From the WebMD Archives

Nov. 3, 2020 -- The coronavirus is surging across the country, and colder weather is driving more Americans indoors, increasing the hours most of us are spending in homes shut up tight against the chill.

The makers of at-home air cleaners have taken notice, with product pitches suddenly as common as fall leaves.

But can a portable air cleaner really protect you from the coronavirus?

The short answer is yes, according to experts like Joseph Allen, DSc, an assistant professor of exposure assessment science and director of the Healthy Buildings Program at Harvard University.

But there are caveats.

“A portable air cleaner, with a HEPA filter, can absolutely help reduce the risk of airborne transmission of COVID-19,” Allen says.

For a few hundred dollars, you can purchase a high-quality home unit that can remove 99.97% of contaminants from the air, including the respiratory droplets that spread the virus.

But before rushing out to buy one, here are a few things you need to know:

  • Not all air cleaners work as well.
  • The size of the room matters, when it comes to air cleaner efficiency.
  • No single strategy -- including a top-rated air cleaner -- is a foolproof silver-bullet solution for combating the spread of the virus.

“The air cleaner has to be employed as part of a multi-layered defense strategy,” Allen notes. “It has to be combined with other strategies and a wholistic risk-reduction approach.”

Portable air cleaners are gaining more attention as infectious disease experts warn that coronavirus cases are likely to continue surging in coming months.

The reopening of schools and colleges has already brought numerous outbreaks in recent weeks, as students return to classrooms in many regions of the country.

With falling temperatures, more Americans are retreating indoors, keeping doors and windows closed and spending less time with visiting family and friends on outdoor porches or patios.

And with the winter holidays right around the corner, health experts worry that increased travel, family get-togethers, and festive parties will put more people in close contact -- driving up COVID-19 cases along with seasonal cold and flu viral outbreaks.

One thing that has increased the focus on portable air cleaners in recent weeks is heightened attention on the role that microscopic aerosolized respiratory droplets play in the spread of the virus.

Linsey Marr, PhD, a Virginia Tech scientist who specializes in the airborne transmission of viruses, notes that these microdroplets are smaller, hang in the air longer, and can travel farther than 6 feet indoors.

As a result, these tiny aerosols represent a greater transmission risk than respiratory droplets, experts say.

When we breathe, sing, sneeze, or cough, we release respiratory droplets, ranging in size from about 100 microns (the thickness of a human hair) down to microscopic levels (1 micron, which is one millionth of a meter), Marr says.

Typically, we emit only a few larger respiratory droplets, which can only travel about 6 feet before falling to ground. But at the same time, we expel hundreds to thousands of smaller microscopic droplets -- aerosols -- that can float in the air for minutes to hours like smoke or fog, and travel 16 feet or more, studies show.

Aerosolized respiratory droplets are generally in the 1- to 10-micron range, Allen says. And although viral particles are much smaller than that -- 0.1 to 0.3 microns or so -- they generally “hitch a ride” on larger dust or water microdroplets to become airborne, “so the virus is never naked,” he says.

The good news, Marr and Allen agree, is that portable air cleaners are very good at filtering out these smaller, more dangerous aerosols.

“They are actually more efficient at capturing smaller particles than larger particles -- 99.97 percent for particles that are 0.3 microns,” Allen says, “but they’re even more effective at smaller particles.”

Evolving Science, Policy

The significant role that microdroplet aerosols play in spreading the coronavirus has evolved since the start of the pandemic.

Allen, Marr, and dozens of other experts have been arguing since February that aerosols can transmit the virus in a big way. But the World Health Organization (WHO) and the CDC resisted the idea at first.

In July, 239 scientists pressed the WHO to change its position in a commentary published in the journal Clinical Infectious Diseases.

The WHO has since altered its stance, and the CDC, earlier this month, fell in line as well.

Allen says it’s “shocking” that it’s taken the CDC so long to acknowledge the scientific evidence of aerosol airborne transmission, despite earlier backing from Anthony Fauci, MD, the nation's chief infectious disease specialist.

But he’s glad the agency has finally come around.

“It’s something that we’ve been pushing for since early February, and it’s been shocking that they did not acknowledge the science [earlier],” he says.

Portable Air Cleaners: A Buyer’s Guide

Shelly Miller, PhD, an environmental engineer at the University of Colorado-Boulder, agrees that portable air cleaners are very good at removing viral particles from indoor air.

But before deciding whether such a device is right for you, she suggests first finding out if you actually need one.

“The most important thing to consider, when purchasing an air cleaner, is whether the space you want to use it in already has good ventilation -- then, the air cleaner isn’t going to add much,” says Miller, whose work focuses on airborne disease transmission.

But for rooms that aren’t getting enough fresh air, such as basements or bedrooms, a portable device “will really improve the air cleaning,” she says.

If you do decide you need an air cleaner for your home, she recommends choosing one with a high-efficiency particulate air (HEPA) filter.

You’ll also need to be sure the unit you buy can handle the size of the room where it will be used.

Most devices on the market rely on mechanical filtration to clean the air and are rated by what’s called a CADR system -- short for clean air delivery rate. They work by using fans to draw in air from a room, passing it through a filter to remove particles, then expelling the filtered air back into the room.

Worth noting: Some devices use electrostatic precipitators or ionization to trap particles. But for home use, experts say this isn’t needed, and they recommend against these types of units because they generate ozone, a harmful respiratory pollutant.

CADR -- A Guide to Unit Ratings

The CADR rating system for portable air cleaners measures both the efficiency of the filter a device uses and how much air moves through it.

Cleaners that use a HEPA filter -- a type of pleated mechanical air filter -- can remove at least 99.97% of dust, pollen, mold, bacteria, pathogens, and other particles with a size of 0.3 microns, according to the U.S. Environmental Protection Agency.

This 0.3-micron designation is a worst-case scenario; particles that are larger or smaller are trapped even better, Allen explains.

He also says filters don’t destroy the virus, the way UV light can, but that’s not critical to clearing the air of COVID-19 particles.

“You don’t need UV or some other process, because the virus is captured on the filter media,” he says. “Once the [viral] particles are captured, they don’t grow and they can’t be released. So, you don’t need anything other than a portable air cleaner with a HEPA filter.

“It’s really simple, just like hand-washing is real simple and basic.”

Room Size Matters

The second key thing to consider, when choosing a portable air cleaner, is the size of the room where you plan to use it.

“If you buy a portable air cleaner and stick it in the corner of a gymnasium, then it’s not going to be very effective,” Allen says. “So, you need to size it right for a living room, or other rooms.”

The higher the CADR, the larger the area it will serve. In general, product packaging will tell you the largest size area or room the cleaner can handle.

Environmental engineers measure indoor air quality by air changes per hour (ACH). This indicates the number of times the air inside a room is filtered every 60 minutes.

As a rule of thumb, Allen says experts recommend four to six air changes per hour for indoor spaces. That means the entire volume of air in a room is being cleaned or replaced every 10 to 15 minutes. That’s enough to remove COVID-19 aerosols from the air and knock down transmission risk.

If you’re willing to do a little math, here’s the formula for matching a device, based on its CADR, to a room where it will be used:

Let’s say you’re looking to buy an air cleaner for a 500-square-foot room --with 8-foot ceilings -- and want to know if a unit with CADR of 300 will do the trick.

First, you multiply that CADR (300) by the number of minutes in an hour (60) -- for a sum of 18,000.

Then you divide that 18,000 by the volume of the cubic feet of space in the room (500 square feet, multiplied by 8) -- for a total of 4,000.

Finally, you divide 18,000 by 4,000 and get 4.5 -- that’s the ACH the air cleaner will deliver, which falls between the four-to-six rate recommended for indoor spaces.

For easy reference, Allen offers this back-of-the-envelope guideline: “Just keep in mind that we recommend four to six air changes per hour, so a quick rule of thumb is to look for a portable air cleaner with a CADR of at least 300 for every 500 square feet of space.”

The Environmental Protection Agency also provides a guide to minimum CADR levels for rooms of various sizes in its online “Guide to Air Cleaners in the Home.” For rooms with 8-foot ceilings, it recommends:

  • A CADR of 65 for 100 square feet
  • A CADR 130 for 200 square feet
  • A CADR 195 for 300 square feet
  • A CADR 260 for 400 square feet
  • A CADR of 325 for 500 square feet
  • A CADR of 390 for 600 square feet

In addition, the Harvard-CU Boulder Portable Air Cleaner Calculator for Schools online tool, created by Allen and Miller to help teachers pick effective air cleaners for the classroom, can give you a rough estimate for rooms of different sizes in your home.

Keep in mind: You need to clean or replace filters regularly for air cleaners to work well. So follow manufacturers’ recommendations on maintenance and replacement if you buy a device with a HEPA filter.

Not a Silver-Bullet Solution

One final point that Allen and Miller stress: A portable air cleaner isn’t a single silver-bullet solution to combating COVID-19 at home.

“It’s not a panacea,” says Miller, noting it should be used with other measures that health experts recommend to guard against the virus.

“We have to layer on defenses,” Allen says. “We should be wearing masks indoors, opening up our windows, bringing in more outdoor air, increasing the level of air filtration, be it through the mechanical systems … or portable air cleaners with HEPA filters. We should keep our social circles small, we should be doing hand-washing.”

In fact, if you’re already doing all of those things, and staying home without many visitors, you may not even need an air cleaner.

The same is true for people who live in apartment buildings or condo complexes, Allen says.

“But if you’re having people over, adding a portable air cleaner with a HEPA filter is a good strategy,” he notes. “It also makes sense to open up the windows even just a couple inches, and mask-wearing is a must.”

Allen bought two air cleaners for his home this year. But he doesn’t consider them a cure-all.

“We don’t use them right now because the weather has been nice, we have our windows open as much as we can, we don’t have people over much, and if we have people over, we’re outdoors and separated and wearing masks,” he says.

Additional resources

  • The Association of Home Appliance Manufacturers website offers a helpful primer on air cleaners and also certifies units, so look for the AHAM Verifide seal when choosing a product.
  • The California Air Resources Board has compiled a list of air cleaners that are certified as safe and effective.
  • Consumer Reports and CNET have put together online guides to the best devices on the market this year, based on their own independent analyses.

Show Sources

Joseph Allen, DSc, assistant professor of exposure assessment science and director, Healthy Buildings Program, Harvard University.

Shelly Miller, PhD, environmental engineer, University of Colorado-Boulder.

Linsey Marr, PhD, professor of civil and environmental engineering, Virginia Tech.

Shelly Miller blog: “A Hopefully Helpful Short Report on Air Cleaners.”

Clinical Infectious Diseases: “It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19).”

Indoor Air: “Ventilation and health in non‐industrial indoor environments: report from a European Multidisciplinary Scientific Consensus Meeting (EUROVEN).”

Environmental Protection Agency: “Air Cleaners and Air Filters in the Home,” “Indoor Air Quality: What is a HEPA filter?” Guide to Air Cleaners in the Home,” “Air Cleaners, HVAC Filters, and Coronavirus (COVID-19).”

Association of Home Appliance Manufacturers: “Facts About Clean Air.”

California Air Resources Board: “California Certified Air Cleaning Devices.”

Consumer Reports: “Best and Worst Air Purifiers of 2020,” “What You Need to Know About Air Purifiers and the Coronavirus.”

CNET: “Best air purifiers for 2020: Dyson, Coway, Molekule and more,” “Do air purifiers protect against COVID-19? What about wildfire smoke?”

Harvard-CU Boulder Portable Air Cleaner Calculator for Schools.

Mayo Clinic: “Can a home air purifier system protect you against COVID-19?”

NPR: “Coronavirus FAQ: How Do I Protect Myself If The Coronavirus Can Linger In The Air?” “Coronavirus FAQ: Is It A Good Idea To Buy An Air Cleaner For My Home?” 

The Conversation: “How to use ventilation and air filtration to prevent the spread of coronavirus indoors.”

Vox: “Coronavirus is in the air. Here’s how to get it out.”

MD Anderson Cancer Center: “Can air purifiers protect you from COVID-19?”

National Library of Medicine (PubMed): “What is an effective portable air cleaning device? A review.”

WebMD: “Colleges and Universities: The New COVID Hot Spots,” “Coronavirus and Apartments: What’s the Risk?” “CDC Says Once Again Breathing Can Spread COVID.” 

CNN: “Fauci: ‘There's good enough data to say that aerosol transmission does occur.’ ”

NBC News: “Fauci calls Amy Coney Barrett ceremony in Rose Garden 'superspreader event.' ”

© 2020 WebMD, LLC. All rights reserved. View privacy policy and trust info