The outbreak of the coronavirus led to a surge in interest in how to disinfect masks, as well as Google searches for “wash N95 mask” and “clean N95 mask.”

Yet answers to this question are in short supply. The standard guidelines say to throw masks out and use new ones.

But the standard guidelines are meaningless for people facing a global pandemic and widespread mask shortages.

Washing Masks Reduces Effectiveness

The simplest solution is to wash masks with soap and water. Yet test data shows washing masks reduces their filtration effectiveness.

Alcohol Disinfection Reduces Effectiveness

Maybe alcohol disinfection is better because it doesn’t require rubbing the mask? Unfortunately, tests found that dipping in rubbing alcohol reduced particle capture by 37%.

Thus, we’re in need of a data-backed way to disinfect masks without harming performance.

A Reliable Way to Disinfect N95 Masks

In times of panic, we often want to take action, yet one of the simplest ways to disinfect masks is to do nothing! Simply allowing masks to dry will kill viruses.

But the critical question healthcare workers need to know is how long we need to dry masks before they’re safe. In other words, how long do viruses remain viable? The question of viability is particularly important, because many studies measure whether viruses are “detectable.”

Viruses can remain detectable for scarily long periods of time, which has lead to incredible-sounding headlines about how long viruses last. But “detectable” is very different from “viable.” Viable is more important for mask users because “viable” means it’s still able to infect someone.

How Long Does It Take to Disinfect Masks?

Researchers at the University of Hong Kong tested how long viruses retain their ability to infect people on different types of surfaces. They sprayed H1N1 flu virus on six common household materials and tested viability up to 24 hours.

Unfortunately, they didn’t test masks, but the closest material they tested was J-cloth. This J-cloth material is somewhat similar to the electrostatic material used in masks.

The tests found that the virus was completely non-viable in under 5 hours.

The virus remained viable the longest on hard, non-porous surfaces—stainless steel and plastic. However, even for the worst surface, stainless steel, the flu virus was non-viable in under 10 hours. The researchers concluded that the H1N1 is “likely to survive up to a few hours.”

Thus, simply leaving a mask out to dry should kill the virus. This eliminates the need to degrade the mask with water or alcohol.

Will Drying Masks Work for Covid19?

The study above is for H1N1 (influenza A), not the coronavirus. The researchers said results should be similar for other influenza A viruses, but they stopped short of saying they’re valid for all flu viruses.

There’s little data on how long the Covid19 coronavirus remains viable. The CDC says that the coronavirus may remain viable for hours to even days. A recent study (that has not been formally published yet) tested the viability of the new coronavirus (HCoV-19) and the “old” coronavirus, SARS (SARS-CoV-1). They sprayed the two viruses on plastic, stainless steel, copper, and cardboard.

They did not test cloth, but the closest material to a mask was cardboard. It took 48 hours for the coronavirus to be completely non-viable on cardboard. On stainless steel, a tiny amount of viable coronavirus remained at 72 hours, That’s 7 times longer than the earlier study found for H1N1.

Based on these two studies, leaving a mask to dry for 48 hours should be enough to kill coronavirus, SARS, and influenza A viruses. Leaving masks for 72 hours would be even more conservative.

However, it is important to remember that virus survival time is not a universal constant like the speed of light. Several factors will influence how long viruses survive:

Viruses survive longer at lower temperatures. For example, one study found that viruses died faster at 20C than 4C, and faster still at 40C.

Studies have found that viruses survive much longer on wet materials than dry materials.

Drying out a mask is more likely to kill the virus. One US hospital is telling its nurses to keep their masks in plastic bags, yet this would prevent the mask from drying and thus allow the virus to live longer.

Bottom Line: How to Disinfect N95 Masks Leaving masks out to dry should disinfect them from the coronavirus, SARS, and influenza A in under 48 hours. 72 hours would be even more conservative.