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.
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“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.
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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.