Worn N95 masks can only be cleaned and reused once before unsafe: new Winnipeg-based study

Nearly two-thirds of worn N95 respirator masks can be reused after sterilization — but they'll only fit properly after one round of cleaning, a new Winnipeg-based study has found.

Previous research into cleaning used fresh masks; St. Boniface study showed effect on worn masks

Previous research suggested masks could be used up to 10 times after sterilization, although those studies didn't factor in the wear and tear of real-world use. (CBC/Radio-Canada)

Nearly two-thirds of worn N95 respirator masks can be reused after sterilization — but they'll only fit properly after one round of cleaning, a new Winnipeg-based study has found.

Researchers and makers of personal protective equipment have already studied ways to sterilize disposable masks and determine what their lifespans can be depending on the cleaning process.

But all of that research was conducted using fresh masks, according to a study published July 31 by the St. Boniface Hospital Albrechtsen Research Centre in Winnipeg.

"No study has yet been done with [N95 respirators] that have actually been worn by workers in clinical environments and sterilized by autoclaving [a sterilization process that combines pressure and extreme heat]," said the study, titled N95 Mask Reuse in a Major Urban Hospital – COVID-19 Response Process and Procedure.

"Our report describes both the sterilization of masks and the logistical plan for collection and deployment. The approaches employed will provide useful guidance for health-care agencies facing an influx of COVID-19 patients and [N95] shortages."

The novel coronavirus that causes COVID-19 is mainly spread through air droplets. Those droplets can be transmitted in a number of ways, including when a health-care worker hooks a patient up to a ventilator — a frequent occurrence in COVID-19 epicentres.

Health-care workers wear N95 respirators because their filters allow them to breathe safely while working with COVID-19 patients.

But the pandemic has created a PPE supply shortage and health-care workers, including those in Manitoba, have had to extend the use of items such as N95 respirators that were only designed to be used once then thrown out.

"The rapid increase in infected patients and global reach of the [COVID-19] outbreak has strained supply chains for not only equipment such as ventilators, but also disposable PPE used in the management of these patients, particularly single-use N95 respirator masks," the study said.

"Assessing the potential reuse of [N95 respirator masks] is thus a rapidly emerging critical issue."

Researchers at St. Boniface Hospital wanted to see how worn masks would hold up after being sterilized via autoclaving — something available in most health-care facilities, regardless of size, that can sterilize N95s while maintaining their filters and having minimal impact on a respirator's structural integrity, the study explained.

To do that, lab workers in the hospital's animal care centre, who worked between two and eight hours in a day, could take off their N95 respirator at a designated area and put it in a resealable bag.

Masks from lab workers who worked between two and eight hours in a day, and weren't damaged or otherwise contaminated, were eligible for the study. (Justin Fraser/CBC)

Masks that were visibly soiled, exposed to harsh chemicals, used while treating patients with other illnesses, or damaged or torn were not accepted for the study.

The masks were then placed into a dryer for a first round of cleaning, prior to autoclaving, to lessen the possibility of workers being exposed to the virus while loading the autoclave.

Once in the autoclave, though, masks were subject to 121-degree C heat for 30 minutes, plus another 15 minutes for drying.

Afterward, masks were fit tested to ensure they were still safe to use, marked with a permanent marker to indicate they had been sterilized, then put in a central storage area where staff could pick them up.

A dozen autoclave loads were studied, and the number of masks in each load ranged from 24 to 295.

The percentage of masks that were safe for reuse after sterilization ranged from nearly 49 per cent to almost 80 per cent, the study said.

This photo shows a scientist using an autoclave, which sterilizes materials at health-care facilities by using heat and pressure. ((Public Health Agency of Canada))

"We found that only a single round of sterilization of [N95 respirators] is possible without fit testing failures, in sharp contrast to studies with unused masks or N95 swatches indicating up to 10 uses," the study said.

"Thus, excluding testing of masks actually worn under working conditions gives the illusion of mask capacity for reuse without actually providing protection."

Staff who were interviewed later said there were no issues breathing normally while wearing the sterilized N95, according to the study. But some noted that the respirator smelled slightly different, its straps were less stretchy, and the mask material felt softer.

After a second round of autoclaving, the N95 respirators started failing fit testing and wouldn't be safe for health-care workers to wear, the study said.

While the study's findings are significant, there are several factors in its methodology that suggest the findings may need to be taken with slight skepticism.

In order for masks to be eligible for the study, they had to be AO Safety Pleat Plus — a make of N95 that is most used at St. Boniface Hospital — and they could not have makeup stains on them.

Researchers did not check whether autoclaving eliminated germs carrying the novel coronavirus from the respirator.

Also, while the masks were worn, they were not worn by people facing the same conditions that front-line health-care workers are facing while battling COVID-19.

The study's findings suggest that the material of N95s themselves could likely tolerate further rounds of autoclaving, but "real world use severely reduces this number," it said.


Nicholas Frew is a CBC Saskatchewan reporter based in Regina, who specializes in producing data-driven stories. Hailing from Newfoundland and Labrador, Frew moved to Halifax to attend journalism school. He has previously worked for CBC newsrooms in Manitoba and Alberta. Before joining CBC, he interned at the Winnipeg Free Press. You can reach him at


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