COVID-19: Guidance on indoor ventilation during the pandemic

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The Public Health Agency of Canada (PHAC) has developed this guide to inform Canadians about how indoor ventilation, in combination with other recommended public health measures, can reduce the spread of COVID-19. This guide also provides practical tips on how to improve indoor air, ventilation and filtration to help reduce the spread of COVID-19. While this guidance is intended to be used generally across indoor environments, its application will depend on:

Canadian public health guidance related to COVID-19 has evolved as our understanding of COVID-19 improves. We continually review the evidence as it's produced and work with our partners across the country and around the world. This ensures that we integrate the most up to date and highest quality information into our guidance.

Key messages

The virus causing COVID-19 is known to spread through droplets and aerosols, which represent a risk particularly to people who are in:

The most important elements in reducing the risk of COVID-19 are preventive measures, such as:

In addition to these practices, adequate ventilation can contribute to reducing the risk of COVID-19 transmission in indoor settings. It's important to remember that good indoor ventilation alone cannot protect people from exposure to the virus, particularly:

We recommend the following actions to help protect you and others from COVID-19 infection in indoor settings.

Limit indoor gatherings

Limit gatherings to your immediate household only. When interacting with those who do not live with you:

Always check with your local public health authority on the specific advice for indoor gatherings for your location.

Open windows and doors

You can improve natural ventilation by opening windows and doors to the outside:

Consult an HVAC professional

If possible, consult a heating, ventilation and air conditioning (HVAC) professional to determine whether your HVAC system:

Consider avoiding the area

If fresh air input from mechanical ventilation is not adequate and natural ventilation isn't possible, consider avoiding the area and moving to a better-ventilated space. If this isn't possible, the use of portable air filtration devices with High Efficiency Particulate Air (HEPA) filters could be considered, if used in combination with established public health infection control measures. We recommend following the manufacturer's direction and, if possible, the advice of an experienced professional before installing these devices in your setting.


Coronavirus disease (COVID-19) is a respiratory illness caused by the SARS-CoV-2 virus. COVID-19 spreads from a person who is infected to others through respiratory particles created when an infected person:

Your first line of defence against COVID-19 continues to be:

Improving indoor air quality through increased ventilation is an additional step.  Ventilation, whether through opening windows or the use of heating, ventilation, and air conditioning (HVAC) systems, can increase the amount of outside air brought inside. This will dilute the number of viral particles in the air, and help to reduce the risk of exposure.

Transmission modes for COVID-19

SARS-CoV-2, the virus that causes COVID-19, spreads from a person who is infected to others through respiratory droplets and aerosols created when a person who is infected:

The droplets vary in size from large droplets that fall to the ground rapidly near the person who is infected, to smaller droplets, sometimes called aerosols, which may linger in the air under some circumstances. Aerosols laden with infectious virus increase the risk of spreading COVID-19, particularly if a person stays within an enclosed indoor space with little air circulation for a long time. Thus, indoor air quality may play a role in COVID-19 transmission and other health conditions by affecting the concentration of pollutants, as well as viral and bacterial particles suspended in the air.

Infectious droplets or aerosols may:

The virus may also spread when a person touches another person (like a handshake) or the surface of an object (also referred to as a fomite) that has the virus on it, and then touches their mouth, nose or eyes with unwashed hands.

Importance of the different transmission routes

Transmission may vary depending on multiple factors such as:

This is why we recommend various public health measures to Canadians as a layered approach to prevent the spread of the virus. The most important are:

Other environmental factors can contribute to the risk of virus transmission. It's important to avoid:

Proper ventilation should always be present in any setting. When indoors, good ventilation can decrease the concentration of aerosols that may be suspended in the air, helping reduce the chance of COVID-19 spread.


In addition to other public health measures, ventilation has an important role in reducing the transmission of COVID-19 indoors. Outbreaks have been linked to poor ventilation where the virus appears to have been transmitted through aerosol production from infected individuals that became concentrated in the air over time. It is important to note that adjusting ventilation is not likely to reduce transmission between individuals in close proximity. Individuals who are physically near a person who is infected remain at risk from both droplet and aerosol transmission. This is due to their close proximity to the infectious source. For this reason, it is important to:

Ventilating a room or indoor space replaces the indoor air with outdoor air. This will dilute and replace any air contaminated with SARS-CoV-2 virus or other air pollutants. Ventilation systems in non-residential settings (like office buildings) may recirculate air through the HVAC system. In this case, some of the indoor air is diluted with outdoor air and filtered before returning to the occupied space. The risk from recirculating the virus through a space serviced by a single HVAC unit is unknown. You can decrease the risk and improve your indoor air quality overall by:

Influence of ventilation on risk of aerosol transmission

If a person who is infected is in an indoor space, build-up of viral particles will depend on:

The smaller the room, the faster the build-up of particles containing SARS-CoV-2 virus. In larger spaces, it may take longer for virus-containing aerosols to build up throughout the room. Good ventilation will:

In any size of room, close proximity can result in high-risk exposure, regardless of ventilation.

Impact of ventilation depends on activities and setting

It's important to seek public health advice before undertaking indoor activities that potentially generate more infectious respiratory droplets or particles than more passive activities. Increased production of aerosols and droplets can be caused by:

Limit or avoid these situations where possible. Ventilation systems in settings where these aerosol-generating activities take place may not dilute the air quickly enough to reduce the risk of spread.

Impact of crowd size on ventilation

To reduce transmission of COVID-19 in indoor settings, it is important to ensure that occupancy is reduced to minimum levels. The likelihood that both an infected individual is present and that a higher number of people become infected increases:

Increased occupancy levels can greatly increase the probability of viral-laden droplet and aerosol exposure. When rooms are in use, maximum ventilation rates must be maintained regardless of the number of occupants.

Improving ventilation

There are many ways to improve ventilation to mitigate the transmission of infectious diseases. The most appropriate measures depend on the characteristics of the particular setting. One way to improve ventilation is by opening exterior doors and windows for a few minutes, ideally with more than one open at a time.

Opening windows in winter may not always be comfortable or possible. Doing so for a few minutes at a time during the day can still improve air quality, with minimal impact on the indoor temperature. If occupants will be indoors for longer periods, for example at schools, occupants should have regular outdoor breaks, to allow for ventilation of the room.

An HVAC system will exchange indoor air a certain number of times per hour as a part of regular operation. To increase ventilation, run your HVAC system fan continuously at a low speed to provide air movement and filtration without unwanted draft. Within non-residential buildings, run the system for 2 hours at maximum outside airflow before and after the building is occupied. Bathroom and kitchen exhaust fans that are vented to the outside can also be used to help remove potentially contaminated air, where appropriate.

Most HVAC systems will recirculate some air through the indoor space, making it important to:

This should be done within the specifications of your HVAC system and in consultation with an HVAC professional.  

Portable or ceiling fans, or single unit air conditioners may circulate air within the room, but they do not exchange air or improve ventilation. If using a window air conditioner unit or a fan is necessary, aim the air stream away from people to reduce the spread of potentially infectious droplets or particles.  

Options if ventilation cannot be improved

When properly used, portable air filtration devices with high-efficiency particulate air (HEPA) filters have been shown to reduce the concentration of some viruses from the air. The use of these devices could be considered as an additional protection in situations where enhancing natural or mechanical ventilation is not possible and when physical distancing can be achieved.

It's important to note that the effectiveness of portable air filtration devices in reducing the transmission of the SARS-CoV-2 virus hasn't yet been demonstrated. As such, they should not be used alone or as replacement for adequate ventilation, physical distancing and hygienic measures. Whenever possible, consider the use of an alternative space, or preferably gathering outdoors rather than indoors, when interacting with people from outside your household.

To select the right air filtration device, it is important to consult an experienced professional to:

When in use, ensure that the air released by the device is not blown directly at individuals as it can increase the spread of droplets.

While humidifiers do not remove SARS-CoV-2 virus from the indoor air environment, they could impact the duration that particles that contain virus are suspended in the air. It is therefore important to maintain an optimal humidity level, between 30% and 50% in indoor settings. Humidifiers can be:

Lower humidity levels can cause droplets to shrink, and smaller droplets can stay suspended in the air for longer. However, increasing humidity too much can lead to condensation on surfaces, as well as inside walls and building areas where it cannot be seen. This can lead to mould growth and the proliferation of mites.

In spaces that are continuously used, like classrooms, windows and doors should be opened regularly where possible. The space should be cleared of people regularly to limit the potential build up of potentially infectious respiratory droplets or particles over time.


Good ventilation includes:

This can help reduce spread of COVID-19 in indoor spaces by preventing the accumulation of droplets and aerosols indoors, but must be combined with other public health measures. We recognize that the ability of the general public to follow this guidance may be limited by various factors.

In addition to improving indoor ventilation, be sure to:


The Public Health Agency of Canada would like to recognize the contributions provided by experts within PHAC and Health Canada in the development of this guidance. PHAC would like to acknowledge the review of this document and valuable feedback provided by the National Collaborating Centre for Environmental Health (NCCEH), Public Health Ontario and the Institut national de santé publique du Québec.



Canadian Agency for Drugs and Technologies

Canadian Committee on Indoor Air Quality

Centers for Disease Control and Prevention (CDC)


Environmental Protection Agency (USA)

Federal Environment Office, Germany

Federation of European Heating, Ventilation and Air Conditioning Associations

Health Canada

Independent Scientific Advisory Group for Emergencies (SAGE)

Institut national de santé publique du Québec

National Collaborating Centre for Environmental Health (NCCEH)

Public Health Ontario

Scientific Advisory Group for Emergencies (UK)


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