Selecting the Air Filter
The type of filter you purchase will limit the potential efficacy of the mask. The MERV (Minimum Efficiency Reporting Value) rating printed on each filter indicates how well it blocks particles of various sizes. MERV ranges from 1-16, with 16 being the best for blocking the smallest particles. The GM Filtrete filter we used had a MERV rating of 12. Its packaging states that the filter blocks 54% of particles between 0.30-1.0 microns; 83% between 1.0-3.0 microns, and 90% between 3.00-10 microns. In comparison, The industry standard N95 mask indicates that it blocks 95% of particles as small as 0.3 microns when it is perfectly fitted.
Table 1: Efficiency at filtering airborne particles
These ratings and estimates are from the American Society of Heating, Refrigeration and Air Conditioner Engineers.
Human coronaviruses are tiny: they are typically 0.1-0.2 microns in size, yet they don’t travel on their own. They generally travel in mucous droplets transmitted through coughs and sneezes. These droplets range in size, but are typically larger than 200 microns. According to the latest research, it would seem that choosing your filter according to its efficiency at filtering particles that are 3.0 microns makes the most sense. When you read the chart on your air filter, it may actually be the case that the 3.0-10.0 microns column is the most relevant to COVID-19. If so, our GM Filtrete filter rated MERV 12 can block 90% of such particles and a MERV 13 can block 96%.
Please know that you are not making a medical device. These homemade masks are clearly not tested by the FDA and don’t have its approval. In times of need, we need to make do with what we can find. If you use MERV-rated air filters to make these masks, you at least have some assurance that the materials have been manufactured according to residential industry standards. And while these masks have not yet been tested clinically, it is likely that they will give much better protection than masks made from t-shirts and also be more comfortable to wear.
Scharfman, B.E., Techet, A.H., Bush, J.W.M. et al. Visualization of sneeze ejecta: steps of fluid fragmentation leading to respiratory droplets. Exp Fluids 57, 24 (2016). https://doi.org/10.1007/s00348-015-2078-4