COVID-19 and airborne aerosols: What you need to know

Wear masks indoors. Avoid poorly ventilated indoor spaces, especially those where people are singing, speaking loudly or laughing. Open windows and avoid crowds.
July 9, 2020

The virus that causes COVID-19 may be more infectious than previously known through airborne aerosols, spurring an international group of 239 scientists to call for new warnings from the World Health Organization (WHO).

man sleeping with mask on a plane - airborne covid
Experts are advising people to be very careful about crowded indoor spaces like airplanes. Photo: Getty Images.

Staying safe from airborne COVID-19 particles. Learn more:

The scientists are concerned that people who are spending time in poorly ventilated indoor spaces like bars, restaurants, churches, salons, classrooms and crowded modes of public transit may not understand how important it is for them to wear masks so they can prevent the spread of SARS-CoV-2, the virus that causes COVID-19.

The group of experts includes Jose-Luis Jimenez, a University of Colorado Boulder expert on aerosols, atmospheric chemistry and air quality. Jimenez has been working with the group that published their paper, “It is Time to Address Airborne Transmission of COVID-19” in the journal, Clinical Infectious Diseases.

Jose-Luis Jimenez researches aerosols and airborn transmission of COVID-19
Jose-Luis Jimenez, a professor at the University of Colorado Boulder.

“The one thing we would like the world to do is to take this seriously, but not to be frightened. We need to explain what is going on. This is not like measles (which is highly transmissible in airborne particles) but, it can go through the air. And that’s why we need to do things like wear masks,” Jimenez said.

The warning from Jimenez and his fellow scientists already has generated action. WHO leaders now have pledged to review their guidance on airborne transmission, a move that Jimenez praised.

“We are glad that WHO is moving towards acknowledging the accumulating evidence and may add aerosol transmission indoors to the likely modes of transmission,” Jimenez said after WHO leaders responded to the scientists’ plea. “This would allow people around the world to better protect themselves and fight the pandemic.”

Jimenez and the other scientists have been urging health leaders to recognize and more openly warn the public about airborne spread of the virus that causes COVID-19.

Staying safe from airborne COVID-19 particles

Avoid the 3 Cs:

  • Closed and poorly ventilated spaces
  • Crowds
  • Close contact

Other safety measures:

  • Always wear masks indoors and in crowded spaces.
  • Stay at least 6 feet away from people.
  • Open windows or use HEPA filters to refresh indoor air.

“There is significant potential for inhalation exposure to viruses in microscopic respiratory aerosols (micro-droplets) at short to medium distances…and we are advocating for the use of preventive measures to mitigate this route of airborne transmission,” the scientists wrote.

“Studies…have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking and coughing in micro-droplets small enough to remain aloft in the air and pose a risk of exposure at distances beyond 1 to 2 meters (or about 3 to 6.5 feet) from an infected individual,” the group said.

“Of course we need to do more research, but the evidence is overwhelming that this happens,” Jimenez said.

Earlier this summer, he also co-authored a paper about a super-spreader event in which 53 members of the Skagit Valley Chorale in Mount Vernon, Wash. became sick. Jimenez said it’s clear that the choir member who was sick could not have personally coughed or propelled the virus onto all the other choir members. Thus, the virus-containing aerosols, which would have stayed suspended in the poorly ventilated room, provide the only reasonable explanation for how so many people became sick. Jimenez and others calculated how the aerosols spread in the room.

“Transmission by the airborne route is likely,” Jimenez and the study co-authors wrote. “It is vital to identify features of cases such as this so as to better understand the factors that promote super-spreading events.”

Jimenez said there is less concern about transmission in well-ventilated facilities like hospitals with good ventilation systems, new buildings or large places like grocery stores that also have good ventilation systems. In addition, by the time patients who are infected with SARS-CoV-2 are sick enough to be hospitalized, the concentrations of viable virus in their respiratory tracts are lower, and thus they likely are spreading fewer virus particles into the air. Researchers believe that people who get COVID-19 are most infectious when they begin to show symptoms. And experts believe that the virus spreads easily when people cough, sneeze, speak loudly, breathe forcefully or sing and are in close contact with other people.

Jimenez, himself, plans to be very careful if he is teaching in-person classes this fall at the University of Colorado in Boulder. He said everyone on campus must wear masks. He said administrators have been doing a great job of reducing capacity for classrooms and installing better air filtration systems in older buildings.

Airborne COVID-19: What is the difference between a droplet and an aerosol?

  • Droplets: When people sneeze or cough, they can expel particles filled with the virus that causes COVID-19. Droplets are relatively heavy. So they usually fall to the ground within two seconds and don’t travel much beyond 6 feet. Masks and social distancing can help prevent the spread of droplets.
  • Aerosols: These particles are lighter, smaller and can float. Aerosols could stay aloft for minutes or hours in poorly ventilated indoor areas, thus causing possible transmission of the coronavirus. Avoiding crowds and wearing masks could greatly help to reduce the spread of airborne COVID-19 particles.

Jimenez also is avoiding flying on airplanes until there’s a safe and effective vaccine for the new coronavirus. While airplanes have very good air filtration systems, Jimenez said it’s possible to be sitting too close to a seatmate or a person in a nearby row and to become infected with the virus before the filtration system is able to neutralize it.

Jimenez does not want people to panic. Rather, he thinks it’s important for health officials to clearly explain to Americans and people around the world why wearing masks, staying at least 6 feet away from people and avoiding crowds is so important.

“These measures are not so different from what we are doing already,” Jimenez said. “What you want to do is avoid indoor places that are crowded with low ventilation and places where people are talking loudly or singing. Always wear masks indoors (aside from your home) and in crowded places outdoors.”

Here are Jimenez’ suggestions for staying safe from airborne particles of the virus:

  • Avoid crowds indoors.
  • Wear masks.
  • Stay at least 6 feet away from people.
  • Open windows or use HEPA filters to refresh indoor air.
  • Inexpensive changes can accomplish a great deal.
  • If needed, upgrade heating, ventilation and air conditioning (HVAC) filters to what are known as MERV 13 filters in for places like commercial buildings and classrooms.
  • Avoid indoor bars, restaurants, salons and churches that are not well ventilated or do not have air filtration systems. Instead go to places where service is offered outdoors.
  • If you are outside, you don’t need a mask if you are not in close contact with people outside of your family, for example on an isolated mountain trail. But, if you are in closer proximity with people on a city street, you might want to wear masks. Imagine that the people you cross are smoking cigarettes. You don’t want to inhale or smell the smoke they exhale without a mask.
  • In general, being outdoors is much safer than spending time indoors since there is almost always air flow and UV sunlight during the day destroys the virus quickly.
  • Follow the advice of Japanese health authorities, which can be boiled down to the three Cs: avoid closed and poorly ventilated spaces, crowds and close contact.

“People are very confused and may not understand why masks are necessary,” Jimenez said.

That’s why he and his colleagues think it’s vital to explain how airborne droplets can be expelled into the air and can then migrate.

Concerns about airborne COVID-19 are increasing. Here, a woman wearing a masks gazes inside an empty restaurant.
Poorly ventilated indoor spaces could put people at greater risk for exposure to airborne particles that spread COVID-19. Photo: Getty Images.

“It also seems that there is a great deal of resistance to the word, ‘airborne’ because medical people think immediately of airborne measles.

“This is not like measles. It’s much less transmissible than measles (which can infect for several hours after an infected person left the room,” Jimenez said.

“COVID-19 doesn’t seem to do that. It goes through the air, but it needs some help to build up sufficient viral load to infect, like poor ventilation,” Jimenez said.

He cautioned that the new information about airborne transmission does not mean that the virus has “suddenly become much more contagious.”

But, studies do confirm that much more consistent mask usage could dramatically cut infections.

“If you take the right measures and you maintain them, and you understand why these measures are needed, then you can be more successful in curbing the pandemic,” Jimenez said.

Some people believe masks inhibit their freedoms.

“This isn’t about freedom at all. It’s about the mechanisms the virus is using to spread. You have to wear a mask because you are exhaling particles that contain bits of your respiratory lining, and they could have viruses in them. And someone else might breathe them in,” he said. “This is something people need to do.”

About the author

Katie Kerwin McCrimmon is a proud Coloradan. She attended Colorado College thanks to a merit scholarship from the Boettcher Foundation and worked as a park ranger in Rocky Mountain National Park during summers in college.

Katie is a dedicated storyteller who loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as an award-winning journalist at the Rocky Mountain News and at an online health policy news site before joining UCHealth in 2017.

Katie and her husband, Cyrus — a Pulitzer Prize-winning photographer — have three adult children and love spending time in the Colorado mountains and traveling around the world.