N95 masks contain multiple layers that are important for the filtering of air. The middle layer consists of polypropylene fibers that mechanically filter air. However, the high filtering efficiency (FE) of N95 masks is achieved by an embedded electrostatic charge of these polypropylene fibers. This electrostatic mechanism employs an electrostatic field that attracts and captures charged and uncharged particles and removes them from the air we breathe. Washing the masks can mechanically damage the fibers and therefore create holes in the filter layer which make the masks ineffective.

Additionally, if the masks get in contact with soap and water or even medical alcohol, the electrostatic charge can be lost and the filtering efficiency is lowered. So even if the mask is washed very carefully and it is ensured that the fibers are not mechanically damaged, the loss in electrostatic charge might still significantly decrease the efficiency of the mask.

However, two different cleaning methods are effective and have only a small impact on the filtering efficiency. The masks can be cleaned in an oven by heating to 158 F for 60 min while hanging. This retains a filtering efficiency of 98.5 %. The masks can also be steamed cleaned for 5 min at a temperature 257 F and will retain between 91.7 % and 98.5 % filtering efficiency. Both of these techniques ensure that the masks stay mechanically intact and that also the electrostatic charge is conserved.