Ventilation in the context of COVID-19

Current as of November 16, 2021

Optimization of Ventilation | Mechanically Ventilated Buildings | Naturally Ventilated Buildings | Air conditioning units, fans, and air purifiers | Ventilation in your building | CO2 Measurement Project | CO2 Measurement Schedule | CO2 Measurement Results

As part of the measures McGill has implemented to ensure the safety and wellbeing of our community throughout the pandemic, we have addressed concerns about ventilation in University buildings, particularly in high density areasHere you can find information about:

  • What the University has done to optimize ventilation 

  • What you can do (and shouldn’t do) to improve ventilation  

  • Measurement of CO2 levels in classrooms without built-in CO2 detectors

Keep in mind that good ventilation is just one of the many layers of protection against COVID-19, which include high vaccination rates, the proper wearing of procedural masks, staying home if you have symptoms, physical distancing, proper cough etiquette and others. 

Optimization of ventilation

McGill has many different types of buildings in use, from recently built structures to historic mansions. Some buildings are mechanically ventilated; others rely on natural ventilation (e.g. windows). As part of the suite of health and safety measures the University has undertaken throughout the pandemic, McGill has optimized its mechanical ventilation systems (also called heating, ventilation and air-conditioning or HVAC systems) and has published recommendations for people using naturally ventilated spaces

McGill has implemented a number of measures to align with guidance from public health agencies and industry regarding the operation of heating, ventilation and air-conditioning (or HVAC) systems, including from:

On July 19, 2021, Quebec’s Ministry of Higher Education strongly recommended that universities implement six measures to optimize ventilation for the Fall 2021 term, based on the Government report on Ventilation and Transmission of COVID-19 in the School and Health-Care Environments.

McGill had already made significant ventilation improvements before the recommendations were published, and so did not need to make further adjustments to meet the Quebec government guidelines.  

Government Recommendations

McGill Measures Taken

  1. Ensure optimal maintenance and operations of ventilation systems, including filtration integrated in the systems.



All mechanical ventilation systems at McGill are equipped with air filters:

  • MERV-13 pre-filters provide an initial filtration that remove large particles and are replaced at least once a year.
  • Air filters are replaced once a year
  1. Optimize outside air based on the capacity of the HVAC system


HVAC systems with CO2 monitoring are set to bring in outside air if CO2 levels rise above 800 ppm. Mechanically ventilated classrooms without CO2 detectors have been measured for CO2 by Subject Matter Experts (SME), and were adjusted when necessary. (See recommendations for naturally ventilated shared spaces, below.)

  1. Avoid maintaining energy conservations strategies (e.g. on demand ventilation initiated and or regulated by a timer) to favour continuous ventilation of spaces


All HVAC systems have been set to run 24/7

  1. Maintain a minimum level of ventilation continuously, even when the space is unoccupied


All HVAC systems have been set to run 24/7

  1. Ensure required air exchanges per hour are met as stated in la “Loi sur la santé et la sécurité au travail”


All HVAC systems operates to meet the appropriate air change per hour as required by the law. All HVAC systems have been designed to follow and surpass the standards of the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE).
  1. Avoid the use of rooms for groups of people where ventilation is deficient or absent


McGill recommends single-person occupancy for rooms where ventilation is deficient or absent and without windows or with windows that do not open. No naturally ventilated classrooms are booked for Winter 2022.

If this is not possible, contact Facilities Call Centre (FCC) at 514-398-4555 or by email at fcc.fod [at] to arrange a ventilation assessment of your naturally ventilated space with no windows or windows that do not open. Requests should only be made by supervisors.

What you can do

If you are in a mechanically ventilated building, follow the guidelines for air conditioning units, fans and air purifiers. Occupants of mechanically ventilated buildings do not need to take any further action, as the ventilation systems are being monitored closely.

If you are in a naturally ventilated space with windows that open:

  • Open them to bring in fresh air when the room is unoccupied (for 10 to 15 minutes at least twice a day), and to maximize the air flow, the door may be opened as well
  • Do not leave the windows open overnight as this may result in frozen and burst pipes and water damage
  • Follow the guidelines for air conditioning units, fans and air purifiers

If you are in a naturally ventilated space with no windows or windows that do NOT open:

  • Limit use of space to one user, unless authorized otherwise by FMAS
  • If the space needs to be used by more than one occupant, contact the Facilities Call Centre (FCC) at 514-398-4555 or by email at fcc.fod [at] to have an assessment of the space before it is used

Using air conditioning units, fans, and air purifiers:

The Institut national de santé publique du Québec has released guidelines for the use of air conditioning units (window or wall), electric fans, and mobile air purification devices in the context of COVID-19. These guidelines are for healthcare settings and for school settings and camps; however, some guidelines are adaptable to the university setting.

Air conditioners and fans

In general, air from a wall or window air conditioning (a/c) unit or fan should not blow directly on people.

  • The air flow caused by fans or a/c units should not be directed towards the exit door of the room or the unit to avoid the dispersion of droplets or aerosols out of the room or the unit.
  • The air flow caused by these devices should not be directed towards the occupants.
  • When two or more people share a room, operate fans or a/c units at low speed and do not direct the airflow towards the faces of the occupants or from one person to another.
  • If one person occupies a room with a fan or a/c unit, close the door if possible or ensure the air flow is directed away from the door.

Note: the guidelines for air conditioning units and fans do not apply to air vents connected to mechanical ventilation systems.

Mobile air purifiers

Mobile air purification devices (or air cleaners) are not advised in the classroom or workplace, as they are unproven in controlling transmission of aerosolized diseases and can potentially spread infectious aerosols if installed improperly.

A report by a group of scientific and technical experts commissioned by the Quebec government to make recommendations on ventilation in schools and health-care settings does not recommend the use of mobile filtration devices (or air purifiers) in schools due to:

  • Their unproven effectiveness at countering the transmission of diseases by aerosols (tuberculosis or other)
  • Their reduced efficiency in a large room (such as a classroom) where sources of potentially infectious particulate matter are diffuse or remote
  • The risk of inappropriate use (possible generation of air flow carrying potentially infectious aerosols)
  • The production of noise that could interfere with concentration

From Ventilation and the transmission of COVID-19 in schools and the health-care setting: Summary of the report from the group of scientific and technical experts (January 2021) (French only)

CO2 Measurement project

CO2 detectors measure the amount of carbon dioxide in the air in internal spaces and can be used to determine if the ventilation in a room is adequate for the number of occupants. As some members of the McGill community have expressed concerns about ventilation at the University during the pandemic, particularly in higher density areas such as classrooms, McGill is measuring CO2 levels in rooms without CO2 monitors. CO2 levels alone are not a guarantee of indoor air quality, but it they are a good indicator of the air exchanges per hour.

Note: most mechanically ventilated classrooms (MVCs) at McGill already have CO2 detectors either built into the central system or installed locally. These detectors measure the amount of carbon dioxide in the air in internal spaces to determine if the ventilation in a room is adequate for its capacity. If the detectors show that CO2 levels have reached 800 ppm, they automatically increase the air exchange in the room.

As of October 4, subject matter experts (SME) began assessing the CO2 levels of the classrooms and exam rooms without detectors, as well as naturally ventilated classrooms, benchmarked with the Ministry of Education’s guidelines. To complete this task the SMEs measure CO2 levels at particular times (i.e. before, during and after class).

The following thresholds have been set in order of priority for managing CO2 measurements:

  1. Rooms that register CO2 measurements up to 1,000 ppm are clear for occupancy. 
  1. Rooms with C02 results above 1,000 and below 2,000 ppm will undergo immediate correction to make them more comfortable.  These rooms are considered "In Review" and mitigating measures are taken. Where manual intervention is required, recommendations are made to improve air quality in these rooms, and they are monitored on an ongoing basis.
  1. Rooms with a measurement above 2,000 ppm will be closed until proper effective corrective measures are applied to bring values at or below 1,000 ppm, as per ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) guidelines. 

To put these levels in perspective, the Quebec Regulation respecting occupational health and safety has set the exposure limit for CO2 in the workplace to be 5,000 ppm for eight-hours of exposure per day and for a 40 hour work week. This is the same value set by the American Conference of Governmental Industrial Hygienists (ACGIH).

Please visit our CO2 Measurement Schedule and CO2 Measurement Results pages for more information.

If you have reason to believe the ventilation system is not working correctly (e.g. too hot/cold, little air movement), please call the Facilities Call Centre (FCC) at 514-398-4555 or email at fcc.fod [at]

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