Effect of gamma sterilization on filtering efficiency of various respiratory face-masks

Three types of respiratory masks viz N95, nonwoven fabric and double layer cotton cloth are being used as an essential inhalation protective measure against COVID-19 by suppressing the entry of respiratory droplets. The filtering efficiency of these masks were tested before and after sterilisation using gamma radiation for the two flow rate conditions corresponding normal breath rate (20lpm) and during sneezing/coughing (90lpm).Sterilisation is carried out using a gamma irradiator containing Co-60 source for the two dose exposures viz. 15kGy and 25kGy.The filtering efficiency for surgical (nonwoven fabric) and double layer cotton cloth mask is found to vary from 18% to 22% for the cumulative particle of size [≥] 0.3{micro}m in both un-irradiated and irradiated condition. The filtration efficiency of N95 mask is found to be reduced to 70% for the most penetrating particle size (0.3 {micro}m) with the flow rate of 20lpm and further reduced for particles in the range of 0.1 and 0.2{micro}m with flow rate of 90 lpm. The reduction in efficiency after gamma sterilization is associated with reduction of electrostatic interaction of filter medium with particles laden in the air stream. Even with reduced filtering efficiency due to gamma sterilisation, the N95 masks are much superior than the surgical and cloth masks. Instead of disposing N95 mask after single use, they can be reused a few times as N70 mask during this pandemic crisis after sterilisation using gamma radiation.