The news recently broke that the retired Taiwanese engineer and professor, although resident in the USA, Peter Tsai, faced with the expansion of covid-19, chose to leave his retirement, which he had been enjoying for two years, to set up a laboratory in his home in order to study options for recycling the much desired and scarce KN95 masks, which he and his research team invented and developed at Tennessee University in 1992.
Thus, after several tests involving long exposure to the sun, among others, he finally came to the conclusion that the most valid way to treat a KN95 mask for reuse is by heating it to 70 degrees Celsius for one hour using dry heat technology.
He made these conclusions public on this link:

In order to help stop the spread of the coronavirus.
Tsai has stated that: “I would rather live the rest of my years in the certainty that I have helped save lives than become rich. This is an example to follow.