To the Editor: We appreciate the thoughtful comments by Dr Torres et al,1 which provide convincing evidence that ultraviolet (UV) C is capable of penetrating through face mask material to inactivate virus.2 We agree that UV irradiation is an acceptable approach for surgical mask decontamination. Reuse of masks is not ideal and should be reserved for situations in which there is no alternative. Further, the best method of decontamination, if the need arises, will vary at different institutions where a variety of equipment may (or may not) exist.
Our report offers a simple solution—a low-cost, ubiquitously available method that does not require special equipment.3 We would like to clarify that the dry heating condition we recommend is 65°C to 70°C, but not 100°C.3 The selection of this condition is based on the facts that (1) coronavirus is effectively inactivated at 65°C or above for 30 minutes,4 and (2) dry heating at 70°C or below has minimal effect on the deformation of melt-blown polypropylene, which comprises the mask filtration material.5 As a result of this limited temperature range, we recommend using a heating apparatus with a temperature adjusting feature, such as an oven, incubator, or blanket warmer. A microwave oven is not recommended because there are metal parts in the face mask, the effects on the filter are unknown, and the temperature is not tunable.
So far, there are no experimental data to compare the effect of dry heating vs UV irradiation on inactivation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A study by Kariwa et al4 showed that the infectivity of SARS-CoV was abolished by dry heating at 56°C for 60 minutes, whereas residual infectivity of SARS-CoV was still detected with UV irradiation of 134 μW/cm2 for 60 minutes. This suggests that SARS-CoV is relatively resistant to UV irradiation. Despite the survival of residual SARS-CoV, the UV irradiation achieved more than 5-log reduction of SARS-CoV (3.8 × 107 to 1.8 × 102). Therefore, UV irradiation is still considered as an effective approach for decontamination of coronavirus.
In sum, we hope that our article is not seen as a criticism of alternative methods that others are using to reduce exposures in personal protective equipment-limited environments. Our goal was to pass on a reliable, simple, and cost-effective method that would be available to anyone in need.
Footnotes
Funding sources: None.
Conflicts of interest: None disclosed.
IRB approval status: IRB approval is not needed for this study.
Reprints not available from the authors.
References
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