Invisible defenses: A guide for cleaner indoor air

What if the air in your home could act as a silent shield against illness? Responding to the ongoing risks of COVID-19 and other airborne pathogens, many people have taken measures to improve the air in their homes and other spaces.

I’m particularly inspired by the New York loft of my friend Anna Stern. Take a quick peek at her pictures in the gallery (below) and you may be inspired, too. She has seven inexpensive — but powerful — air filter machines hidden away, five of them behind her living room sectional. Together, these machines can achieve over 12 complete air changes per hour when set on high, meeting recommendations from top air quality experts.

Clean air is a vital line of defense, yet navigating filters and ventilation can feel overwhelming. With so many options, from do-it-yourself (DIY) filters to high-end HEPA machines, how do you determine what’s right for your space? How much does filtering air actually help if you have kids or roommates who aren’t masking? How long does the virus survive in the air in the first place?

For this explainer, I talked with aerosol experts as well as smart laypeople who have installed filters in their homes, in the hopes of making it as easy as possible for readers to clean their air and protect their health.

Table of contents (click to jump to each section):

• Filtration and ventilation: Powerful together
• CO₂: The secret to prolonging airborne infectivity
• How do I choose a HEPA or other filtration device?
• Online tools that make it easy to evaluate options

Filtration and ventilation: Powerful together

Filtration and ventilation of a room or home can be a powerful deterrent to infection from SARS-CoV-2 (the virus that causes COVID-19) as well as other pathogens that spread through the air. Filtration physically removes airborne particles and pollutants by passing air through a filter. Ventilation involves the exchange of indoor air with outdoor air to dilute pollutants and particles, but the outdoor air must be relatively clean for this to help improve health.

Examples from research:

• Portable air cleaners (PACs) with a clean air flow rate of 2.6 air changes per hour (ACH) in a classroom can reduce the amount of airborne particles a person breathes in, by up to 66%, according to one study. Experts recommend placing PACs at the center of a room.
   

• Air purifiers in a classroom set to clean the air around six times an hour (6 ACH), lower risk of SARS-CoV-2 infection after two hours to one-sixth of the risk with no filtration, another study found.
   
• Increasing the filtration rate from 2 liters per second per square meter to 4 liters per second per square meter might reduce the relative risk of infection even more. In a 10 by 10 foot room, for example, you would be increasing ACH from about 3 to 6. Open-plan offices, classrooms and well-ventilated restaurants are safer than small office and meeting rooms.

There is no hard and fast consensus on how many air changes you need in a room. Instead, it depends on the number of potentially infected people present, how crowded the room is, and what activity is taking place. For example, reading quietly in a library does not generate nearly as many aerosols as practicing in a choir or participating in an aerobic exercise class.

The newest guidelines from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) take this into account. They recommend that about six air changes per hour is a good minimum to aim for in your home, while 12 is excellent. For instance, the California Department of Public Health has recommended six to 12 ACH to reduce far-field aerosol transmission in health care facilities. Far-field is a catch-all term for anything beyond six feet. (And for comparison, ASHRAE specifies that operating rooms in hospitals have a minimum of 20 ACH).

Along with filtration, ventilation is a powerful aid. In fact, just moving air is important. Ceiling fans are one useful tool: they move air up and down in a space and help disrupt pockets of warm air, while spreading particles over a larger volume. Ceiling fans also help prevent riskier ‘dead spaces’ where air is stagnant.

Similarly, vortex-style fans generate a spiraling column of air, forming a vortex that travels across the room and eliminates stagnant pockets.

CO₂: The secret to prolonging airborne infectivity

In the early days of the pandemic, some studies suggested that the SARS-CoV-2 virus could remain viable and infectious for at least three hours or even as long as sixteen hours after being breathed out. But more recent research by Allen Haddrell, an environmental chemist at the University of Bristol Aerosol Research Center, has shown that the virus loses 90% of its ability to infect people within 20 minutes, with most of that loss in the first five minutes. 

In their research, Haddrell and his colleagues use small electrical charges to eject fluid, forming uniform liquid droplets instantly trapped in mid-air. With a laser and camera, scientists can measure the size and chemical composition of the particles, and then allow the droplets to fall into a Petri dish for culturing. Videos on Haddrell’s YouTube channel explain the process more concretely.

However, Haddrell also had disturbing news: he found that as carbon dioxide increases in a room, the virus persists longer. Bicarbonate in our saliva helps generate carbon dioxide as we breathe out, and this creates a humid and buffered aerosol that allows the virus to persist. If the carbon dioxide in a room is high, the virus can retain infectivity for a longer time.

According to Haddrell, a small increase in CO₂ in a room, to only 800 parts per million (which is considered an indication of good ventilation, according to OSHA and ASHRAE) markedly increases the virus’ stability. In an April 2024 paper, Haddrell and his colleagues reported that at higher concentrations of CO₂, such as in crowded and poorly ventilated spaces, ten times more virus remained infectious after 40 minutes compared to clean air. 

Unfortunately, those higher concentrations are common in homes, offices, and public spaces. “My house often has over 1000 ppm,” Haddrell explained. “And that’s just from me and my partner and my cat.” Opening the windows might be a solution in some places, he said, but in areas of high pollution, that’s not a good option.

That’s why many people now use small, portable CO₂ monitors. “If CO₂ is low, it’s a marker for low viral load,” Haddrell said. If there is high ventilation in a big space, the risk is much lower than if CO₂ is, say, 2000 in a crowded theater where you might be sitting for three hours.

However, there is one important caveat: HEPA and other filters do not filter out CO₂. So it’s possible that a space can register high levels of CO₂ and actually be filtering out pathogens. This is true of airplanes once they are at cruising altitude, for instance. If you want to monitor for that possibility, find a monitor that measures particulate matter as well as CO₂. If CO₂ is high, but particulates are low, there’s a good chance the air in the space is well-filtered.

There is no hard and fast rule for what level of CO₂ is ‘safest’ indoors. But outdoor air, where viruses are quickly diluted, usually registers around 400 ppm. 

One last tip: viruses survive better at lower humidity indoors. New research from Stanford University suggests that a relative humidity of 40-60% may help kill more aerosols than lower humidity readings. Given that mold growth can occur more easily at indoor relative humidity of 50% and higher, keeping your indoor air at about 40-45% might be ideal.

How do I choose a HEPA or other filtration device?

HEPA air cleaner machines can be expensive, with some high end models as costly as $1,000 each. Not everybody can afford one, let alone one for each room in their home. But filtration can be affordable, thanks to DIY options. The most popular and well-studied option is the Corsi-Rosenthal box, a joint venture of the nonprofit foundation started by Richard Corsi, Dean at the College of Engineering at the University of California, Davis and Jim Rosenthal, a Certified Air Filter Specialist and owner of Tex-Air Filters. The boxes are easily made with a box fan, furnace filters, and some duct tape, for under $100 each.

The Corsi-Rosenthal boxes, when on high, may be too loud for the classroom, sleep, or concentrated work. But leave it to human ingenuity to develop other, quieter DIY versions: quiet personal computer fans (or PC fans, which keep computer hardware cool) instead of box fans to power the devices. They are efficient, sleek, attractive, and easier to fit in smaller spaces, though the clean air delivery rate is a bit lower.

You can buy kits or preassembled devices at Clean Air Kits. For travel, an innovative Chinese engineer named Adam Wong developed a collapsible CR-style filter box using PC fans.

Another innovator of PC fan filter creations is Alex LeVine, a Los Angeles inventor who has Crohn’s disease. LeVine built PC-fan boxes as chandeliers (called “fandeliers”) to render family dining and entertainment safer; as well as tabletop and nightstand filters he calls “warp core.” “My father was a Harvard anthropologist,” LeVine told The Sick Times, “and anytime anyone was sick his solution was cross-ventilation and aspirin.” When LeVine first saw a Corsi-Rosenthal box in 2022, he “marveled at the fact the most popular video on how to build one featured a nine year-old girl. That’s how easy it is.”

Online tools that make it easy to evaluate options

If you don’t have the time or energy to build an inexpensive device, there are several sites that can help you evaluate other options. One of my favorite sites for reviewing air filters is House Fresh. The owner, Danny Ashton, lives in the United Kingdom, and offers honest reviews of air filter devices informed by his personal testing. 

“A lot of brand name filters come out promising the world and don’t even share their CADR,” Ashton told The Sick Times. “The only real way to know is to test it yourself, so I started a business doing just that. And I’ve discovered that a lot of these name brands are taking advantage of consumers.” Another issue is that name brands may require people to buy replacement filters from the manufacturer, at a high price.

Clean Air Stars is another helpful site which offers an easy-to-use online tool that will tell you how to achieve the ACH you want in a room, and how many of various recommended devices you’ll need.

In my case, I decided I wanted six ACH in my living room and a moderate noise level since this is where I spend most of my time. To use the tool yourself, you’ll need to know your room volume — which means multiplying the square feet by the height. But, a pro tip from HVAC engineer Joey Fox: even if you have very high ceilings, you can ‘cap’ your ceiling calculation at 9 feet high. High ceilings dilute air; your actual risk is down near the ground where you live, sit, eat, walk, talk, and sleep. 

Following Clean Air Stars’ recommendation, I spent about $200 for four devices that now  keep my living room air nice and clean. I add ventilation to the filtration by keeping some windows cracked open and my ceiling fan always on low. Check the picture gallery to see my fantastic air quality now — measured by an inexpensive air quality meter.

Final bit of advice from Corsi: “If someone in the house is sick, try to develop an isolation zone in their bedroom. Make sure you have air blowing out the windows of that room if you can, so that air does not flow from their room into the rest of the house.” It also helps, he says, to use MERV13-rated filters and high settings for any HVAC machines you have, as well as, of course, masking in common areas.

This approach really helps mitigate spread: Liesl McConchie, who has three children and is PTA president at her children’s school in San Diego, revealed last September that one of her kids had contracted COVID-19. But air filtration and careful masking protected the rest of the family, Liesl told The Sick Times: “We have so many different air filters running through the house, along with ceiling fans on all the time, windows open.” 

CO₂ levels stayed under 500 PPM for the two weeks, she said. Her son masked in an N95 when he walked through the house to spend time outdoors, and Liesl and her husband masked when going into his room. “Everybody joined in on Zoom for family reading time and games, and my son built dozens of Lego sets and did tons of cool art.” 

And that, says Corsi, is the beauty of filtration and ventilation. “I’ve been infected once,” he said. “This virus is going to be with us for a long time, and I anticipate that, even with precautions, I will get infected sometime again. From my perspective, prolonging the time between infections as long as possible is the goal.”

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Gillian Neimark is a journalist and children’s book author. Her most recent picture book, Forest Joy (Blue Jasper Editions), just won a Family Choice Award, and her poem on the evolution of chloroplasts appears in the December issue of Scientific American.

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