The US Food and Drug Administration on Sunday changed its policy on decontamination for some N95 masks mainly used by health professionals, saying certain masks made in China should not be reused.

Shortages of N95 masks in the midst of the coronavirus pandemic have prompted loosening of some rules by the FDA in the form of emergency use authorizations. The masks, which are intended for use by health care workers and front-line responders, can filter out viruses, unlike cloth and surgical masks, which the public is encouraged to use to limit the spread of larger droplets that can carry the novel coronavirus.

As concerns arose about shortages across the United States, the agency allowed the use of masks that were being used in other countries but had not been approved by the National Institute for Occupational Safety and Health, and it also allowed reuse of N95 masks after decontamination.

Now the agency says that certain masks made in China and not approved by the agency are still OK for emergency use but should not be reused. The list of masks that are authorized but should not be reused includes a number of models from 3M that are manufactured in China.

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For health care workers, the need for masks, and the ones that actually stop viruses, has never been in dispute. But public mask-wearing was controversial even before it became politicized. At first, health officials were doubtful of the value of simple masks in protecting the user. But over time, they have agreed that widespread mask-wearing reduces the spread of the coronavirus. The World Health Organization waited until Friday to endorse the widespread use of face masks by the public.

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In an announcement, the FDA said that testing by the National Institute for Occupational Safety and Health, part of the Centers for Disease Control and Prevention, had shown that some respirators manufactured in China might “vary in their design and performance.”

The agency has been reconsidering and revising decisions on masks as it gains new information, the announcement said.

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