Science Policy

Advanced air filtration may help limit the spread of COVID-19 when combined with other protective measures

02.12.21 | 4 min read | Text by Federation of American Scientists

Given the pervasiveness of COVID-19 throughout the U.S., the risk of infection to transportation workers and passengers is significant. For instance, in a survey of over 600 bus and subway workers in New York City, almost one quarter reported contracting COVID-19, and 76 percent personally knew a coworker who had died from the disease. During last week’s hearing, the House Transportation and Infrastructure Committee discussed best practices for protecting transportation workers and passengers from COVID-19, with a particular focus on preventing the spread of the coronavirus through the air.

Transmission of COVID-19 via aerosols

In early October 2020, the Centers for Disease Control and Prevention (CDC) updated its guidance, confirming that COVID-19 can be transmitted via aerosols in addition to larger respiratory droplets. When an individual with COVID-19 coughs, speaks, or breathes, tiny coronavirus-carrying droplets can travel over distances longer than six feet and stay suspended in the air for up to several hours. For the coronavirus, most transmission via aerosols occurs in enclosed, poorly ventilated spaces, when a person is exposed to respiratory particles for an extended period of time. Mass transit vehicles can be one such pathway for infection since people from different households share the same spaces while either working or riding to their destinations.

Protecting people from COVID-19 involves implementing measures to keep virus particles from entering individuals’ noses and mouths. Scientists have found that wearing a face covering can limit the amount of droplets an individual releases, and thus also reduce the amount of virus particles in the air. Masks can also provide some degree of protection to the wearer by providing a barrier between coronavirus-carrying droplets and the person’s nose and mouth. The CDC also suggests that buildings and transportation systems examine the quality of their ventilation and filtration systems to reduce spread of COVID-19. Effective ventilation quickly dilutes the amount of virus particles in the air and allows clean air to quickly circulate in enclosed spaces. Advanced filtration systems can help catch and retain virus-carrying particles on tightly woven inserts, keeping them from reentering the space. While any one of these methods alone is not sufficient to protect people from the coronavirus, a layered approach that combines many safeguards can reduce the ability of respiratory diseases like COVID-19 to spread.

Using advanced filters to remove coronavirus-carrying particles from enclosed spaces

Several Members of the Committee noted the importance of developing and implementing advanced filtration technologies on transportation systems and in buildings. Scientists estimate that the coronavirus can spread even via airborne particles under 5 microns in diameter. (For comparison, a single raindrop is typically about 2,000 microns in diameter.) Most buildings have filters with a Minimum Efficiency Reporting Value (MERV) rating of between 7 and 8, which means they can filter up to 84.9 percent of particles between 3 and 10 microns in diameter. Subway cars also use these filters. The highest rated filters (MERV 16 to 20) can capture over 75 percent of particles that are between 0.3 and 1 micron in diameter, and high efficiency particulate air (HEPA) filters, which are used on airplanes, can theoretically remove at least 99.97 percent of particles 0.3 microns in diameter and larger. To better protect workers and passengers, transportation systems like Washington, DC’s Metro and the Bay Area Transit system in San Francisco are already testing out more advanced filtration technologies through pilot programs funded by the Federal Transit Administration.

Benefits of advanced filters beyond reducing spread of COVID-19

Widespread adoption of advanced filtration technologies can be beneficial not only to reduce the amount of coronavirus-carrying particles in the air, but also to trap other harmful aerosols. During the hearing, Dr. David Michaels from George Washington University and Dr. William Bahnfleth from Penn State University both noted that investing in better filters now can also protect people from inhaling harmful particulate matter from other sources. For example, wildfires contribute about 30% of all fine particulate emissions in the U.S., with many of these being 2.5 microns or smaller. Inhaling these harmful particles can be associated with cardiovascular and respiratory issues, as well as premature mortality, particularly in vulnerable groups such as the elderly, children, and pregnant women.

Enhanced air filtration is a useful tool to help slow the spread of COVID-19, especially when used alongside other measures like wearing masks, improving ventilation, social distancing, and hand washing. As the new administration and Congress work toward ending the pandemic, practices such as the widespread adoption of more robust filters are likely to be examined in more detail. We encourage our community to get involved in the effort to counter COVID-19 by engaging in future congressional hearings through our Calls to Action.