Heating may be the best way to disinfect N95 masks for reuse, according to a study which found that doing so preserves their filtration efficiency for 50 cycles.
Since the outbreak of the COVID-19 pandemic, N95 face masks have been in short supply, said the researchers, including those from Stanford University in California, US.
Health care workers, in particular, desperately need these masks to protect themselves from the respiratory droplets of infected patients, they said.
Because of the shortage, many have to wear the same mask repeatedly, according to the researchers.
The study, published in the journal ACS Nano, tested several methods for disinfecting N95 materials, finding that heating them preserves their filtration efficiency for 50 cycles of disinfection.
The researchers noted that N95 masks contain a layer of "meltblown" polypropylene fibres that form a porous, breathable network.
To help capture smaller particles that could slip through the holes, the fibres are electrostatically charged, they said.
The US Centers for Disease Control and Prevention has recommended several methods for disinfecting N95 masks, such as heating, ultraviolet (UV) radiation and bleach treatment, the researchers said.
However, so far these methods have not been tested extensively, especially for multiple rounds of disinfection, they said.
The researchers wanted to compare five of the methods that could reasonably be used within a hospital setting to see how mask materials hold up to repeated disinfections.
While analysing N95 masks, they examined pieces of the meltblown fabric used to make these masks.
The researchers treated the material with a particular disinfectant and compared its ability to filter aerosol particles -- resembling respiratory droplets, but lacking coronavirus -- before and after disinfection.
The team found that spraying the fabric with an ethanol or chlorine bleach solution drastically reduced the filtration efficiency after only one treatment, from about 96 per cent to 56 per cent (ethanol) or 73 per cent (bleach).
A single steam treatment maintained filtration, but five steam treatments led to a sharp decline in efficiency, the researchers said.
UV radiation allowed up to 20 cycles of disinfection, however, administering the exact dose of UV that kills the virus without damaging mask materials could be problematic, they said.
The researchers said best disinfection method appeared to be heating.
For example, heating at 85 degrees Celsius for 20 minutes allowed the fabric to be treated 50 times without loss of filtration efficiency, they said.
However, frequently donning and removing N95 masks could affect fit, which also impacts performance, the researchers said.
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