Three types of respiratory masks viz N95, non-woven 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 sterilization using gamma radiation for the two flow rate conditions corresponding normal breath rate (20 lpm) and during sneezing/coughing (90 lpm). Sterilization 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 (non-woven fabric) and double layer cotton cloth mask is found to vary from 18% to 22% for the cumulative particle of size ≥ 0.3µm 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 µm) with the flow rate of 20 lpm and further reduced for particles in the range of 0.1 and 0.2µm 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 sterilization, 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 sterilization using gamma radiation.

The filtration efficiency of masks is determined for atmospheric aerosols of size ≥ 0.3 µm and is shown in Fig. 3.The pressure drop across the filter is measured and is given in Table 2. The pressure drop indicates condition for usage during breathing and is found to be in the accepted range (inhalation and exhalation resistance limit is 35 and 25 mm w.g. respectively). It is observed from Fig. 3 that particulate filtering efficiency is less for high flow rate (90 lpm) when compared to the low flow rate (20 lpm) for all three types of mask. The surgical and cloth mask are performed more or less same or slightly decrease in efficiency after sterilisation when compared to without gamma sterilized condition.

The filtering efficiency found to vary from 18% to 22% in both un-irradiated and irradiated condition for the cumulative particles of size ≥0.3µm.In the case of N95 masks, the filtering efficiency is found to be reduced to 70% from 99.6% and about 62% from 95% for the two irradiated conditions for 20 lpm and 90 lpm respectively. Further, it is also noticed that there exists about 1-2% reduction in efficiency for all the three types of masks between 15 kGy and 25 kGy exposed doses. Various types of facemask viz. N95, surgical mask and self-made double layer cloth mask have been tested for particulate filtering efficiency for two-flow rate condition viz. 20 and 90 lpm before and after sterilization using gamma radiation. The sterilization was carried out for two-dose condition viz. 15kGy and 25kGy.The masks were tested using ambient aerosols and using mono dispersed polystyrene latex particles of size 1.0 µm and in particular,N95 mask is tested with 102.7 nm particles.

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