Can you open a window please?

Air quality and ventilation – a key element for safe indoor spaces

We often take the air we breathe for granted, with little thought to the quality, despite its critical nature to our very survival. Perhaps one of the benefits of the Covid-19 global pandemic (admittedly the “benefits” are few - but we believe in trying to find the silver linings and useful learning even in the worst of times!) is that many if us have become more acutely aware of the importance of the quality of the air we breathe. As individuals we now wear masks to protect ourselves from Covid-19 while the war against Covid-19 continues and we continue to hope for the benefits of a world class vaccine campaign to reap rewards. However, we are also experiencing an increase in COVID-19 cases due to the more contagious delta variant and colder weather is approaching which will undoubtedly increase indoor activities which are known to increase the risk of transmission. Further, there are individuals that are either unable (e.g., medical/religious grounds or children 12 and under), or unwilling to be vaccinated (hopefully the unwilling will continue to decline with vaccine mandates being rolled out by governments and the private sector) and many will be concerned or even hesitant about returning to indoor work settings in general now that we are more aware how easily diseases (yes there unfortunately may be other similarly serious diseases in a post Covid-19 world) can be transmitted. Given this, what can we do to make sure that the risks are minimized for everyone regardless of the disease in question or vaccination status?

Ventilation is going to be key! Both air filtration and proper ventilation working together are critical to indoor air quality and this has never been so evident with the spread of Covid-19. Think of the air filtration system as the mask for the building while upping the ventilation is the closest thing to bringing the outdoors in to reduce the risks. Keep in mind that circulation is different from ventilation, and it is important to have adequate exchange of fresh air and not just recirculate the same air within an indoor space.

But how does one know if they are getting filtration and circulation right? It can be tough and air quality tests can be quite expensive. For air filtration we can look at the filter’s Minimum Efficiency Reporting Value (MERV) rating. The MERV rating is based on a scale of 1 to 20 and tells you how effectively your filter traps the small particles you don’t want to circulate through your indoor space. The higher the MERV rating, the higher the amount of particles that will be trapped by the filter. The higher the MERV rating the better the air filtration so it’s best practice to choose a filter with as a high a MERV rating as your system will allow. Filters with a MERV rating between 16 to 20 are typically only found in hospitals or other spaces needing extra levels of filtration. I personally use a MERV 16 at home which helps those with allergies or asthma.

When it comes to ventilation that can be more challenging to measure. Many are turning to the use of carbon dioxide monitors as a proxy for indoor air quality with respect to proper ventilation. The theory behind using carbon dioxide monitors as a proxy does make some logical sense. When we breathe, we expel carbon dioxide. Without ventilation, carbon dioxide levels in an enclosed space will climb. Add more people to that space and the carbon dioxide levels will rise higher faster. By introducing ventilation, the carbon dioxide levels will be dissipated as fresh air is exchanged and fresh air is brought in. The ventilation required will vary depending not only on the size of the space, but also based on the number of people in the space and the rate at which the people are breathing (think of a spin class versus a yoga session in the same space – the spin class participants are likely breathing harder and faster increasing carbon dioxide levels comparatively higher and faster). If we introduce (inadvertently of course!) someone with Covid into a space, the risk others may become sick would most certainly increase in an enclosed space with no ventilation since the individual breathes out contagious particulates or aerosols along with the carbon dioxide as they breathe. Without proper ventilation these particulates and aerosols accumulate.

One can surmise, therefore, that the higher the levels of carbon dioxide in an enclosed space, the higher the risk one might catch Covid-19 from an infected individual in that space. Proper ventilation may therefore help to decrease the risk. There is no set consensus on what the proper levels of carbon dioxide would indicate sufficient mitigation of risk of Covid-19, but it does seem likely the lower the levels of carbon dioxide the better. Outside carbon dioxide levels are typically less than 450 ppm. In general, Health Canada has adopted a long-term indoor exposure guideline of 1000 ppm of carbon dioxide for residential and other settings1. This may be viewed as more of a comfort-based measure and may not be suitable for pandemic conditions so should be viewed as a maximum threshold. The U.S. Centre for Disease Control recommends maximum indoor carbon dioxide levels of 800 ppm during the pandemic and supports using portable CO2 sensors with a logging function to monitor indoor spaces2.

Using a carbon dioxide monitor can be used to help monitor the air in an enclosed space so that appropriate actions can be taken to keep carbon dioxide levels reduced to the greatest extent possible. If carbon dioxide levels are becoming too high, some possible corrective actions include, but are not limited to:

  • Increase fresh air supply via the HVAC system
  • Keep fans on the HVAC system running continuously or at least more frequently, including after hours
  • Open windows and doors
  • Use fans to improve circulation if needed when opening window or doors
  • Reduce overall occupancy
  • Avoid high-intensity activities to the greatest extent possible
  • Use an air filters and/or cleaner in addition to proper ventilation

Keep in mind that using a carbon dioxide monitor is only a proxy for the air quality and ventilation and does not directly link to the risk of contracting Covid-19, nor can it definitively mitigate the risk of transmission, but it is certainly a better approach than nothing and will certainly increase your awareness of the relative level of ventilation of your indoor space so that you can take corrective actions in order to mitigate potential risks.

1Health Canada. Residential indoor air quality guidelines: carbon dioxide. Ottawa, ON: Government of Canada; April 2021 (Residential indoor air quality guidelines: Carbon dioxide -
2U.S. Centers for Disease Control and Prevention. Ventilation in buildings. Atlanta, GA: U.S. Department of Health & Human Services; 2, June 2021 (Ventilation in Buildings | CDC)

Photo by Chris Barbalis on Unsplash
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