Air Purifiers, Air Filters, and the Best DIY Hacks to Reduce the Coronavirus Risk

Illustration of a box fan placed by a door blowing air outdoors.
Illustration of a box fan placed by a door blowing air outdoors.
Illustration: Matija Medved

Efforts to prevent Covid-19 infections have focused largely beyond the home, emphasizing crowded indoor public spaces. But after our son attended an event where he could have been exposed to the coronavirus, and we were told it could take a week to get test results, I began a quest to understand what to do if someone brings the virus home without knowing they are infectious, and how to create a sustainable defense for the “just in case” scenario, amid so many uncertainties.

First question: What are the odds family members will get infected?

“It clearly happens,” says Ashish Jha, MD, a practicing internist and professor of medicine at Harvard Medical School. “The attack rates are nowhere near 100%, they’re maybe 15 to 20%.” In other words, on average across all households with an infected person, 15% to 20% of family members would get the disease. A review of 40 studies on the topic, published August 1 but not yet peer-reviewed by other scientists, put the figure at 18.8%. Of course, many factors can affect these odds, including how many people are living in a given home and how crowded it is.

Second question: How can we lower the risk?

Improving ventilation in your home and filtering out the airborne coronavirus particles lowers the risk of Covid-19 infections, experts agree. Solutions range from cheap do-it-yourself hacks to not-so-cheap filter upgrades for a whole-house heating or air conditioning system, to somewhat more expensive but surprisingly useful portable air cleaners. Some tactics are most useful when you know there’s a sick person at home. Others can be used continuously just in case — perhaps particularly wise with schools opening, given that we now know kids can be infected and highly infectious.

All the approaches can be seen as layers of prevention that, when added together, further reduce risk. As a bonus, these same techniques will help reduce the spread of a cold or the flu. Also, allergy sufferers may find significant relief when the indoor air is cleaner, and everyone can enjoy the removal of smells, be they from tobacco smoking or bacon frying. The basic principles can be employed in small offices and other buildings, too.

Feel free to skip down to the tactics. But to better understand the best strategy for your home and your budget, a crash course in how the virus gets around will prove helpful.

What you’re dealing with

The coronavirus is expelled in respiratory droplets when an infected person talks or just breathes, and more so if they sing or shout. Heavier, sometimes visible droplets fall to the ground, typically within about six feet. Smaller droplets, called aerosols, are mostly invisible and can waft through the air, like cigarette smoke or hairspray, carrying virus particles around a room and even through a forced-air heating or air conditioning system, studies show.

The coronavirus can remain viable in aerosols many minutes or possibly hours, research suggests. The risk is known to be much greater indoors, especially in poorly ventilated buildings, compared to outside.

Infection can occur when these particles are inhaled by another person or they otherwise get in the eyes, nose, or mouth. Surfaces can harbor the virus and cause infections, but the most common method is through the air, experts say, and they urge a layered approach to lowering risks — clearing or diluting indoor air, masking up, distancing, and disinfecting surfaces.

“I think it is possible in a house, if you don’t have proper filtration, to actually spread the virus.”

“There is definitely a risk of airborne transmission in a home, but at the moment, we do not know the importance of any particular transmission route,” says Linsey Marr, PhD, an expert in airborne transmission of viruses at Virginia Tech. “Given the evidence, it seems wise to take a precautionary approach and take measures to reduce the risk of transmission via respiratory droplets of all sizes, including those that can be inhaled, and contaminated objects.”

Several factors affect airborne infection risk, including:

  • Dispersion and dilution: Airborne virus particles become more diluted the farther they waft from the source and the more they mix with fresh air or pass through filters.
  • Dose and duration: Odds of infection increase with proximity to a source and time spent amid airborne virus particles. Exact thresholds are not known.
  • Breathing rate: Singing, shouting, or exercising all increase the number of viral particles exhaled by an infected person or inhaled by someone nearby.

A person who is exercising might expel or take in 20 times as many viral particles in a given time period as someone breathing normally, says Richard Corsi, PhD, an engineer at Portland State University who studies indoor air quality. “This is one of the reasons gyms are particularly risky environments.”

Back home, armed with that information, you’re ready to take a layered approach to the coronavirus capture and dispersion, with each of several tactics further reducing the risk of infection should someone bring the pandemic home.

Create negative pressure

If someone in your home has Covid-19, isolation is the top priority, all experts agree. The sick person needs to hole up in a single, enclosed room as much as possible, and wear a mask if they come out, and then keep their distance from everyone. Others who bring food or are for whatever reason in the same room with the infected person should wear masks, keep distance, and get out quickly.

“Get a box fan and put it in a window blowing out.”

But here’s a clever hack, whether a person you live with tests positive for Covid-19 or has been exposed and you simply wish to be cautious: Borrow a technique used in biohazard laboratories and create a crude version of a negative pressure room.

“Get a box fan and put it in a window blowing out,” Corsi explains. Viral particles will follow the airflow out of the isolation room rather than into the rest of the house.

This blew my mind. So simple. So not touted.

Open the windows

Whether someone in the house has Covid-19 or you just want to be cautious in case they might, opening windows is an excellent first step. Inviting fresh air, when practical weatherwise, is a very effective way to dilute and disperse the coronavirus aerosols, Corsi says, just as a good breeze can reduce the smell of cigarette smoke.

The goal is to exchange all the air in the house with fresh air as rapidly as possible. In a typical home, opening two or three windows might force a complete exchange in 30 minutes to an hour, Corsi tells Elemental, but that can vary greatly depending on outdoor wind speed and how well the air moves through your home. There are no simple formulas here, but suffice to say any internal breeze will help.

Note, however, that bringing in fresh air could introduce pollen and other pollutants that exacerbate any allergies or asthma — a double-edged sword for some families. Also, a recent study found that people who live near high-polluting industrial plants suffer worse health in general, and also greater risk of death from Covid-19, so keeping pollution out of the house can be vital for many people.

Tip: Layering window-openings with better indoor air filtration can mitigate multiple problems at once. So…

Upgrade your air filters

If you have a room air conditioner that’s stuffed into a window, it may or may not use replaceable filters. If they’re replaceable, read on. If not, pull them out and clean them every month or two, otherwise, they get clogged and less air flows through them.

If you have a centralized, forced-air heating and air conditioning system, it’s time to change the filters. Any filter will trap a particle that hits a fiber. But the cheapest ones, which can cost as little $2, won’t trap many of the aerosols containing virus particles, simply because the holes in their fiber mesh weave are too big.

A study of infections during a single lunch at a restaurant in China illustrates what can happen if air is not properly filtered. An infected diner is presumed to have been the source who spread Covid-19 to five people at other tables that were in the direct flow of an air conditioning vent.

“I think it is possible in a house, if you don’t have proper filtration, to actually spread the virus,” Corsi says.

It’s less clear whether the virus might travel through, say, an entire apartment, condo, school, or office complex through a central HVAC system. “Can the virus waft up and down buildings via air ducts or pipes? Maybe,” writes Marr, the Virginia Tech professor of civil and environmental engineering, “though that hasn’t been established.”

Choosing air filters: What to look for

If you have a whole-house system, look for AC filters that can not only catch the coronavirus particles, but also more of the dust, dander, mold, smoke, smog, and other things that are bad for your lungs.

Look for filters labeled MERV 13 or the equivalent. The Minimum Efficiency Reporting Values (MERV rating) is based on tests approved by the American Society of Heating, Refrigerating, and Air Conditioning Engineers. These filters may claim to capture 99.97% of particles, but that’s a general claim, under ideal conditions with no leaks, and it does not speak specifically to the tiny coronavirus particles. In reality, a MERV 13 filter probably captures around 80% of the coronavirus particles, Corsi says, but “80% is pretty high in overall risk reduction.”

Some stores confuse filter choices by using a proprietary rating system instead of MERV. The highest-rated of these can, however, be equally effective, says Pratim Biswas, PhD, an aerosol science and engineering researcher at Washington University in St. Louis who is currently transitioning to the University of Miami. Make sure the label explicitly states that the filter traps virus particles or virus carriers.

There are two basic approaches to achieving MERV-13 efficiency:

HEPA (high-efficiency particulate air) filters use tight pleated mesh weaves to mechanically trap particles on impact. Compared to cheap filters, these have very tight mesh, so even tiny virus particles struggle to pass through.

Electrostatically charged filters are made of polymer-plastic materials that retain both positive and negative electrical charges. Viruses, bacteria and other biological particles all carry some small charge, as do dust and many other particles, Biswas explains in a phone interview. The charged particles are attracted to the filter’s fibers, creating a higher level of capture with a less-dense mesh, which allows a higher level of airflow, without a large pressure drop.

When in doubt about a filter’s capabilities, opt for the higher ratings or shop brands that use the MERV 13 rating. Price can be a rough guide: MERV-13 filters or equivalents for home systems are all in the $15 to $20 range.

Either type of filter must be changed frequently. Most claim to last up to three months. By then, they get too clogged to allow enough air to pass through, and electrostatic filters tend to lose their charge.

Keep in mind that whole-house systems are designed to turn on and off to keep the temperature relatively constant, and when they are off, any viruses being exhaled will build up.

Tip: Some whole-house systems have a “fan mode” that circulates air through the filters without heating or cooling, allowing you to clean the air for any reason all the time.

Level up: Buy a portable air cleaner

You’ll see them marketed as “air purifiers.” Scientists prefer the term “air cleaners.” They work, but they’re not purely perfect.

Very good ones can be found for around $150 for small rooms and $200 to $250 for larger spaces.

You might hear they work only in small rooms, and must be properly sized for the room. That’s partly true. But Corsi, who has tested them in his laboratory and uses them at home, says even in a large living area, a portable air cleaner might knock the concentration of the coronavirus down by 50%, which is, again, a helpful layer of protection. And meanwhile, everyone is breathing fewer allergens and other pollutants.

If someone in your home has Covid-19, a portable air cleaner in their room is highly recommended, though if you can create a negative vacuum, per above, the air cleaner may be less necessary, he agrees. You could also use an air cleaner to protect others from infection. If a sick person is properly isolated, but they have to come out now and then to use the bathroom or get a late-night snack or whatever, “using a portable air cleaner in other parts of the house can really help,” Corsi says.

Tip: Weather and temperature permitting, opening windows while running an air purifier and/or a whole-house system with high-grade filters can offer multiple layers of protection.

Biswas says even in a large living area, having an effective portable air cleaner very nearby will be particularly effective at cleaning the air being breathed by you or even several people sitting together.

Make sure the air cleaner you pick has a HEPA filter. The units will typically have a carbon filter, in front of the HEPA filter, to capture large particles and pet hair and neutralize odors.

A top-pick HEPA air cleaner tested by Wirecutter reduced particulate pollution by 70% in one hour in a 10,000-cubic-foot conference room — more than twice as much space as it was rated for. It was much more efficient and effective in a 135-square-foot room, of course. The unit costs $200, and Wirecutter estimates you’ll spend another $270 on filters and electricity over five years. Wired suggests two models at different price points, one for small rooms and one for larger spaces.

For a space with 1,000 square feet or less and a typical home ceiling height, Corsi suggests looking for a Clean Air Delivery Rate (CADR) of 300 cubic feet per minute or higher. He just ordered 12 HEPA units online, in the $220 price range, for rooms in the Portland State engineering department. Corsi knows his choice will do the job, but he prefers to maintain objectivity as an expert and not to advocate for the particular model, but now you know his thought process. (On Twitter, he offers a deep-dive into the math for sizing a unit to a room.) There are other, less expensive options for small rooms and some that are said to be quieter than others.

Shelly Miller, PhD, an environmental engineer at the University of Colorado Boulder who researches indoor air pollution, has never owned a portable air cleaner. Until now. Miller bought one to prevent aerosol transmission should anyone in her home catch Covid-19. She recently asked a grad student to create this detailed guide to the products, including specific recommendations for units geared for different room sizes, all of which she and her colleagues reviewed.

Tip: Weather and temperature permitting, opening windows while running an air purifier and/or a whole-house system with high-grade filters can offer multiple layers of protection, and you can also use different tactics in different rooms separated by closed doors, depending on who might be at greatest risk for allergies, asthma, or Covid-19. You may choose to balance that with decisions about overall energy use, of course.

Make your own air cleaner

It turns out a certain, common, boxy room fan, which costs between $20 to $40 online and in big-box stores, is about the same size as a 20x20-inch AC filter. Though far from a perfect solution, you can buy a high-grade filter (see above) and simply tape it to such a fan.

The idea isn’t new. This nine-year-old video offers an analysis of how effective it can be.

I was dubious of this hack, especially given the seriousness of the coronavirus, so I asked Biswas if he would recommend it to people who can’t afford air purifiers. “Absolutely,” he says. It will leak, “but it’s better than not doing anything.” Think of it as an imperfect face mask, he suggests: The best one will stop most or all respiratory particles, but even a homemade cloth mask will capture some.

In fact, the hack has grown more popular of late. As I was finishing the draft of this article, Jim Rosenthal, CEO of Tex-Air Filters, posted the results of a test he just conducted. In an employee break room, the box fan with a MERV 13 filter made “meaningful reductions,” removing 74.6% of the one-micron-sized particles and 80% of the five-micron-sized particles — thought to be roughly the upper threshold for the coronavirus-packing aerosols that stay aloft.

“The ‘Box Fan with a MERV 13 Filter’ should be a useful tool in the fight against Covid-19,” Rosenthal says. He also notes, however, that the one-inch-thick filter strains the fan motor. A four-inch-thick filter, sold for commercial uses, catches small particles as effectively but allows air to pass through more easily, increasing overall efficiency and preserving the life of the fan.

Tip: Tape the filter to the intake side of the fan, so the air sucks it in tight rather than trying to blow it off.

A final thought

Early on in the pandemic, at the height of my spring allergies, I switched our AC filters to the expensive kind. I did not expect what happened. Within days, my years-long battle with daily sniffing, sneezing, and watery eyes cleared up almost entirely, and I stopped taking the over-the-counter allergy meds that weren’t working very well anyway.

Anecdotes are not science, and your mileage may vary. But those filters have almost surely improved my health and well-being, not to mention peace of mind.

And then, given what I learned researching this story, I ran out and purchased two portable, $150 air cleaners. They’re small, designed for 200-square-foot rooms. One is for my home office, the other is for our son, who just headed back to college and a cramped apartment with three other guys and, yeah, a whole lot of uncertainties.

Explainer of things, independent health and science journalist, author, former editor-in-chief of LiveScience and Space dot com.

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