Metrolab Blog

Particle Counters Knowledge Hub 11

Claim: HEPA filters cannot capture viruses like coronavirus because efficiency is based on 0.3micron and coronavirus is about 0.01micron

Reality; This is not a true statement.

Filters are a combination of different filtration mechanisms (see graph below). The diffusion mechanism serves quite well and can filter smaller than 0.1micron size particles with almost 100% efficiency. However, when we are considering combined efficiency, we should define MPPS value first. MPPS stands for Most Penetrating Particle Size and defines a particle size that falls between filtration mechanisms and considered as the weakest link between particle size spectrum. So, efficiency value should be defined based on this particle size (mostly between 0.3 to 0.5 micron for HEPA’s). For HEPA H14 class filters according to EN1822, the efficiency limit for this MPPS is 99,995%. Assume this MPPS value is 0.4 micron. This doesn’t mean that particles smaller than 0.4micron will have the worst efficiency than this. This is the weakest point and in many cases, filtration efficiency is way much better in smaller sizes thanks to the diffusion mechanism. You can see this correlation and increased efficiency in this graph.

Filter efficiency "most penetrating particle size"