Along with wearing masks and ensuring frequent hand washing, new research suggests that effective indoor ventilation is just as important in preventing the spread of highly contagious coronavirus disease 2019 (COVID-19).

When the pandemic began, investigators believed that the virus was largely transmitted via large droplets spread through sneezing, coughing, or close contact.1 Although these droplets are mostly limited to approximately 6 feet, researchers quickly began investigating whether there is a transmission risk beyond that distance.

“Multiple studies provided quickly strong scientific evidence for successful indoor airborne transmission of COVID-19 in inadequately ventilated environments,” said Jarek Kurnitski, head of the TalTech Nearly Zero Energy Buildings Research Group, in a press release. “The virus is transmitted via saliva droplets with a size from 0.5 micrometer up to a few thousand micrometers produced by a person talking, sneezing, coughing, or even just breathing.”2

According to the study, small and large droplets act differently. Whereas large droplets above 100 micrometers do not travel farther than even 1.5 meters when coughing, small droplets can remain airborne and follow airflow streamlines for longer distances. Normal exhalation mostly produces droplets with a diameter around 1 to 10 micrometers.1

This understanding of droplet behavior is relatively new, however. Until recently, aerosol physicists believed that droplets larger than 5 micrometers fall down at the distance of up to 2 meters, which led to the widespread belief that a 2-meter distance would ensure safe social distancing.1 In addition to disproving this belief, investigators have also found that the COVID-19 virus can remain infectious in aerosol particles for up to 3 hours.1

“This means that you can get the virus in 2 ways: in close contact, where the concentration of aerosols and larger droplets in close proximity of the infected person is very high, or farther away in inadequately ventilated rooms, where the concentration of aerosols remains so high that a person can get an infectious dose, for example, within an hour spent in the same room with an infectious person,” Kurnitski said.2

Several controls targeting airborne transmission should be applied in indoor spaces, according to Kurnitski. Sufficient outdoor air ventilation should be the main control, whether entering the space through a ventilation supply duct or an open window. If the building lacks a functioning ventilation system, portable air cleaners with fine particle filters should be used, and viral contamination can be deactivated by applying short-wavelength ultraviolet light.1

Notably, ventilation systems with recirculated air present a particular challenge because they leave virus particles recirculating in the ventilation system. In such systems, Kurnitski said increasing outdoor air supply is necessary, and filtration of extract air should be improved by installing more effective filters.2

According to the press release, the impact of engineering controls, such as ventilation systems, is greater than the impact of the most widely applied measures, such as wearing personal protective equipment. Though still important, ensuring appropriate ventilation can decrease the risk of transmission when working in a building with others.1

  1. Morawska L, Tang J, Bahnfleth W, Bluyssen P, et al. How can airborne transmission of COVID-19 indoors be minimized? Environmental International; September 2020. Accessed October 23, 2020.
  2. Effective ventilation may be a key factor in preventing the spread of COVID-19 [news release]. EurekAlert; October 20, 2020. Accessed October 23, 2020.