Here Comes the Sun—Methylene Blue in Combination with Sunlight Sanitises Surgical Masks Contaminated with a Coronavirus and a Tenacious Small Non-Enveloped Virus

In the context of the SARS-CoV-2 pandemic, the reuse of personal protective equipment, specifically face coverings, has been recommended. Reuse of such items necessitates procedures to inactivate contaminating human respiratory and gastrointestinal pathogens. We previously demonstrated decontamination of face coverings contaminated with either infectious SARS-CoV-2 and animal coronaviruses or a highly resistant, non-enveloped norovirus via a novel photochemical treatment. Contaminated materials were coated with photosensitive methylene blue dye and were subsequently exposed to a visible bright light source (LED-equipped light boxes) to trigger the generation of virucidal singlet oxygen. A possible factor restricting the widespread use of such photochemical decontamination is its reliance on the availability of electricity to power light sources. Here, we show that natural sunlight can be used in lieu of artificial light. We demonstrate efficient inactivation of a SARS-CoV-2 surrogate, porcine respiratory coronavirus, via 10 µM dye coating in conjunction with short outdoor exposures of 5–30 min (blue sky to cloudy day; mean 46,578 lx). A tenacious human norovirus surrogate, murine norovirus, is inactivated via methylene blue solar decontamination involving 100 µM dye concentrations and 30 min of high-illuminance sunlight (blue sky; mean 93,445 lx) or 2 h of mid- to low-illuminance (cloudy day; mean 28,558 lx). The protocol developed here thus solidifies the position of methylene blue solar decontamination as an important equitable tool in the package of practical pandemic preparedness.