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Decontaminating Covid-19 Masks Raises Safety, Efficacy Concerns

July 9, 2020, 6:10 PM

As coronavirus infections rise and N95 masks to protect health-care workers remain in short supply at least until the fall, many are decontaminating and reusing the masks that are designed to be used once.

The use of decontaminated masks has raised safety concerns among nurses, but mask manufacturers such as 3M Co., and government agencies, such as the Occupational Safety and Health Administration, say the sterilized masks are safe, although they’d prefer new ones to be worn.

“We’ve seen a rush to embrace these decontamination processes,” said Jane Thomason, lead industrial hygienist for National Nurses United. “They’ve not been studied. They’ve not been fully evaluated.”

Thomason said that rather than allowing decontaminated masks, the government should have pushed hospitals to use other types of proven masks, such as the half- and full-face masks designed for multiple uses.

The White House Covid-19 Task Force briefed the Senate in June that about 165 million N95 masks are needed each month for health-care workers and others whose jobs could lead to coronavirus exposure, such as prison guards. Domestic production and imports will make about 135 million new masks available in July, the task force said. By winter, the White House estimated, domestic production will reach 180 million a month.

Test, Verify

Richard Peltier, an aerosol chemist and associate professor at the University of Massachusetts-Amherst, is researching the effectiveness of different decontamination methods and how many cycles a mask can go through before its filtering abilities drop below the N95 respirator’s required threshold. The research is funded by the National Science Foundation.

To meet the government’s N95 designation, a mask must filter out at least 95% of particles that are 300 nanometers or larger—enough to trap a single coronavirus.

Peltier said he first started looking at whether a decontaminated N95 mask continued to be effective at the request of a local hospital that was considering exposing masks to a gas—vaporized hydrogen peroxide—a process often used to sterilize medical equipment.

Using a device that measures whether an N95 mask is filtering out the microscopic particles, Peltier said he confirmed the mask was still functioning after one decontamination cycle.

That led to the NSF project where Peltier evaluated masks that had gone through the vaporized hydrogen peroxide process or several other decontamination methods through up to 10 cycles.

The study is undergoing peer review, Peltier said, and he can’t yet discuss his conclusions. However, he did say some sterilization methods did lower masks’ effectiveness below the N95 threshold.

Companies manufacturing N95 respirators and performing the decontamination work also have been looking at the effectiveness of N95 masks after multiple sterilizations.

Doesn’t Fit, Don’t Wear

Dr. Nikki McCullough, a health and occupational safety leader in 3M’s Personal Safety Division, said 3M has evaluated some of its masks after decontamination to determine through how many cycles a mask stays effective.

3M has approved masks going through up to 20 cycles. The limits are updated regularly in a bulletin 3M publishes.

McCullough cautioned that other factors must be considered on whether a mask is still protective. “The number of cycles that a particular respirator will withstand will depend on how many times it has been donned, stored and duration and conditions of use,” she said.

If a user can’t get the mask to form a proper seal on the user’s face, the mask shouldn’t be worn, McCullough said.

The Centers for Disease Control and Prevention published guidance for the maximum number of times N95 masks can be sterilized using a wide range of methods from steam heat generated inside a microwave oven to exposing the masks to ultraviolet light.

1.2 Million and Growing

The largest decontamination effort is overseen by Battelle Memorial Institute, an Ohio nonprofit company.

Will Richter, a microbiologist and principal research scientist with Battelle, said the company has decontaminated about 1.2 million N95 masks at centers across the nation.

In April, Battelle was awarded a contract from the Defense Department, worth up to $415 million, to open and operate 60 decontamination centers that would sterilize respirators sent by health-care providers.

Richter said Battelle found that 20 different models of N95 mask, and 10 foreign-made masks, remain effective through 20 cycles. The company has also cooperated with 3M and the CDC’s National Institute for Occupational Safety and Health to evaluate the Battelle system.

Battelle’s systems use vaporized hydrogen peroxide. Each system can sterilize about 5,000 masks at a time in a process that takes about 24 hours, Richter said.

While the CDC has approved Battelle’s methods for up to 20 sterilization cycles, Richter said, the company has its own quality controls. Masks that are stained, often from makeup, or otherwise appear soiled, aren’t processed. Also pulled out are masks with apparent material flaws such as broken straps.

After a batch of masks has been decontaminated and “air washed” to allow residual gas to dissipate, workers use gas detectors to determine if the amount of hydrogen peroxide gas is below the OSHA permissible exposure limit, Richter said. The OSHA limit is 1.0 parts per million, but Battelle enforces a stricter limit, 0.8 parts per million.

Of the 1.2 million masks Battelle has processed and returned, the company has received complaints about fewer than 0.5% of the masks, Richter said.

Smell Test

One complaint from users has been about the smell of the sterilized masks.

“The first thing that we noticed when we opened the packaging was that they had ... a very strong chemical smell,” said Amy Arlund, an intensive care unit nurse with Kaiser Permanente in Fresno, Calif.

The nurses stopped using the decontaminated masks when the hospital was able to provide new masks, Arlund said.

Richter said Battelle isn’t sure what’s causing the odor complaints. It isn’t the vaporized hydrogen peroxide because the allowable residual gas amount is more than 100 times below what the detectable smell level is, Richter said. It could be smells from the packaging or minute amounts of other chemicals in levels far below OSHA limits.

Richter said anyone not satisfied with the condition of a decontaminated mask shouldn’t wear it.

“If they think it doesn’t fit or has a noxious odor, get rid of it and grab the next one,” he said.

To contact the reporter on this story: Bruce Rolfsen in Washington at

To contact the editors responsible for this story: Karl Hardy at; Martha Mueller Neff at