Indoor Air Quality

• gas and oil furnaces, fireplaces, stoves, or other appliances that are not properly maintained or not vented to outside of the building
• portable or unvented space heaters or generators
• car engines in attached garages
• gas-powered grills, lawnmowers, or other small engines
• tobacco smoke

• Install a carbon monoxide detector
  • It is legally required in many homes in Ontario. The requirements for when and where to install them are expanding January 1, 2026: be sure to review the updated fire code.
• Ensure all gas and oil-burning cooking and heating appliances are properly maintained AND inspected annually by a qualified technician
• If you have an attached garage, never use a remote starter when your car is inside it or start your car when the door between your garage and house is open

Health Canada has set the long-term exposure limit for CO₂ in residential settings (i.e. in your home) at 1000 ppm, averaged over a 24-hour period. However, it is normal for CO2 levels to fluctuate and serious health effects are not expected below 5,000 ppm.

For example, if you are monitoring CO₂ in your home, you may see higher readings if you have friends over for a social gathering. The increase in number of people, talking and laughing increases the production of CO₂. If CO₂ levels begin to rise, this indicates not enough fresh air is coming in for the number of people present. Generally, you should aim to keep CO₂ levels below 1000 ppm, when possible.

CO₂ levels can be affected by many different things, such as:

• Age of a building (not designed with current ventilation needs in mind);
• Design of a building (built for one purpose, but now used for another);
• Function of the building (e.g., windows that cannot open)
• Size of the building (may not allow for people to spread out)
• Number of people in the building;
• Other CO₂ sources (e.g., smoking, stoves, furnaces, water heaters, pets).

Increasing air circulation, using air filtration, and removing pollutants can improve indoor air quality. The risk of illness (from exhaled breath that might have viruses or bacteria in it) can be reduced with good ventilation and air filtration. Reducing the number of people in the room will also decrease risk. Air filtration units (i.e. HEPA filters*) will only remove particles from the air, not CO₂.

*HEPA filter = High Efficiency Particulate Air filter

CO₂ sensors should be placed:

• On a wall at a height of 1-2 metres;
• Well away from windows or air supply vents; and
• At least 2m away from people or open flames

Once installed, you should see the CO₂ levels change frequently  —as people enter and leave the space, or when windows/doors are opened.

Using a CO₂ monitor helps to identify 3 kinds of risk:

• The negative effects of CO₂ exposure
• The potential for other pollutants to accumulate in a space
• The risk of being in a poorly ventilated space during respiratory illness seasons

High CO₂ levels can mean that ventilation is insufficient for the number of people present, which might also be causing other indoor air quality issues. High indoor CO₂ levels can cause tiredness, headaches, increase your risk of acquiring airborne illnesses (e.g., COVID-19, influenza, RSV), and other symptoms. The reading on a CO₂ monitor is not a direct indicator of risk for these illnesses, but it will help you determine if you should take simple steps to lower the risk of transmission.

Also consider:

• CO₂ sensors can tell you if the ventilation is okay, but dangerous indoor air pollutants can still be present even if CO₂ levels are low.
  • Remove other sources of indoor air pollutants using Health Canada’s IAQ resources
• Wildfires, extreme heat, and other sources of outdoor pollution may impact indoor air quality. If outdoor pollution is present, indoor air filtration becomes more important

As part of legacy work during the COVID-19 pandemic, we are proud to work with the Peterborough Public Library and other partners to offer CO₂ Monitors that can be borrowed at the library.

We are proud to have worked on this collaboration – a North American first, we believe – to increase access to these otherwise costly devices. A special thanks to these partners that helped made this project a reality:

• Danielle J. Cane, London School of Hygiene and Tropical Medicine
• Dr. Angela Eykelbosh, Environmental Health and Knowledge Translation Scientist, National Collaborating Centre for Environmental Health
• Barry Hunt, Prescientx
• Kate Nyhan, Cushing/Whitney Medical Library, Yale University
• Dr. Comilla Sasson, Colorado School of Public Health and University of Colorado
• Cheryl White, Masks4Canada

Remember the WATCH acronym to improve ventilation in any space:

• Windows
  • Open windows as much as possible. Even a crack helps!
  • Keep doors open to circulate air
  • Two open windows with a fan to blow out one window is optimal
• Air Movement
  • Check if you feel air coming from the diffusers or air vents
  • Attach a ribbon to the vent for a visual cue
• Thermostat
  • Keep the FAN setting on when a room is occupied
  • AUTO is okay when the room is unoccupied
• CO₂ Levels
  • Use a CO2 monitor with a nondispersive infrared (NDIR) sensor
• HEPA Filter or Corsi-Rosenthal Box
  • Use the highest setting (noise permitting)
  • Move 1.5ft away from walls & corners
  • Place as close as you can to the center of the room
  • Avoid blowing directly at anyone
  • Disable features like: ionization, plasma, UV with catalyst, auto
  • Keep away from open windows, air vents, other HEPA filters

If your CO₂ monitor is showing higher levels of CO₂, you may want to consider one or more of the following:

• Reducing the number of people in the building/room;
• Avoiding strenuous activities (singing. dancing, shouting);
• Open windows and doors (where it won't pose a risk to people or impact HVAC system);
• Install screens so that windows and doors can be opened without pests coming in;
• Servicing or upgrading your HVAC system;
• Consider using HEPA filters in your HVAC system to reduce particles in the air that may contain viruses or bacteria;
• Increase the amount of air being drawn into your HVAC system (decrease circulation of "stale" air);
• Place portable air filtration systems throughout the building; and/or
• Increasing the humidity of your space