Since 2020, workplaces have implemented a wide range of control measures to reduce the risk of a worker’s exposure to COVID-19. There is no single control measure that exists that will fully protect a worker from exposure. Frequently, multiple control methods are used. Many employers have focused on personal protective equipment (PPE) as the first and last line of defense to protect workers. In fact, PPE is considered the least desirable means of controlling a hazard.

The hierarchy of controls illustrates the types of measures and procedures best placed to eliminate or reduce a worker’s exposure to hazards in the workplace. Where elimination and substitution of a hazard are not possible, the next level of effective control is through engineering control, including ventilation.

In terms of COVID-19, ventilation is a critical engineering control to ensure that any infectious virus that may be aerosolized is diluted and exhausted from the workplace. Essentially, when the air is clean in the workplace, the risk of exposure to the virus is lowered. Clean air can be achieved through frequent air changes and the use of filters.  A filter with a minimum efficiency reporting value (MERV) rating of at least MERV-13 should be effective to capture virus laden airborne particles; however, High-Efficiency Particulate Air (HEPA) filters are preferred only when the air system can accommodate this level of filter.  Before switching to a higher rating of filter, an assessment must be undertaken by an HVAC technician to ensure any increased air resistance does not hamper the performance of the system.


Under the Occupational Health and Safety legislation, an employer has the broad duty to take every precaution reasonable in the circumstance to protect a worker. This requirement of due diligence can include adopting standards for airflow and air exchanges in heating, ventilation, and air conditioning (HVAC) systems. Certain jurisdictions (NL, NB, BC, Federal sector) require employers to ensure that ventilation systems meet the design and performance criteria set out in the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) standard ASHRAE 62-1989, Ventilation for Acceptable Indoor Air Quality. Specifically, there is a requirement to ensure outdoor air is supplied at a rate of 15 to 20 cubic feet per minute (cfm) per building occupant. In other jurisdictions, the function and design of the HVAC system of buildings should generally conform to the standards from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). In general, HVAC systems should operate at least two (2) hours before and after occupied. Ventilation in washrooms should function at full capacity.

Health care settings will often have additional regulatory requirements. Example: In Ontario, ventilation systems must be inspected at least every six (6) months. Details of the inspection can be found here:  see

Assessment of the Ventilation Systems

A ventilation system can be modified to ensure a greater supply of outside air while reducing the recirculation of air in a room, another room, or an open-air plenum (ceiling space). HVAC systems are complicated and many factors, such as the outside air temperature, humidity, airflow, and air direction must be considered before modifying a system. An HVAC technician or engineer must perform any modification or balancing of a system. Improperly balanced HVAC systems have the potential to reduce air changes, potentially causing contaminated air to build up in a room or spread into a hallway and other rooms.

During the pandemic, most workplaces should have their ventilation systems modified to increase outside air and inspected for performance issues by a qualified technician or engineer. A ventilation checklist has been developed by the Occupational Health Clinics for Ontario Workers (OHCOW) which includes pertinent questions that a local and joint health and safety committee can use to assess the suitability of ventilation in a school/building; see

Use of Fans and Portable HEPA Air Purifiers/Cleaners

There are a few considerations that must take place before introducing a fan or similar device into the workplace. Local and central ventilation systems are optimized and balanced to provide consistent air exchanges. A fan has both the potential to disrupt air changes and could result in small airborne droplets from a student’s sneeze or cough to disperse towards the face of a worker or other students.

There have been cases of coronaviruses that were spread by improper ventilation. In the article below, the currents of an air conditioning unit in a restaurant blew COVID-19 air-suspended droplets towards people and surfaces in its direction.  We would expect similar results if a fan was present in certain workplaces and therefore would generally not recommend the use of any fan that would blow air in a direction that could create cross currents between students or workers.

The use of portable HEPA air purifiers/cleaners can enhance the air changes in a ventilated room.  They are not a substitute for a balanced and optimized HVAC system. The approval to use one or more standalone HEPA air purifiers/cleaners in a room would be based on a risk assessment for the spread of the virus. Important to success of air purifiers/cleaners to remove particles is dependant on the size of the room, placement, and air flow of the unit. An industrial hygienist or experienced HVAC technician must be involved in this assessment.

Air purifiers use a different technology (i.e., UV lamps, ion and ozone generators) to remove impurities in the air. Certain types of air purifiers have the potential to release harmful ozone. Air cleaners rely on filters to remove particles in the air.

Maintenance, including filters changes, of air purifiers/cleaners must be carried out by trained workers familiar with the unit’s manufacturer’s requirements.

Guidance for Rooms with Natural or Limited Ventilation

Rooms with limited or no mechanical ventilation should only be used when necessary. In these instances, other measures and controls need to be put in place to reduce exposure. The selection of controls must be based on the hierarchy of controls. The controls are determined by an assessment. It is important to remember that the implementation of a control measure shouldn’t create new hazards and must allow the workers to work without discomfort.”

Engineering controls:

  • opening of windows and doors*
  • use of portable HEPA air purifiers/cleaners+
  • use of portable fans to draw air in and out
  • ceiling fans to dilute/disperse air
  • plexiglass barriers

Administrative Controls

  • decreased room occupancy
  • increased disinfecting of surfaces
  • full dilution of air when workplace is unoccupied (i.e., opening all doors and windows) *

* assess for other risks and hazards such as security, violence, air pollution, pollen/allergen counts, insects, outdoor temperature, and humidity.
+ assess for increased noise

Air Quality Resources



Guidance for Building Operations During the COVID-19 Pandemic


Reducing the Risk of COVID-19 using Engineering Controls


Ventilation guidance

Public Health Ontario,air%20and%20potentially%20reduce%20exposure.

Morawska, et al. How can airborne transmission of COVID-19 indoors be minimized?

Environment International (2020)