Open this photo in gallery The emergency department at St. Michael's Hospital, located at the corner of Shuter St. and Victoria St. in downtown Toronto, is photographed on Oct. 20, 2016. Fred Lum/The Globe and Mail

Nine Toronto-area hospitals have to scrap their policies forcing unvaccinated nurses to wear surgical masks, a labour arbitrator has ruled in a binding decision that concluded the hospitals couldn't provide scientific evidence to back up the practice.

The arguments that witnesses for the hospitals presented were “insufficient, inadequate, and completely unpersuasive,” arbitrator William Kaplan wrote in a 53-page decision last Thursday.

The decision focused on a grievance filed by the Ontario Nurses' Association against St. Michael's Hospital.

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However it is also binding on eight other institutions with the same policy: Sunnybrook Health Sciences Centre, Mount Sinai Health System, St. Joseph’s Health Centre, Michael Garron Hospital, Women’s College Hospital, North York General Hospital, Baycrest Centre for Geriatric Care and the Centre for Addiction and Mental Health.

St. Michael's already has a policy requiring employees with infectious illnesses to stay home. Those with flu symptoms have to report them.

The hospital failed to prove that nurses who didn't have flu symptoms were a significant source of infection and therefore would be required to wear masks if they weren't inoculated, Mr. Kaplan said in his decision.

He noted that St. Michael’s own director of infection prevention and control, Dr. Matthew Muller, testified that “the likelihood of transmission is dramatically higher when you’re coughing or sneezing.”

Mr. Kaplan also heard an expert, Lisa Brosseau, a professor of environmental and occupational health sciences, testify that surgical masks fit poorly and aren't an effective form of protection.

The arbitrator said it was illogical to force nurses who aren't immunized to be masked when St. Michael’s isn't as strict with unvaccinated visitors.

"If masking is truly effective as source control, how can it be that they too are not required to mask? The answer to this question reveals that the masking part of the policy is, as one St. Michael’s witness admitted, 'weak,' ” Mr. Kaplan wrote.

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In a statement, St. Michael's said it was disappointed with the ruling and underlined that Mr. Kaplan didn't find the policy had a coercive aim.

"The policy was put in place to protect everyone during flu season and give staff and clinicians the choice to either be vaccinated or to wear a mask," St. Michael's said, adding that it would now focus on promoting vaccination for all its staff.

The Ontario Hospital Association was also displeased. During flu season, hospitals are congested places with vulnerable patients, the OHA said in a statement. "Hospital leaders enacted this policy in good faith to protect these populations from what could be a potentially fatal illness."

It was the second time in three years that an arbitrator ruled against mask requirements for unvaccinated employees in Ontario hospitals. A 2015 decision against the Sault Area Hospital found that the requirement was in fact an undeclared way to force more nurses to get flu shots.

Following that earlier decision, several hospitals in Toronto continued to have mask policies. The nurses' union said the requirement singled out unvaccinated nurses and made patients think they were infectious.

A key part of the arguments presented by St. Michael’s rested on four scientific controlled trials showing that mortality in long-term care facilities dropped substantially when health-care workers were vaccinated. But Mr. Kaplan heard other experts question the reliability of those studies.

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Two of the hospital’s experts made arguments with no scientific support or misquoted citations, Mr. Kaplan said. “Everyone makes mistakes, but this went beyond the pale.”