While masks are in short supply during the coronavirus pandemic, the shortage is prompting people to ask if UV light disinfects masks. This is a critical question for frontline healthcare workers, as they scramble for ways to safely re-use masks. 

Here’s the problem. We know that UV-C light kills bacteria and viruses, but we also know that UV-C light degrades fibers. That means it’s crucial for healthcare workers to know whether UV light reduces the effectiveness of masks. 

Putting It to The Test: Does UV Light Harm Mask Performance?

Researchers got hard data on this question by testing four N95 masks before and after UV sterilization.

They tested four models of N95 masks: the 3M 1860, 3M 9210, Gerson 1730, and Kimberly-Clark 46727.

They exposed masks to five different doses of UV light (also called “UVGI”) from 120 to 950 J/cm².

UV Light Disinfects Masks, But at What Dose?

These are high doses. Even the lowest dose is 66 times higher than a dose that researchers found is sufficient to deactivate 99.99% of viruses on masks (1.8 J/cm²).

To test how well the masks could capture particles, they shot tiny pieces of salt from 0.02 to 0.4 microns at the masks and measured what percentage penetrated the masks. This range of particle size covers the size of flu, ebola, and coronavirus particle sizes.

Related: Are UV light air purifiers effective at disinfecting viruses? →

They tested 10 different sizes of particles and used the data for whichever sized particle penetrated the mask the most.

UV Light Harms Mask Effectiveness Less Than 2%

The data showed almost no harm to performance. For most masks and most dosages, the degradation in particle capture was less than 1%.

In only three tests was the harm above 1%. And even in the worst case, the UV light caused degradation of under 2%.

UV Light Sterilization Harms Straps More Than Filtration

However, capturing particles isn’t the only things mask have to do. They also have to keep their shape and fit to the face. Thus, the researchers also tested how the UV affected the structural integrity of the masks.

One test they did was to pull the straps as hard as they could until they broke. The tests found that, at higher and higher doses of UV light, less and less force was needed to break the mask straps.

Besides the effect of UV light, the data also showed that masks with rubber band straps (3M 9210) broke far more easily than masks with elastic straps (3M 1860). This strength is also important for people who need to re-use masks. Masks with fabric straps will last longer on average than masks with rubber band straps.

A Simple Way to Tell If Mask Needs to Be Disposed

If healthcare workers are going to use UV light to disinfect masks from COVID-19, they’re going to have to solve another question: how many times can I disinfect the mask? One dose–even a strong dose–had only a small effect on performance, but surely repeated doses will have a cumulative effect.

The researchers suggested the answer lies in integrity. They found that, besides the straps, the UV light “dramatically affected” the structural body of the masks. The researchers suggested that (at least for some masks) masks will fall apart before they lose their ability to filter particles.

This is important because healthcare workers don’t have laser particle counters to continually test how well their masks are performing. So how do they know when it’s time to throw them out?

The scientists suggested people can use the structural degradation as a signal. Put simply, when the mask starts to fall apart, that’s a sign that they should throw the mask away while knowing that filtration performance has likely not dropped severely.

Better Options Than Disinfecting Masks With UV Light

This finding is similar to results of washing masks with soap and water, which also harms performance. Instead, research shows that  simply drying and waiting as better options for disinfecting masks.

Read more: This proven disinfection method deactivates viruses with zero harm to mask performance.