The PM2.5 name comes from these filters' original use as protection against Particulate Matter 2.5, the smallest and most dangerous form of air pollution.  Airborne virions like the Coronavirus are even smaller than PM2.5 (the Coronavirus's lipid envelope is about 120 nanometers across). However, research has shown the material to capture nanoparticles of a similar size with a high degree of efficiency (approx. 95%), and lab tests show a PM2.5 filter with a properly-fitted cloth mask can reduce airborne particulates by as much as 90%