More on Medical Devices

Faced with shortages, researchers combine heat and humidity to disinfect N95 masks

Because the process is so simple, it may be possible to automate so hospitals can stretch existing supplies.

Jeff Lagasse, Associate Editor

As the COVID-19 pandemic swept around the world earlier this year, shortages of protective equipment such as N95 masks left healthcare workers little choice but to reuse the masks they had – increasing the risk of infection for both them and their patients.

Now, researchers at the Department of Energy's SLAC National Accelerator Laboratory, Stanford University and the University of Texas Medical Branch may have a solution: Using a combination of moderate heat and high relative humidity, the team was able to disinfect N95 mask materials without hampering their ability to filter out viruses.

What's more, it should not be too difficult to turn the new results into an automated system hospitals could use in short order. Because the process is so simple, it might take just a few months to design and test a device. That would give hospital staff the ability to have their own personal collection of up to a dozen masks, which could be decontaminated over the course of a coffee break.

The team reported its results September 25th in the journal ACS Nano.


Hospitals and health systems are still dealing with N95 mask shortages, and a September 21 Washington Post report has recounted efforts among provider organizations to petition the federal government for more help. The Post reported on survey results showing 68% of nurses were still required to reuse respirators, and one nurse in Texas said she's still wearing the same five N95 masks she was given in March.

Facing a shortage of the masks, researchers considered a number of ways to disinfect them for reuse, including ultraviolet light, hydrogen peroxide vapors, autoclaves and chemical disinfectants. The problem is that many of those methods degrade N95 masks' filtering abilities, so at most they could be reused a few times.

In the new study, University of Texas Medical Branch virologist Scott Weaver and Stanford/SLAC professors Yi Cui and Wah Chiu and colleagues focused their attention on a combination of heat and humidity to try to decontaminate masks.

Working at the World Reference Center for Emerging Viruses and Arboviruses, which has biosafety measures in place for working with the most contagious viruses, the team first mixed up batches of the SARS-CoV-2 virus in liquids designed to mimic the fluids that might spray out of our mouths when we cough, sneeze, sing or simply breathe. They next sprayed droplets of the brew on a piece of meltblown fabric, a material used in most N95 masks, and let it dry.

Finally, they heated their samples at temperatures ranging from 25 to 95 degrees Celsius for up to 30 minutes with relative humidity up to 100%.

Higher humidity and heat substantially reduced the amount of virus the team could detect on the mask, although they had to be careful not to go too hot, which additional tests revealed could lower the material's ability to filter out virus-carrying droplets. The sweet spot appeared to be 85 degrees Celsius with 100% relative humidity. The team could find no trace of SARS-CoV-2 after cooking the masks under those conditions.

Additional results indicate masks could be decontaminated and reused upwards of 20 times, and that the process works on at least two other viruses: a human coronavirus that causes the common cold and the chikungunya virus.

Although the results are not especially surprising – researchers have known for a long time that heat and humidity are good ways to inactivate viruses – there hadn't been an urgent need for a detailed quantitative analysis of something like mask decontamination until now. The new data, authors said, "provide(s) some quantitative guidance for the future."

And even after the coronavirus pandemic is over, there are likely benefits, in part because of the method's application beyond SARS-CoV-2 to other viruses, and because of the economic and environmental benefits of reusing masks.


Data published in August in JAMA Internal Medicine shows that reused, sterilized N95 masks and very out-of-date N95 masks retain their effectiveness at protecting healthcare workers from COVID-19 infection. They provide greater than 95% effectiveness at keeping the wearer from inhaling very small airborne particles that may carry SARS-CoV-2, the virus that can cause COVID-19. These masks also retain such effectiveness many years beyond their expiration dates.

Meanwhile, in July, a Premier survey showed nearly 90% of healthcare providers are contributing to stockpiles of critical medical supplies and drugs intended to last as long as 90 days. But as new COVID-19 hotspots emerge, the survey showed local efforts to build stockpiles must be supported with a national strategy to avoid redirecting supplies away from front-line caregivers, and exacerbating ongoing product shortages. Either the health system or the state is directing the majority of stockpiling efforts, the survey found, although product backorders are inhibiting requests to replenish stockpiles and provide timely care.

The products providers cited as heavily backordered include N95 masks and bouffant caps (both cited by 53% of respondents); isolation gowns and shoe covers (49%); testing swabs and test kits (40%); surgical gowns (35%); exam gloves (32%); surgical masks (30%); and syringes (7%).

Twitter: @JELagasse
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