Once the role of airborne/aerosol transmission of COVID-19 became more recognized, lots of places starting putting HEPA filter devices into offices, classrooms, and various other locations. HEPA (High Efficiency Particulate Air) filters were initially created in the 1940s to help remove radioactive materials from air in labs and manufacturing spaces (during the development of the atomic bomb). Since then they have found common use in labs, manufacturing and other spaces were fine particles need to be controlled, and this includes removal of biological pathogens from air. Generally, a HEPA filter is one that can remove at least 99.97% of 300 nm (or 0.3 micrometre) sized particles from air that travels through it.

At first glance, 99.97% efficiency seems quite impressive and a good level of protection from bacteria and viruses. However, the reality is somewhat more complicated. The basic question is whether your HEPA device sitting in the room is significantly reducing pathogen exposure or not? Like many engineering questions, it depends on the context and here we will explore some of those factors.