UV-C light with a wavelength around 260 nm (DNA, RNA show the maximum UV absorption at 260 nm) is known to be most effective to inactivate pathogens because the stronger UV light with shorter wavelength has a potential risk to generate Ozone which is hazardous to the human body. Lore et al. (2012) presented the H5N1 virus on 3M 1860/1870 FFR showed > 4-log reduction by UV-C irradiation with 1.8 J/cm2 for 15 min. Mills et al. (2018) presented similar results using a lower intensity of UV-C light (1.0 J/cm2). Heimbuch and Harnish (2019) irradiated 1.0 J/cm2 UV-C light on various influenza strains (H1N1, H5N1, H7N9) and coronavirus strains (MERS, SARS) to prove effective inactivation (> 3.95 log reduction) for all organisms. Also, they found no significant fit and filter performance degradation after 10-20 treatments, except for some fit performance degradation from donning/doffing cycles. However, Lindsley et al. found the stronger intensity of UV-C irradiation (> 120 J/cm2) can cause physical damages to PPEs. Lowe et al. suggested the UVGI decontamination protocol for health care facilities using commercialized UVGI towers (ClorDiSys UVGI Light System). When PPEs are decontaminated by UVGI irradiation, it is important to make sure the UV light reaches all contaminated areas without being blocked by any shadows. Also, it is critical to measure the decontamination efficacies with all possible PPE models because different PPE materials can show different degrees of UV light transmission to the inner PPE layers.
Hydrogen peroxide (H2O2) is a strong sterilant, reacting with many biological substances to produce reactive oxidation to destroy membrane lipids, proteins, and DNA/RNA. The advantage of H2O2 decontamination is its ability to penetrate small space, like filter inner layer where UV light cannot reach, and the final products of H2O2 after the oxidation are harmless water (H2O) and oxygen (O2). However, H2O2 vapor itself can be harmful so operators cannot access the decontamination room until the H2O2 concentration decreases below 1 ppm, which requires longer operation time. Also, the VHP method is not recommended with respirators with cellulose materials since H2Os can be easily absorbed by cellulose and hard to remove.
As of April 2020, the FDA (Food and Drug Administration) approved the N95 respirator/face mask reuse using VHP (Vaporous hydrogen peroxide) method based on Battelle’s report in 2016. Battelle evaluated 3M N95 respirator decontamination efficiency and damages using commercialized VHP device (Clarus® R HPV generator form Bioquell, utilizing 30% H2O2. There was no significant filtration efficiency degradation up to 50 cycles and FFR fit started degraded after 20 cycles from respirator rubber strap deformation. VHP is a promising method with a potential for high capacity throughput. Battelle claims that the decontamination capacity is 10,000 N95 respirator/masks per chamber load, with the machine able to process four chambers per sterilization process, which makes total 80,000 masks decontamination per machine per day (two sterilization processes per machine per day). 
Kenney et al. decontaminated three bacteriophages (T1, T7, and Phi) on 3M 1870 FFRs using VHP generated from the Bioquell’s BQ-50 system and showed >99.999% of deactivation efficacy of all. Many other researchers proved the VHP decontamination capability and there is no significant damage on mask/respirator by VHP decontamination. [9–13]
Moist heat has been known to be an effective way to inactivate viruses, and the temperature should be high enough to inactivate pathogens (> 60 °C) and low enough to prevent mask damages (< 100°C). Also, McDevitt (2010) presented higher RH is more effective in deactivating the influenza virus by testing under various temperatures (55, 60, 65 °C) and RH (25, 50, 75 %). Bergman and Viscusi team published many papers regarding biological efficacy and filter performance degradation by various respirator/face mask recontamination methods. 30 min incubation at 60°C, 80% relative humidity (RH) of moist heat caused minimal degradation in the filtration and fit performance of the tested FFRs up to three cycles.[10,15,16] Heimbuch et al. disinfected FFRs contaminated with H1N1 influenza using moist heat (65°C and 85% RH) and provided .4-log reduction of the viable H1N1 virus. However, one limitation of the moist heat method is the uncertainty of the disinfection efficacy for various pathogens, especially it is less effective on bacteria and mold spores.