HVAC systems; as upper room ultraviolet germicidal irradiation; and as in-room UV recirculation units.
UV aerosol disinfection has been used in high-risk healthcare settings to prevent and control respiratory disease transmission. Its use in non-healthcare settings is more limited due to the higher risk to human health in these less-controlled environments.
Installation of UV disinfection devices requires careful consideration and extensive professional consultation for a range of factors including occupational health and safety, material durability and design of space.
6.3. Air circulation
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).
Fans can support existing mechanical and natural ventilation by distributing air evenly. Pedestal or portable fans should be placed in dead spots or areas with poor airflow. A fan can be placed in front of an open window (facing to the outside) to push indoor air out.
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.
Fans should not be directed to blow air from one person directly onto another person.
6.4. Community 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.
Breathalysers are safe to operate in either active or passive modes. In passive mode, the person speaks closely into the device but does not contact it directly. In active mode, the person blows with one long continuous breath into a disposable mouthpiece attached to the device. Where possible, the active mode test should be conducted outdoors, with maximum distance between the police officer and the person blowing into the device.
The operator conducting breath testing should wear a mask.
The breathalyser mouthpiece can be disposed of into a normal rubbish bin. The police officer should perform hand hygiene after disposal of the mouthpiece. All reusable devices and equipment should be cleaned and disinfected between each use, according to the manufacturer’s instructions.
Some instruments pose a higher risk of aerosol generation than others. Compared to the aerosols produced in normal speaking and breathing, wind instruments can be categorised as low risk (for example, tuba), intermediate risk (bassoon, piccolo, flute, clarinet, bass clarinet, and French horn) and high risk (trumpet, trombone, and oboe).
When higher risk instruments are played, they generate more respiratory particles of a smaller aerosol size compared to lower risk instruments and speaking. It is this generation of greater numbers of respiratory particles (which may contain infectious virus) that increases the risk of airborne disease transmission.
Other high-risk routes of transmission of COVID-19 when playing instruments include:
- breath condensation and saliva collection in some instruments after playing
- sharing and touching reeds.
Condensation and saliva should always be collected and disposed of hygienically. Hand hygiene should be performed before and after playing shared musical instruments, and surfaces should be cleaned between each use.
These additional measures are recommended when playing wind and brass instruments to reduce the risk of infection transmission:
- Reduce the number of people in an indoor space. This might require changed seating arrangements for different musical activities involving the use of wind and brass instruments, including orchestras, bands, or music classes.
- Increase physical distancing between the musicians. Musicians playing high-risk instruments should be 2 metres apart whenever practicable, and the distance between wind instrument players and other musicians should be maximised.
- Distance the audience as far as practical from brass and wind musicians.
- Maximise ventilation in enclosed spaces.
- Monitor and clean breath condensate (the ‘spit valve’) regularly. Musicians must drain this fluid, dispose of it in a rubbish bin and then perform hand hygiene.
- Do not share wind instruments unless thoroughly cleaned and disinfected between use.
- Use a barrier cap on the bell end of a brass instrument. This can significantly reduce the release of respiratory aerosols. This may be considered a mitigation method for playing in groups, especially in hard-to-ventilate spaces.
Singing in group settings, such as choirs
During singing, droplets and aerosols are emitted and can follow ambient airflow patterns in a space. If a person is infectious, they may transmit COVID-19. The longer the singing, the greater the risk.
Measures that may reduce the risk of infection transmission include:
- singing outside or in a well-ventilated room
- physical distancing between singers.
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.
6.5. 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 ventilation and air-cleaning strategies to prevent COVID-19 transmission.
Ventilation and air cleaning strategies include:
- Barrier air flow from a COVID-19 patient’s room (that is, hospital isolation room) should be actively ducted to the atmosphere outside of buildings and maintained at negative pressure (that is, clean air flows into the room passively and contaminated air is extracted out).
- Barrier air flow from a COVID-19 patient ward/zone (that is, 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 (that is, clean air flows into the ward/zone passively and contaminated air is extracted out).
- Dilution – the supply of outdoor air to HVAC systems should be increased as far as reasonably possible to achieve 100% outside air. HVAC systems should supply the maximum amount of outside air and maximise filtration to prevent recirculation of aerosol droplets. Care should be taken to limit re-entry of contaminated air from energy recovery devices, outside air intakes and other sources.
- Filtration – to remove any suspended aerosol particles that may return to the air handling unit. F8 or F9 (ideal) high-efficiency particulate air (HEPA) filters are examples.
- Temperature and relative humidity (RH) – maintain indoor temperatures of 24 to 27 degrees Celsius for cooling during the warmer months and RH of 50-60 per cent. HVAC systems should not be set to ‘cold’ temperatures below 21 degrees or ‘dry’ low humidity settings below 40 per cent.
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 recommended. In negative-pressure isolation rooms, 12 ACH is recommended.
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.
For minimum maintenance schedules for air handling systems, see Maintenance standards for critical areas in Victorian health facilities.
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.
6.6. Ventilation strategies for residential care settings
Whenever possible, these strategies should be implemented in consultation with an occupational physician or ventilation professional.
In red zones (see under section 5.2 'Isolating, cohorting, zoning' on Personal protective equipment (PPE)), the air should be actively ducted to the atmosphere outside the buildings.
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 full 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.
6.7. Ventilation strategies for community and workplace settings
Mechanical ventilation strategies
For centralised HVAC systems and individual air conditioning units, the following strategies may be adopted to reduce the risk of aerosol transmission.
- All components of an HVAC system (including filters and amenity exhaust ducts) should be inspected, cleaned, maintained and serviced as part of regular maintenance schedules and as per the manufacturer’s instructions.
- HVAC or air conditioning units should utilise as much outdoor air as reasonably possible within the site or facility.
- Run pre and post-occupancy purge cycles to flush a space with clean air after occupancy (for example, before and after business or operating hours where staff will be on-site). This should be for the time taken to complete a total change of air in a space.
- Where possible, recirculation of air (recycled air) between rooms or spaces should be turned off.
Filters may be upgraded to provide a higher level of filtration; however, a higher-grade filter should not be installed if this compromises or reduces the ventilation rate. Consult a ventilation professional and manufacturer before upgrading filters.
If an HVAC system does not have filters, these may be retrospectively installed.
Temperature and humidity set points should be maintained as per standard settings. Staff and occupant thermal comfort and safety should be prioritised.
Disable or turn off ‘demand-controlled’ ventilation controls. These reduce air supply based on environmental factors (for example, occupancy, carbon dioxide levels or temperature).
Ventilation in bathrooms and kitchens at a workplace site or facility (provided by exhaust fans or windows) should operate continuously during occupancy, such as in business operating hours. Ideally, they should also operate for two hours before and after occupancy as a purging cycle.
If mechanical ventilation is not available, there should be at least one functional natural ventilation opening such as a door or window. Two or more openings is ideal. Cross ventilation, that is, through two openings opposite to each other, is effective in creating airflow and clearing airborne contamination.
Open external windows and doors as much as possible to increase outdoor airflow into the space if it is safe to do so. Keeping windows and doors open may also create issues relating to outside noise levels. In these circumstances, a temporary compromise, such as temporarily closing a door until noise has subsided, should be considered.
The use of natural ventilation will depend on outdoor weather conditions. In hot or cold weather, consider opening windows and doors intermittently for short durations (for example, 10 minutes every hour). During very hot or cold weather or adverse events such as bushfires and severe thunderstorms, some natural ventilation strategies may not be practical or feasible. This will reduce the natural ventilation rate, but thermal and occupant comfort should be prioritised in these situations.
Allow a break time between occupancy of a room by 2 groups of people (for example, a meeting room). During this break, windows and doors should be opened to maximise the air exchange and fans may be used to promote air movement.
A box fan or exhaust fan may be retrospectively installed in a window to improve air movement out of a space and dilute aerosol particles.
In hallways and corridors, windows and doors should always remain open where feasible.
Whirlybirds or extractor fans may be installed to enhance the effects of other ventilation strategies.
Electrical fans may be used to increase air recirculation and movement within a space.
Air recirculation and filtration devices such as portable air cleaners and HVAC system filters can augment ventilation by improving indoor air quality and promoting uniform air mixing and movement. They supplement and enhance ventilation but should not be used in place of other mechanical and natural ventilation strategies.
Reviewed 26 May 2023