How Better Ventilation Can Help ‘Covid-Proof’ Your Home

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Yves here. Even though most readers are already very well versed in the topic of improving ventilation, this guide has some simple ideas. Consider circulating it to relatives and friends who might be receptive to doing more to lower their Covid exposure.

By Liz Szabo, Kaiser Health News Senior Correspondent, previously wrote for USA Today and The Virginian-Pilot in Norfolk, Virginia and has won several national journalism awards. Originally published at Kaiser Health News

For two years, you beat the odds. You masked, kept your distance, got your shots.

Now, despite those efforts, you, your child, or someone else in your home has come down with covid-19. And the last thing you want is for the virus to spread to everyone in the family or household. But how do you prevent it from circulating when you live in close quarters?

The Centers for Disease Control and Prevention recommends isolating covid patientsfor at least five days, preferably in a separate room with access to their own bathroom, as well as diligent mask-wearing for both patient and caregiver. But for many families, those aren’t easy options. Not everyone has an extra bedroom to spare, let alone a free bathroom. Young children should not be left alone, and the youngest can’t tolerate masks.

“For parents of a young child, it’s pretty difficult not to be exposed,” said Dr. Preeti Malani, chief health officer at the University of Michigan. “You have to work back from the perfect to the possible and manage your risk the best you can.”

But take heart. Scientists say there is still a lot people can do to protect their families, chief among them improving ventilation and filtration of the air.

“Ventilation matters a lot,” said Dr. Amy Barczak, an assistant professor of medicine at Harvard Medical School. “If you’re taking care of someone at home, it’s really important to maximize all the interventions that work.”

To understand why good ventilation can make a difference, it helps to understand how the novel coronavirus spreads. Scientists have learned a lot in two years about its infectious mechanisms.

Viral particles float through the air like invisible secondhand smoke, diffusing as they travel. Outside the home, viruses are quickly dispersed by the wind. Inside, germs can build up, like clouds of thick cigarette smoke, increasing the risk of inhaling the virus.

The best strategy for avoiding the virus is to make your indoor environment as much like the outdoors as possible.

Start by opening as many windows as the weather allows, said Joseph Fox, a heating, ventilation, and air conditioning engineer for a large school district in Ontario, Canada. If possible, open windows on opposite sides of the home to create a cross breeze, which can help sweep viruses outside and bring fresh air inside.

For extra protection, place a box fan in the patient’s window, facing outward, to draw germy air outside. Seal any openings around the sides of the fan, said Jim Rosenthal, CEO of Tex-Air Filters, a company that manufactures air filtration products in Fort Worth, Texas.

“It’s real simple, and it’s cheap,” Rosenthal said.

To prevent infected air from seeping out of the sickroom, Fox suggests wedging towels in the gap under the bedroom door. People should also cover return air grills with plastic. These grills cover vents that suck air out of the room and recycle it through the heating or cooling system.

Fox also suggests turning on bathroom or kitchen exhaust fans, which can shuttle germy air outside. Although running exhaust fans while taking a shower is relatively safe, Fox said, it’s important to open windows when running the fans for more than 10 minutes. That’s to avoid depressurizing the house, a circumstance that could result in carbon monoxide being pulled into the home from the furnace or water heater.

Coronaviruses thrive in dry air, and increasing the amount of moisture in the air can help deactivate them, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech. Marr suggests increasing humidity levels to somewhere between 40% and 60%.

Using portable air cleaners can provide additional protection. Research shows that high-efficiency particulate air filters, or HEPA filters, can remove coronaviruses from the air. If people have only one HEPA filter, it’s best to place it in the sickroom, to trap any virus the patient exhales.

“You want to put the filter as close to the source [of the virus] as possible,” Fox said.

If affordable for families, additional air cleaners can be used in other rooms.

Store-bought air purifiers can be expensive, with some models costing hundreds of dollars. Yet for about $100, people can build their own portable air cleaners using a box fan, four high-efficiency air filters, and duct tape. These do-it-yourself devices have been dubbed Corsi-Rosenthal boxes, after their co-inventors, Rosenthal and Richard Corsi, dean of the college of engineering at the University of California-Davis. The low-cost boxes have been shown to work just as well as commercial air purifiers.

Rosenthal said the pandemic motivated him to help design the air purifiers. “We’re not helpless,” Rosenthal said. “We need to provide tools that people can use right now to make things better.”

Although nursing a loved one through covid puts the caregiver at risk, the danger is much smaller today than in the first year of the pandemic. An estimated 95% of the population has some immunity to the coronavirus, due to vaccines, prior infections, or both, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

Nonetheless, a recent study found that half of the people living in an infected patient’s household also contracted the virus.

Given that older people and those who are immunocompromised are at higher risk from covid, they might consider staying with a friend or neighbor, if possible, until the sick family member has recovered, said Priya Duggal, a professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

Patients can be considered covid-free after a negative PCR test, Barczak said. Because patients with even tiny amounts of residual virus can continue to test positive on PCR tests for weeks, long after symptoms disappear, patients can also use rapid antigen tests to assess their progress. If antigen tests are negative two days in a row, a person is considered less likely to be contagious.

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  1. BurntOutNA

    I recently brought an unidentified severe respiratory bug (likely covid given timing and local resurgence) home from the facility where i work, and the other people did not get ill in my household following very similar practice to that suggested here. Upon realizing exposure I set up a negative pressure room with a fan facing outward in the window. I put towels around the door and had a high volume hepa filter just outside my quarantine room. I wore a tight fitting n95 when I left the room, and washed hands thoroughly before using common areas. I picked up meals and used the bathroom in this manner for the days I was exhibiting symptoms of respiratory inflammation. I used a mixture of elderberry syrup, garlic, oregano oil and cold pressed cbd to reduce symptoms and hopefully contagiousness. In this manner, I walked out of the rehab where I work with a likely case of covid and gave it to none of my family. N=1, and internet anecdote, but I feel my evidence greatly supports this wonderful article! Don’t assume you have to give your family COVID!

  2. Alyosha

    As a practicing Indoor Air Quality professional, I second almost all of this advice. I quibble with the statement that running a bathroom or kitchen exhaust fan will overpower the convection exhaust of direct fire pilot units like water heaters or furnaces, unless you’ve installed commercial grade ventilation in those locations. A high powered, whole-house exhaust fan could do that but only if you have the sort of house that’s so tightly sealed you can’t light a fire in the fireplace without opening a window.

    You also can’t put a HEPA filter on a box fan. The static pressure across a HEPA is far too much for a standard box fan to draw an appreciable amount of air through that filter. You can however buy a centrifugal duct fan and mount a HEPA to it. This is going to be more expensive but can likely be done for <$150 and some DIY effort. If you're using it in a single room, a 4" fan is going to be more than enough. Always check the Cubic Feet per Minute (CFM) rating of the fan and deduct 25-30% if you're mounting a HEPA, just to be on the safe side. You have excellent ventilation (or filtered recirculation) if you can change the air in a space four times each hour. That's the level required for doing hazardous material remediation. An AC infinity 4" fan with filter reduction should still move 150 CFM (9,000 CFH) and a 10*20*8' room is 1600 cubic feet.

    Also, buy an old house. Old houses almost always have better IAQ than new construction because they're just not as tight.

    1. XXYY

      Corsi & Rosenthal recommend MERV 13 air filters arranged in a hollow square to provide more surface area. MERV 13 is equivalent to an n95 mask. HEPA filters are MERV 15 to 20.

      Millions of people are currently running the MERV 13 configuration with a 20″ box fan (the standard Corsi-Rosenthal box). It works fine.

      This site goes into the subject in exhausting detail.

  3. Raymond Sim

    Quibbles and complaints:

    If you have vulnerable people in your household, sending them away after a positive test in a person they’ve been exposed to may actually compound your problems.

    Any given grill in any given room of any given house may be outputting air rather than ‘sucking’ it.

    A lot of households won’t have the option of providing a sickroom with a window you can easily place a fan in.

    Put my previous two quibbles together and you can see that stuffing towels under doors may not cut it at all. We live in a world with duct tape people! A Corsi box in the room and necessity-mothering-invention deployment of tape, towels, cardboard, plastic trash bags, furniture, family pets, etc can work wonders*, but it ain’t necessarily easy, and it’s not something you’re necessarily going to do well on the fly while under stress.

    The last paragraph of the article will be misread by many, and is thereby sub rosa perpetuating the “PCR stays positive foreeeeever, antigen tests will tell you when you’re safe.” myth.

    *Ask anybody who’s been living in fire country these last few years

  4. twonine

    40-60% RH is certainly the sweet spot. However, also the sweet spot to rot out your house if you try and maintain it all winter in a northern climate. I keep our bedroom at 40% RH on all but the coldest nights, but the room is on the southwest corner, where the sun keeps the interior of walls warmer; and the walls have dense-pack cellulose insulation with a high hydric buffer capacity.

    “Build tight. Ventilate right.” Heard it first from Lstiburek, but he’d likely credit those that came before.

  5. Dave in Austin

    Airfree Iris 3000. Works for me. A small 3″ ceramic cube villed with iny vertical holes, each one with a toaster wire. It heaats the cube up to 450+ and the air circulates via convenction. The inventor 20+ years ago, a Portugese engineer, is very carefull to say “Cleadrs 600 sq ft in 1-2 days but if circulation is bad you need fans. In my 650 sq ft Austin place I have had two since the beginning of COvid. Any carbon-based life form is turned into dust. How it works with a coughing COvid person near you is a different matter. $230 at Amazon.

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