Public Health Agency of Canada issues updated guidelines for at-home ventilation
TORONTO -- The Public Health Agency of Canada (PHAC) has updated guidelines on at-home ventilation during the pandemic, encouraging the use of high-efficiency central air filters and opening windows while urging caution when it comes to ceiling fans and portable air purifiers.
In an update Monday, the agency encourages the use of central air filters with higher Minimum Efficiency Reporting Values (MERV). Filters with higher MERV values have a greater ability to capture particles and can reduce the amount of small particles in the air, the PHAC said.
Setting the HVAC system to the "Fan On" position will also increase the amount of particles that get filtered or diluted, though this may result in higher electricity costs, the PHAC said.
Depending on the weather, the agency says opening windows is another good way of promoting ventilation, provided that there are no air pollution advisories and allergens are at safe levels and that multiple windows should be opened in order to ensure cross-ventilation.
Having a box fan in a window or turning on bathroom and kitchen exhaust fans can remove contaminated air, but another window should be opened to provide fresh replacement air, the PHAC said.
PHAC warns that pedestal fans, ceiling fans and single unit air conditioners only circulate air, but do not exchange air.
"Fans can blow infectious droplets and particles further from their source, which may have contributed to some COVID-19 infections," the PHAC said.
Window air conditioning units should be positioned to avoid blowing air directly at people, the PHAC said.
The World Health Organization, in its advice published last month, says that devices can be used to ensure the air is well mixed and possible contaminants are diluted, but they should only be used if there's already adequate ventilation.
Humidifiers also don't remove the virus from the environment, but they can be used to keep the indoor humidity at ideal levels, which is between 30 to 50 per cent, the PHAC said. The agency said dryer air could lead to small particles suspended in the air for longer, increasing the risk of virus transmission, while too much humidity could lead to water condensation on surfaces.
The PHAC is also urging caution when it comes to high efficiency particulate air (HEPA) air purifiers and cleaners, noting a lack of "direct evidence that portable HEPA air cleaners are effective in reducing SARS-CoV-2 transmission in closed spaces."
" As such, they should not be seen as a replacement for adequate ventilation, physical distancing and hygienic measures, but could be considered as an additional protection in situations where enhancing natural or mechanical ventilation is not possible and in rooms that are not crowded," the PHAC said.
The WHO also says portable air purifiers should only be used "if no other strategy can be adopted."
A lack of ventilation can make a huge impact when it comes to indoor transmission of SARS-CoV-2, even if social distancing measures are put in place. An Australian study looking at a church-related outbreak last July found that a choir singer with COVID-19 led to 12 infections, even though they were sitting as far as 15 metres away. The ventilation system was not turned on, and windows and doors were largely closed, suggesting that a lack of ventilation led to airborne transmission of the virus.
However, the PHAC warns that ventilation shouldn't be the sole strategy to prevent COVID-19.
"It is important to recognize that although ventilation is an important part of an overall strategy to reduce potential risk of COVID-19 transmission, ventilation is not likely to reduce transmission between individuals in close proximity, which is the predominant mode," the agency said.