Decontamination of N95 masks for re-use employing 7 widely available sterilization methods

Abstract

The response to the COVID-19 epidemic is generating severe shortages of personal protective equipment around the world. In particular, the supply of N95 respirator masks has become severely depleted, with supplies having to be rationed and health care workers having to use masks for prolonged periods in many countries. We sought to test the ability of 7 different decontamination methods: autoclave treatment, ethylene oxide gassing (ETO), low temperature hydrogen peroxide gas plasma (LT-HPGP) treatment, vaporous hydrogen peroxide (VHP) exposure, peracetic acid dry fogging (PAF), ultraviolet C irradiation (UVCI) and moist heat (MH) treatment to decontaminate a variety of different N95 masks following experimental contamination with SARS-CoV-2 or vesicular stomatitis virus as a surrogate. In addition, we sought to determine whether masks would tolerate repeated cycles of decontamination while maintaining structural and functional integrity. All methods except for UVCI were effective in total elimination of viable virus from treated masks. We found that all respirator masks tolerated at least one cycle of all treatment modalities without structural or functional deterioration as assessed by fit testing; filtration efficiency testing results were mostly similar except that a single cycle of LT-HPGP was associated with failures in 3 of 6 masks assessed. VHP, PAF, UVCI, and MH were associated with preserved mask integrity to a minimum of 10 cycles by both fit and filtration testing. A similar result was shown with ethylene oxide gassing to the maximum 3 cycles tested. Pleated, layered non-woven fabric N95 masks retained integrity in fit testing for at least 10 cycles of autoclaving but the molded N95 masks failed after 1 cycle; filtration testing however was intact to 5 cycles for all masks. The successful application of autoclaving for layered, pleated masks may be of particular use to institutions globally due to the virtually universal accessibility of autoclaves in health care settings. Given the ability to modify widely available heating cabinets on hospital wards in well-resourced settings, the application of moist heat may allow local processing of N95 masks.

Fig 1. Inactivation efficacy of various decontamination methods against SARS-CoV-2 on experimentally contaminated N-95 masks.

Coupons of inoculated N-95 masks (n = 4 models) were subjected to (A) autoclaving at 121°C for 15 minutes; (B) peracetic acid dry fogging; (C) exposure to 70°C + 22% RH for 2–3 hours; (D) exposure to 75°C +22% RH for 2–3 hours; and (E) VHP treatment for 1,2 or 5 hour cycle times. Virus recovery post-treatment was determined by elution of coupons in culture medium and endpoint titration in Vero E6 cells via TCID₅₀ assay. Inoculated, untreated drying controls of each mask material were included as positive controls in all experiments. Results indicate means +/- standard deviations of three biological replicates. Dotted lines indicate quantification limits of the TCID₅₀ assay.

Fig 2. Penetration of UV-C through N95 masks.

The degree of UV-C penetration through a layered N95 mask was demonstrated using photochromic UV-C Dosimeter Disks. A 3M 1870+ Aura respirator mask was cut in half, and dosimeter disks were placed directly on top (left disk), half-way under the first fabric layer (top center disk), or half-way under the thick middle layer (lower central disk) of material. A color change from yellow (unexposed) to deep pink was achieved on exposed portions of all disks. The lighter orange color, consistent with reduced UV-C exposure, was revealed on the disk partially covered by the top layer of mask material during UV-C treatment, while the disk placed half beneath the thick middle fabric layer showed no color change from yellow, indicating a lack of exposure to significant UV-C radiation.

Fig 3. Impact of autoclaving on ink print on 3M 1804 filtering facepiece respirator.

A single cycle of autoclaving consistently resulted in varying levels of ink bleeding on 3M 1804 and 9210 masks (a before; b after). Minimal further bleeding of ink was inconsistently observed after additional autoclave cycles. a) Before autoclave. b) After single cycle of autoclave.