We know that people carry the SARS-CoV-2 virus with them from one country to another, one city to another, on planes. However we haven’t traced many outbreaks to contact on planes as a significant vector of spread. One of the reasons that’s usually cited for this is the use of high efficiency particulate (HEPA) air filters.
These filters are essentially sheets of fiberglass fibers, with diameters between half a micrometer and 2 micrometers, randomly arranged. A fan pushes air through the filter, and particles get trappd inside. The very smallest of particles collide with molecules of gas, which slows them from passing through the filter and increasing the chance of their becoming trapped.
HEPA filters aren’t really a kind of filter, though, the name speaks more to their standard of effectiveness – capturing at least 99.97% of airborne particles that are larger than 0.3 micrometers (or 0.00001 of an inch). Bacteria are easily trapped by HEPA filters. Viruses are smaller than a HEPA filter’s pores, but they often travel in clusters, on mucus, or respiratory droplets
This level of air filtration was developed in the 1940s. The Manhattan Project used HEPA air filtration to limit the spread of radioactive material. They ‘spread’ into commercial use in the 1950s to catch viruses, pollen, bacteria, and other particles in the air.
Even with these filters on planes, passengers and flight attendants have historically had higher rates of cold and flu infection, which may be traced to surfaces – and just coming into contact with more people. That’s why I hope current increased cleaning regimens last past the current outbreak.
Research into the spread of tuberculosis has found some ‘drift’ of air between rows – aircraft don’t appear to increase the risk of TB spread compared to other indoor settings, however.
Many smaller planes, including 50 seat regional jets, do not have HEPA air filtration. However DASH-8 turboprops can now be retrofit to have them.
This Delta video shows a HEPA filter being replaced. As filters age airflow reduces, but aren’t necessarily less efficienct capturing virus particles.