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Exposing contaminated N95 respirators to vaporized hydrogen peroxide (VHP) or ultraviolet (UV) light appears to eliminate the SARS-CoV-2 virus from the material and preserve the integrity of the masks' fit for up to three uses, a National Institutes of Health (NIH) study shows.

Dry heat (70° C) was also found to eliminate the virus on masks but was effective for two uses instead of three.

Robert Fischer, PhD, with the National Institute of Allergy and Infectious Diseases in Hamilton, Montana, and colleagues posted the findings on a preprint server on April 15. The paper has not yet been peer reviewed.

Four Methods Tested

Fischer and colleagues compared four methods for decontaminating the masks, which are designed for one-time use: UV radiation (260 – 285 nm); 70° C dry heat; 70% ethanol spray; and VHP.

For each method, the researchers compared the rate at which SARS-CoV-2 is inactivated on N95 filter fabric to that on stainless steel.

All four methods eliminated detectable SARS-CoV-2 virus from the fabric test samples, though the time needed for decontamination varied. VHP was the quickest, requiring 10 minutes. Dry heat and UV light each required approximately 60 minutes. Ethanol required an intermediate amount of time.

To test durability over three uses, the researchers treated intact, clean masks with the same decontamination method and assessed function via quantitative fit testing.

Volunteers from the Rocky Mountain laboratory wore the masks for 2 hours to test fit and seal.

The researchers found that masks that had been decontaminated with ethanol spray did not function effectively after decontamination, and they did not recommend use of that method.

By contrast, masks decontaminated with UV and VHP could be used up to three times and function properly. Masks decontaminated with dry heat could be used two times before function declined.

"Our results indicate that N95 respirators can be decontaminated and re-used in times of shortage for up to three times for UV and [VPH], and up to two times for dry heat," the authors write. "However, utmost care should be given to ensure the proper functioning of the N95 respirator after each decontamination using readily available qualitative fit testing tools and to ensure that treatments are carried out for sufficient time to achieve desired risk-reduction."

Reassurance for Clinicians

The results will reassure clinicians, many of whom are already using these decontamination methods, Ravina Kullar, PharmD, MPH, an infectious disease expert with the Infectious Diseases Society of America, told Medscape Medical News.

Kullar, who is also an adjunct faculty member at the David Geffen School of Medicine of the University of California, Los Angeles, said the most widely used methods have been UV light and VPH.

UV light has been used for years to decontaminate rooms, she said. She also said that so far, supplies of hydrogen peroxide are adequate.

A shortcoming of the study, Kullar said, is that it tested the masks for only 2 hours, whereas in clinical practice, they are being worn for much longer periods.

After the study is peer reviewed, the Centers for Disease Control and Prevention (CDC) may update its recommendations, she said.

So far, she noted, the CDC has not approved any method for decontaminating masks, "but it has said that it does not object to using these sterilizers, disinfectants, devices, and air purifiers for effectively killing this virus."

Safe, multiple use of the masks is critical in the COVID-19 crisis, she said.

"We have to look at other mechanisms to keep these N95 respirators in use when there's such a shortage," she said.

Integrity of the fit was an important factor in the study.

"All healthcare workers have to go through a fitting to have that mask fitted appropriately. That's why these N95s are only approved for healthcare professionals, not the lay public," she said.

The study was supported by the National Institutes of Health; the Defense Advanced Research Projects Agency; the University of California, Los Angeles; the US National Science Foundation; and the US Department of Defense.

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