“I feel I’m not poisoned to death by the smog, but am choked to death by the smell of the mask,” another person wrote on the Weibo microblogging platform. “Is it fake?”

The report said that the masks seized in Shanghai — which bore the logo of the American company 3M, the manufacturer of one of the most popular models of masks in China — were made with inferior materials and that they would offer no protection from air pollution. In fact, the report said the masks would actually pose an added threat to the health of those who wore them, though it did not explain how.

“The greatest risk from a fake mask is that people will be outside feeling protected but actually breathing in much more pollution than they’re aware of,” said Dr. Richard Saint Cyr, a physician in Beijing whose health blog is popular among expatriates.

He said he believed 3M masks — real ones, that is — were indeed beneficial. “There’s a lot of published research which shows that a well-designed mask can greatly decrease your inhaled intake of PM 2.5,” he said, referring to a particularly dangerous category of pollutant.