Should You Buy an Air Purifier for Covid-19?
By: Chris Baraniuk
Tags: Health, Coronavirus, Public Health, Air Purifier, Covid 19

Should You Buy an Air Purifier for Covid-19?

They can help, but there are some things you need to know

Image: NosUA/Getty Images

Over the summer, the owners of Boedecker Cellars in Oregon could leave their doors open to let fresh air circulate while socially distanced customers enjoyed glasses of the company’s prize pinot noir. But now winter is coming.

“We wanted to make sure that every table in here had coverage,” says co-owner Athena Pappas. So she and her partner, Stewart Boedecker, decided to buy portable air purifiers (some call them air cleaners), which continually cycle air through filters that catch tiny particles, including viruses. These can reduce the indoor transmission of diseases like Covid-19 by lowering the amount of virus floating in the air on tiny droplets called aerosols.

The couple measured the size of their venue and calculated that they would need eight air purifiers, based on the square footage covered by each. The business has continued to avoid any links to Covid-19 cases, and Pappas says customers seem to appreciate the devices. The state of Oregon has just lifted a two-week restriction on indoor dining, and Pappas hopes her customers continue to feel protected now that they are returning.

“Our number one goal is safety over money right now,” she says.

Across the United States, from school buildings to bars famous for their drag queen shows, staff are installing air purifiers in an effort to reduce the spread of Covid-19 and reassure people that indoor spaces are still safe to visit. Homeowners are also turning to them to protect their families. (This year’s massive wildfires have also increased demand.)

As a result, manufacturers say they’ve never been busier. Austin Air Systems in Buffalo, New York, has ramped up production to nearly 2,000 purifiers per day, says Michael Domon, vice president of sales at the firm, which supplies schools, banks, call centers, and even the Department of Defense. “Everything I produce, every single day, I ship out the very next day.”

Across the United States, from school buildings to bars famous for their drag queen shows, staff are installing air purifiers in an effort to reduce the spread of Covid-19 and reassure people that indoor spaces are still safe to visit.

This summer, evidence mounted that the coronavirus could be transmitted via aerosols as well as larger droplets expelled when people cough or sneeze but which fall to the ground relatively quickly. Everyone produces aerosols as they breathe or speak and the virus-laden mist, invisible to the naked eye, that is emitted by an infectious person can build up inside a room if the air supply is not refreshed or cleaned.

To offer readers a sense of how indoor conditions increase risk, German newspaper Die Zeit has come up with an interactive tool that explains how aerosol load increases depending on factors such as the size of room, how well ventilated it is, and how loudly people are talking.

The Centers for Disease Control and Prevention (CDC) notes that air purifiers, when used appropriately, can reduce airborne contaminants including viruses. But mask-wearing, distancing, and other measures are not done away with by air purifiers. “By itself, air cleaning or filtration is not enough to protect people from exposure to the virus that causes Covid-19,” says the Environmental Protection Agency (EPA). If you have a well-ventilated, uncrowded home, or you work in a building with a central ventilation system that cycles air rapidly and has good-quality filters installed, you might not need any purifiers. But if, in other circumstances, you want to add a helpful tool to your arsenal to help you lower the risk of airborne transmission, ahead is a guide detailing what you should consider when buying an air purifier — and how to think about them as potential components in your wider arsenal of defenses against Covid-19.

How to pick an air purifier

If viruses are what you want to remove from the air, ventilation experts generally recommend you get an air purifier or cleaner with a high-efficiency particulate air (HEPA) filter. These thick, spongy filters contain a mass of fibers that trap airborne material. They cover a wide range of particle sizes and are 99.7% efficient at snaring matter that is 0.3 microns in diameter. Thanks to the strange physics of tiny particles, those 0.3-micron particles are harder to catch than either smaller or larger ones. For specks and droplets that are smaller or larger than 0.3 microns, including bioaerosols of between 0.7 and 10 microns, HEPA filters are even more than 99.7% efficient.

When buying an air purifier, avoid terms such as “HEPA-style” and similar — it could mean the filter inside the device is not as effective as a true HEPA filter. Some central ventilation filters are labelled with minimum-efficiency reporting value ratings (MERVs). But for portable air purifiers, HEPA filters are generally considered the gold standard. Also, if you have a purifier with a lower-quality filter, it’s unlikely you can modify it to accept a HEPA filter, says Joachim Curtius, PhD, an aerosol scientist at Goethe University Frankfurt.

Purchasers will also want to consider the square footage of the space in which they want to use the purifiers — manufacturers generally advise what coverage you can expect for a given purifier, and you might need more than one. And another really important factor is the air changes per hour (ACH) rating, which tells you how frequently the purifier will cycle the air in its allotted space — five ACH means five changes per hour, and so on. Hospital rooms might have 20 or even 40 air changes per hour via commercial systems. For general use, six is a decent rule of thumb, though, suggests William Bahnfleth, PhD, a professor of architectural engineering at Penn State University. It means you can expect the air to be cleaned roughly every 10 minutes.

Finally, there’s the clean air delivery rate (CADR), a rating on some purifiers that have been certified by the Association of Home Appliance Manufacturers (AHAM). The CADR defines how quickly a purifier can rid the air of specific types of particles, including dust and smoke — which may be roughly in the same size range as aerosols bearing SARS-CoV-2. These ratings don’t refer specifically to virus-cleaning capabilities, but experts suggest using a purifier with a tobacco smoke CADR that is roughly two thirds of the square footage of the room you want to use it in. So, a CADR of 200 in a room that is 300 sq ft, for instance, would indicate the device can be expected to efficiently remove bioaerosol-sized particles from the air.

To calculate this effect in a little more detail, after 15 minutes, a HEPA purifier with a CADR score of 300 would clear 99% of tobacco smoke particles from a room measuring 10 by 10 by 10 feet, according to a paper published in the journal Otolaryngology–Head and Neck Surgery.

If you’ve found a purifier with a HEPA filter and a decent CADR and ACH, you might still wonder whether it will really do the job of reducing airborne SARS-CoV-2, the virus that causes Covid-19. Some scientific studies offer confidence, however.

How well do purifiers actually work?

One series of experiments in a room occupied by two Covid-19 patients at the University of Florida Health, described in the International Journal of Infectious Diseases, applied HEPA filters to air sampling devices. The researchers did this to ensure the devices were not gathering residual SARS-CoV-2 as they collected multiple samples over time. While they found SARS-CoV-2 RNA in many of their air samples, they didn’t find it when sampled air had passed through the HEPA filters — suggesting that they did indeed block the pathogen effectively.

Another study, in a German high school, evaluated the efficiency of portable air purifiers in a classroom used by 27 students and their teacher, all of whom were wearing masks. Class carried on as normal while the purifiers quietly hummed away and detectors monitored aerosol levels. The findings have been published as a preprint and are not yet peer-reviewed, but they back up the fact that HEPA filters decrease aerosols of a wide range of sizes in a relatively busy indoor space.

“In less than 30 minutes, you can reduce the aerosol load by more than 90%,” says co-author Curtius.

“My opinion is that these purifiers work,” he adds. “It’s also practical in a school room — it’s not too loud, it’s not producing a large draft.”

Although the study did not test specifically for the presence of SARS-CoV-2 in the air or filters post-experiment (just aerosols in general), it is “solid” evidence that HEPA filters work as intended, says Jose-Luis Jimenez, PhD, a professor of chemistry at the University of Colorado, Boulder. That’s important to confirm, he says, because some people have questioned whether HEPA filters really are effective against SARS-CoV-2.

“The fact that someone had to actually do that experiment and write the paper and make all that effort is just crazy,” he says. “And it’s just because of all these flat Earthers that are denying that filters work to remove the virus.”

Other factors to consider

When buying your own purifier, you might want to consider the noise or decibel rating of a particular purifier. Check how loud it is when the fan is at maximum to ensure it won’t become a nuisance. Finally, some models also come with UV lights inside that are intended to kill biological pathogens. It’s an extra layer of protection, but a HEPA filter alone can be very effective.

Once you’ve chosen a model, you then have to decide where to place it in a room. You should pick as central a position as possible — don’t stick it in a corner, says Jimenez. Curtius and his co-authors add in their preprint that the space around the device should be free of obstacles, so avoid placing it under a desk, for example.

That aside, there are few rules about how much proximity to potentially infectious people in a room matters. In a bar, for instance, is it better to have purifiers next to every table or in the middle of the space?

“That’s one thing that’s a little bit of an open question,” says Bahnfleth. “We don’t have, I would say, really clear guidelines on how to do it.”

Giving your purifier the best chance to do its job is important, especially if you buy a relatively simple one that will work at a steady pace — no matter how many people are in the room. Some wall-mounted commercial models can, however, attempt to respond to an increased density of people within a space, including those made by Fellowes.

Think of [air purifiers] like having an airbag in your car — it doesn’t mean you can dispense with your seatbelt.

“It’s monitoring the sound levels and then it’s motion-tracking with infrared sensors,” explains Mike Booth, global market director for air treatment at Fellowes. “We’re able to adjust our fan speed automatically based on changes in the density in the space.”

The bottom line

Again, air purifiers really aren’t silver bullets in the fight against Covid-19. As Curtius puts it, think of them like having an airbag in your car — it doesn’t mean you can dispense with your seatbelt. So too with the virus — all the other precautions are still important, wherever possible, such as masks, distancing, hand-washing, and shielding coughs.

But as thousands of air purifiers go into service around the country and worldwide, it might be worth thinking of their potential benefits in a broader sense. They won’t prevent every case of Covid-19 — but perhaps they will lead to a statistically significant difference in the places where they are used.

“This is about a public-health-level approach,” says Kevin Van Den Wymelenberg, PhD, director of the University of Oregon’s Biology and the Built Environment Center.

Bahnfleth adds that the public’s growing awareness of how airborne pathogens spread could lead to air-cleaning technology becoming much more common.

“I’m not sure I see the future as one with portable air purifiers everywhere,” he says. “I would think, in new buildings, the technology would probably be deployed in a more centralized way.”

During the 1918 flu pandemic, high-rises in New York were fitted with robust heating systems that allowed people to open their windows for ventilation in the winter. Today, we could see similar architectural imprints left behind by the Covid-19 pandemic. If so, many of us could end up breathing cleaner air indoors in the future — and wondering why it wasn’t always the norm.