The COVID-19 pandemic shed new light on how viruses spread. During the pandemic, scientists learned that viruses can linger for a long time in the air. In fact, this may be the main way that many of them infect people.
Researchers are now taking a closer look at how viruses spread through the air and testing ways to clear out germs.
A Sea of Aerosols
Every time you talk, sneeze, sing, or cough, you breathe out more than just air. You also exhale lots of tiny fluid particles. These come in a wide range of sizes. The larger ones, called droplets, fall quickly to the ground. These typically travel less than 3 to 6 feet. But the smallest particles, called aerosols, can hang in the air for minutes or even hours. They are able to travel through the air further than 6 feet.
Just as you breathe out aerosols, you can also breathe in aerosols that others exhale. And some viruses can hitch a ride in them. The more viruses that are in the air, the more likely you are to breathe them in and become infected.
Studies have found SARS-CoV-2, the virus that causes COVID-19, in aerosols. This is now thought to be the main way the virus spreads.
Dr. Donald Milton, who studies airborne viruses at the University of Maryland, recently measured how often the virus is exhaled by people with the flu. He found that about 80% exhaled some influenza, the virus that causes flu. Most of the virus was found in the tiny airborne aerosols. People didn’t have to cough or sneeze to expel these viruses into the air. The flu virus was detectable in the air after normal breathing and talking.
Other illnesses are also caused by viruses that are airborne. These include the common cold, respiratory syncytial virus (RSV) infections, measles, and severe acute respiratory syndrome (SARS).
Exactly how many particles you need to inhale to get sick isn’t clear. And how many you breathe in can depend on many different factors.
Spreading Indoors and Out
How much virus is floating around in the air depends on where you are. Indoor spaces tend to be poorly ventilated compared to the outdoors. Outdoors, aerosols can float off or blow away. In a building with poor ventilation, there’s nowhere for the aerosols to go.
When aerosols collect in the air, you’re more likely to breathe them in and get infected. That’s why many outbreaks of COVID-19 were traced to places with poor ventilation where people were talking loudly or singing—such as restaurants, bars, or places of worship.
Good airflow can quickly clear indoor air of aerosols. This lowers the chance that people will breathe in enough viruses to become infected.
Some viruses, like the flu, are known to spread more rapidly during the colder seasons. People tend to spend more time indoors when the temperature drops. But other factors may also affect how easily viruses spread during cold weather.
Certain viruses, including the flu, last longer in cold temperatures. And humidity, or how much water is in the air, drops in colder air. Humidity can affect how well viruses spread. One reason may be that moisture aids the mucus flow in our airways. This flow helps to clear out viruses.
Studies also suggest that the flu spreads better at lower humidity. Dr. Seema Lakdawala, a flu researcher at Emory University, has found that flu viruses in aerosols can survive over a wide range of humidity levels. So, humidity might not harm the viruses themselves. Instead, it may affect their ability to stay in the air.
At lower humidity, Lakdawala explains, water from aerosols will evaporate. This causes them to shrink, allowing them to stay in the air longer and travel farther. At higher humidity, aerosols might absorb water from the air, causing them to fall faster.
Preventing Infection
With all these viruses floating in the air, what are the best ways to stay healthy? “A lot of the lessons we learned from the COVID-19 pandemic are tools that we can apply every day,” Lakdawala says.
Getting CDC-recommended vaccines can help your body fight off germs. Maintaining a healthy lifestyle can also help keep you from getting ill. That includes a healthy diet and enough physical activity.
Face masks trap fluid droplets coming from your mouth and nose. This can stop viruses from spreading through the air. Masks can also protect the people wearing them.
Researchers are studying other ways to help lower the number of viruses in the air. These include improving building ventilation, air filtration, and even disinfecting the air with ultraviolet light.
Dr. Peggy Sue Lai, a lung doctor at Massachusetts General Hospital, is studying how to improve indoor air quality in schools. “We have detected multiple respiratory viruses in the air of classrooms,” she explains.
If school buildings aren’t well-ventilated, these viruses can build up. But upgrading a building’s heating, ventilation, and air conditioning (HVAC) system to improve ventilation can cost a lot of money, particularly in older buildings. Schools might not always have the funds to make the necessary upgrades.
Lai is testing how well portable air purifiers with high-efficiency particulate air (HEPA) filters reduce virus levels in classrooms. She hopes this could be a less costly alternative to upgrading an HVAC system. But to work effectively, they need to be used correctly. Different air purifiers are designed for different-sized rooms.
“A small air purifier you buy for your bedroom, for example, is not going to work in a large, crowded public space, like a school classroom,” Lai explains. It also needs to be placed properly to ensure that all the air in the room reaches the purifier.
Lakdawala points out that no single strategy will be 100% effective at preventing infections. So she recommends using a combination of strategies to slow the spread of viruses.
Reduce the Spread of Airborne Viruses
- Maintain a healthy lifestyle to help your body fight germs.
- Stay up to date with all CDC-recommended vaccines.
- Stay home when you’re sick so you won’t spread viruses. Reduce close contact with people who are sick to avoid getting and spreading illnesses.
- Improve indoor ventilation and air filtration. Open windows, use portable air purifiers with a HEPA filter and turn on fans. Change your HVAC system filters frequently.
- Wear masks or respirators when in high-risk settings, such as in health care facilities, or if you are at high risk of getting very sick.
