Dr. Peter Tsai, the INVENTOR of the filtration fabric in the N95 mask. N95 masks are made of polypropylene material and are designed to tightly fit over your face with little leakage around the edge of the mask.
When reusing N95 masks, leave a used respirator in dry, atmosphere air for 3-4 days to dry it out. Polypropylene in N95 masks is hydrophobic and contains zero moisture. COVID-19 needs a host to survive–it can survive on a metal surface for up to 48 hours, on plastic for 72 hours, and on cardboard for 24 hours. When the respirator is dry in 3-4 days, the virus will not have survived.
Take four N95 masks, and number them (#1-4).
On day 1, use mask #1, then let it dry it out for 3-4 days.
On day 2, use mask #2, then let it dry out for 3-4 days.
Same for day 3, and day 4…
You can also sterilize the N95 mask by hanging it in the oven (without contacting metal) at 70C (158F) for 30 minutes-it is reported that COVID-19 cannot survive at 65C (149F) for 30 minutes.
Use a wood clip to hang the respirator in the kitchen oven to do the sterilization.
Experimentally though this time is much less when incubating virus in transport media
With the incubation temperature increased to 70°C, the time for virus inactivation was reduced to 5 mins.
When sterilizing N95 masks, be wary of using UV light–keep N95 masks away from UV light / sunlight. N95 masks are degraded by UV light because it damages the electrostatic charges in the polypropylene material. It is unclear how long the masks can be exposed to UV light before they are ineffective.
Read more details at the link
We do have more data now on UV and what levels are safe to use
Ultraviolet germicidal irradiation
UVGI disinfection involves placing respirators in direct line with UV-C lamps. Any exposed surface of the mask will be disinfected. This process generally requires the masks to be flipped after some period to ensure both sides of a mask are disinfected or requires a setup involving two UV-C lamps on either side of the respirator.
For samples of FFR filters treated with SARS (severe acute respiratory syndrome) and MERS (Middle East respiratory syndrome) viruses, no viable virus was found following a dose of 1 joule per square centimeter (J/cm2) of short wavelength (254 nanometers) UV-C light, even in the presence of artificial skin oil and saliva.11 Researchers found no impact of doses ranging from 3 to 20 J/cm2 on filter performance.9-12
A similar dose range did not compromise fit on three N95 FFRs worn by 10 volunteers.12 Doses ranging from 3 to 6.5 J/cm2 did not alter fit performance on human subjects; doses up to 20 J/cm2 did not significantly change fit performance on respirators mounted on a static head form connected to a breathing machine.11
Lindsley et al13 tested the impact of much higher doses, ranging from 120 to 950 J/cm2, on respirator filter samples and 590 to 2,360 J/cm2 on respirator straps. For most samples, filter efficiency decreased after UVGI treatment but remained above 95% at all dose levels for the four tested respirator models. Strength of the different layers decreased at all doses, with more layers showing decreased strength with increasing dose. The breaking strength of straps also decreased at all doses, ranging from a loss of 10% to 21% at the lowest dose of 590 J/cm2 and 20% to 51% at the highest dose of 2,360 J/cm2. Respirators treated with a single decontamination of UVGI showed no significant difference from untreated respirators in odor, donning ease, comfort, and respirator fit.9
Altogether, these data suggest that respirators could be decontaminated by UVGI for up to 20 cycles at a dose of 1 J/cm2 per cycle, if the fit performance data from Heimbuch and Harnish11 using a static head form are representative of fit on humans. Limited human data on a small number of N95 FFR models suggest that 3 cycles at this dose will not degrade fit.
While respirator filter performance (and perhaps fit) were not adversely affected at the higher doses studied by Lindsley et al,13 because the lowest dose level of 120 J/cm2 caused a loss of strength of at least one filter layer, extending UVGI treatment beyond 20 cycles does not seem appropriate at this time.