Fact or Fiction: Will cleaning air ducts curb COVID-19 transmission?

Are you really having a regular pandemic week if your phone hasn’t buzzed at least two to three times with sales calls from air duct cleaners?

This sudden interest in air quality control during a global pandemic comes as no surprise to a Toronto professor, though he wishes it fell on the public’s mind sooner.

“I’ve had more conversations in the past year about filtration and ventilation than I’ve had in my entire career,” said Jeffrey Siegel, professor of civil and mechanical engineering and aerosol dynamics at the University of Toronto.

“We’ve known for centuries that ventilation is critically important for infectious disease. … We don’t know enough yet to kind of set a ‘safe’ level of ventilation, but we know more is better.”

Siegel said ventilation may simply mean moving air from one spot to the next to avoid stagnancy.

But just because the air is moving, doesn’t mean it will become cleaner. For that, you’d need a filtration device.

“It’s really double-edged,” said Zhongchao (Chao) Tan, director of the Green Energy and Pollution Control Research Lab at the University of Waterloo. “When the air moves, it also carries the virus and the bacteria from one space to another, if there is no filtration to catch them.”

An incident at a Chinese restaurant in 2020 paints a similar scene.

According to the CDC, 10 individuals who ate at the restaurant between January and February of that year got infected with COVID-19. In a research letter published in July, the CDC concluded that the virus had become airborne, and that “droplet transmission was prompted by air-conditioned ventilation.”

Public Health Ontario has since scrutinized that letter, saying that its authors’ “weakness” was that they “did not conduct any aerodynamic testing to support their hypothesis.” However, the health agency still maintains that infectious disease transmission is possible through ventilation and air movement, and that filtration can mitigate that.

Almost every ventilation system has some kind of filtration, said Tan. But different filters are designed to catch different particle sizes. For a filter to specifically capture infectious disease particles, it has to be distinctly designed for their size and large quantities.

“A virus like COVID-19 can be captured by a normal filter … but how many (particles)? If you have 100 coming in, can you capture 10? Can you capture 20? Or can you capture 99? That’s why it has to be specially designed to capture as many virus particles as possible.”

This is critical, according to Siegel and Tan, in hospital settings, where ventilation coupled with high-efficiency filtration has been used to curb the concentration and spread of viruses for years.

But your home is probably not designed with a system like this — and for good reason, said Siegel — because that’s a huge load on your ventilation system, and your home is not where virus transmission is usually happening anyway.

So where does cleaning your air ducts come into play in all this?

Tan and Siegel say it really doesn’t.

“Cleaning your air ducts has been shown in study after study to not make a difference in infectious disease transmission,” Siegel said.

In fact, Siegel said that the effect that cleaning ducts would have on COVID-19 would be the same as mopping your floors. It won’t make a difference in transmission, and if you care about mitigating the spread of COVID-19, this shouldn’t be on your list of priorities at all.

Furthermore, Tan said if the virus were to actually be lingering in your ducts, it will die. So there’s no point in flushing the air passages out.

Does that mean there’s no point in cleaning your air ducts at all? 

Not necessarily.

Both experts say cleaning ducts is a good option to get rid of an “indoor source” such as mould, or large amounts of debris leftover from a construction project.

It could also be beneficial if you, or someone you live with, has a dust allergy.

But even so, that means you should be cleaning your ducts once every six to 10 years, according to Tan. Once you do, it is normal for dust in the duct to become airborne and aggravate allergies, but Tan said it will soon settle.

So what exactly can I do to limit the spread of COVID-19? 

Both experts emphasize you should implement health guidelines before thinking about anything else. Improving HVAC systems alone will not work.

“I don’t want anyone to think that they’re the magic pill that will save us,” Siegel said.

Siegel said you should maintain physical distancing, mask-wearing, hand-washing, and crowd control in spaces shared by others.

Then you can begin to take steps in improving ventilation.

Health Canada recommends opening windows or doors to dilute indoor air. Siegel and Tan say you can also invest in portable HEPA filters, or look into installing higher-efficiency MERV 13 filters in your central ventilation. Make sure these filters are installed properly and frequently cleaned.

A good method is to also identify poorly ventilated rooms and suspend their use. (Hint: Siegel said these areas tend to feel stuffy and are often smelly.)

Lastly, Siegel and Tan say the public should continue to think about air quality even after we’re out of the COVID-19 woods. A big part of that? Health agencies investing in more studies that illustrate how and when aerosol transmission happens through ventilation systems.

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