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How the Coronavirus Spreads: Biggest Threats Are in Air, Not on Surfaces

CDC ties greatest risk for COVID-19 to airborne particles, droplets from coughs and sneezes

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The Centers for Disease Control and Prevention (CDC) has once again updated its guidance on how COVID-19 spreads, this time acknowledging that airborne particles from basic breathing and talking play a key role in virus transmission.

The agency's website now explains that when a person infected with COVID-19 exhales, a mix of larger droplets and smaller particles that contain the virus are released into the air. Gravity pulls the largest droplets quickly to the ground, but the smaller virus-containing particles can stay suspended in the air “for minutes to hours” and can be breathed in by others — even by those who are more than 6 feet away. Previously, the CDC maintained that COVID-19 was thought to only “sometimes” spread this way.

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Two other principal modes of transmission are also highlighted in the latest CDC guidance: Splashes and sprays produced when an infected person coughs or sneezes can land on a nearby person's eyes, nose or mouth and get them sick. You can also get COVID-19 if your hands have virus particles on them and you touch your eyes, nose or mouth.

How COVID-19 Spreads

Health officials say COVID-19 spreads by three main ways:

Airborne particles: Breathing in small virus-containing droplets and particles that have been exhaled by an infected person.

Splashes and sprays: Having these small droplets and particles land on the eyes, nose or mouth. This can happen when an infectious person coughs or sneezes, for example.

Direct contact: Touching your eyes, nose or mouth with hands that have the virus on them.

Source: CDC

The importance of good ventilation

Even though our understanding of how the coronavirus spreads has shifted, “the ways to prevent infection with this virus have not,” health officials say. Frequent handwashing is still recommended; so is social distancing and mask wearing in public spaces for people who are not fully vaccinated.

But experts who have long championed for this most recent change say the update really pushes the need for good indoor ventilation to the forefront — especially at a time when schools and offices are bringing people back to meet in person.

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This is because ventilation, where indoor and outdoor air is exchanged, helps to ensure that the virus is not “building up,” thereby increasing the likelihood that others in the space will breathe it in, explains Linsey Marr, the Charles P. Lunsford professor of civil and environmental engineering at Virginia Tech and an expert in aerosol science. Her advice? “If the weather allows it, open windows are great. If not, then it involves adjustments to the HVAC (heating, ventilation and air conditioning) system so you're bringing in as much outdoor air as possible,” she tells AARP.

Opening windows can also keep your home environment healthy — especially if you've having people over who are not part of your household, says Richard Corsi, dean of Portland State University's Maseeh College of Engineering & Computer Science and an expert on indoor air quality. Upgrading filters in indoor spaces — aim for MERV 11, 12 or 13 — and firing up portable high-efficiency particulate air (HEPA) cleaners can also lower the concentration of virus particles in the air, Corsi adds. (MERV, or minimum efficiency reporting value, ranges from 1 to 16; the higher the rating, the better the filter is at capturing smaller particles.)

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Just make sure you select an air cleaner, also known as an air purifier, that is right for your space by paying attention to the clean air delivery rate (CADR): You want it to meet or exceed the square footage of the room. “Let's say a room is 100 cubic feet. If the air cleaner only does 20 cubic feet per hour, that's not enough,” Marr says. Look for the CADR rating on the cleaner itself or on its packaging, or check the manufacturer's website.

One more tool that works: Upper-room ultraviolet radiation, “where you just sterilize the air,” says Donald Milton, M.D., a professor of environmental and occupational health at University of Maryland's School of Public Health. This can be especially helpful in school lunchrooms, for example, where kids are eating without masks on, he says. The CDC notes this intervention also makes sense in areas where there's an increased likelihood of sick people — like a hospital waiting room or school clinic.

Masks, distancing still a priority indoors for unvaccinated

Another takeaway from the CDC's latest update on COVID-19 transmission: Masks are still an important tool in indoor settings, especially if you are unvaccinated or not yet fully vaccinated. (Some experts encourage masks for everyone in indoor public spaces, particularly if you are going to be around unvaccinated people, but the CDC on May 13 updated its guidelines to say that fully vaccinated individuals no longer need to wear a mask indoors, except in certain situations like in health care settings or on planes, trains, buses and other forms of public transportation. They should also be worn where required by local businesses and workplaces.)

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If an infected person wears a mask, then the “rate of release of these aerosol particles in an indoor environment can be much lower,” Corsi explains, which means there's essentially less virus in the air to be inhaled by others. Masks can also protect the wearer by blocking fine droplets and particles from being inhaled.

While not a solution on its own, keeping a distance from others is another key to reducing risk of infection by airborne particles, especially if you are not vaccinated. Infections through inhalation at distances greater than 6 feet from an infected person can happen — it's been documented in super spreader events and studies, Milton and his Stop COVID Study team note.

"If there's an infector in the space that you're in, there's no distance that's perfectly safe,” Corsi adds. That said, the risk of catching COVID-19 is greater the closer you are to an infected person, “because in close contact, the concentration is going to be higher,” Corsi explains.

Think of aerosol transmission of COVID-19 like cigarette smoke, Marr says. “It's most concentrated near infected people and dissipates as you get farther away.”

Washing hands trumps washing surfaces

Remember the early days of the pandemic when disinfectant wipes and sprays were in short supply? It turns out, all of the worry over wiping down counters and surfaces — even groceries and takeout containers — may have been overkill.

It's still possible to pick up COVID-19 from a surface, but the chance of that happening is low — “generally less than 1 in 10,000,” the CDC now says.

Washing your hands with soap and water — or an alcohol-based hand sanitizer when those are not around — is a reliable way to prevent surface transmission. Cleaning surfaces with soap or detergent also does the trick. Disinfectants are really only needed in indoor public settings where there has been a suspected or confirmed case of COVID-19 within the last 24 hours, the CDC says.

How to protect yourself from COVID-19

• Get a COVID-19 vaccine as soon as you can

• Wear a properly fitted mask that covers your nose and mouth

• Stay at least 6 feet away from people who don't live with you

• Avoid crowds and poorly ventilated indoor spaces

• Improve ventilation (here's how to do that)

• Wash your hands often with soap and water

Source: CDC

Michelle Crouch contributed reporting.

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