Relative survival of Bacillus subtilis spores loaded on filtering facepiece respirators after five decontamination methods

Abstract

This study determines the relative survival (RS) of Bacillus subtilis spores loaded on an N95 filtering facepiece respirator (FFR) after decontamination by five methods under worst-case conditions. Relative survival was obtained by testing after decontamination and after storing the FFRs at 37°C and 95% relative humidity for 24 hours. The decontamination methods involved ethanol, bleach, ultraviolet irradiation (UVA 365 nm, UVC 254 nm), an autoclave, and a traditional electric rice cooker (TERC) that was made in Taiwan. Without decontamination, 59 ± 8% of the loaded spores survived for 24 hours. When 70% ethanol was added to the N95 FFR at a packing density of 0.23, the RS was 73 ± 5% initially and decayed to 22 ± 8% in 24 hours. Relative survival remained above 20% after 20 minutes of UVA irradiation. The other four decontamination measures achieved 99%-100% biocidal efficacy, as measured immediately after the methods were applied to the test FFRs. Relative survival is a useful parameter for measuring sterilization or degree of disinfection. Bleach, UVC, an autoclave, and a TERC provide better biocidal efficacy than ethanol and UVA. Not only a higher filter quality but also a lower value of RS produced the most decontaminated FFR.

Keywords: N95; bioefficacy; decontamination; respirator; spore; survive.

Figure 1

Experimental system setup.21, 22 A filter holder contains one piece of N95FFR

Figure 2

Sample treatment flowchart

Figure 3

Relative survival as a function of concentration of 0.4 mL ethanol for Bacillus subtilis spores loaded on N95 filtering facepiece respirator. Error bars represent one standard deviation

Figure 4

Relative survival as a function of aqueous packing density (α_aq) of 70% ethanol for Bacillus subtilis spores loaded on N95 filtering facepiece respirator. Error bars represent one standard deviation

Figure 5

Relative survival as a function of concentration of 0.4 mL NaOCl for Bacillus subtilis spores loaded on N95 filtering facepiece respirator. Error bars represent one standard deviation

Figure 6

Relative survival as a function of wavelength and duration of exposure to ultraviolet for Bacillus subtilis spores loaded on N95 filtering facepiece respirator. Error bars represent one standard deviation

Figure 7

Relative survival of Bacillus subtilis spores loaded on N95 filtering facepiece respirator. Error bars represent one standard deviation. (TERC: Taiwan traditional electric rice cooker)