Long after most nations urged their citizens to wear masks, and after months of hand-wringing about the quality of the evidence available, the World Health Organization today endorsed the use of face masks by the public to reduce transmission of the coronavirus.

Since the beginning of the pandemic, surprisingly, the WHO had refused to endorse masks. The announcement was long overdue, critics said, as masks are an easy and inexpensive preventive measure.

Even in its latest guidance, the WHO made its reluctance abundantly clear, saying the usefulness of face masks is “not yet supported by high quality or direct scientific evidence” but that governments should encourage mask wearing because of “a growing compendium of observational evidence.”

The WHO also provided an exhaustive list of the potential disadvantages of wearing a mask, including “difficulty with communicating clearly and “potential discomfort.”

A study funded by the WHO concluded this week that respirator masks, like the N95, are better than surgical masks for health care workers. It also found that face shields, goggles and glasses may offer additional protection from the coronavirus.

But the WHO did not budge from its previous recommendations for medical workers, saying that respirator masks are only needed if such workers are involved in procedures that generate virus-laden aerosols — droplets smaller than 5 microns.

Apart from those circumstances, transmission of the virus so far has only been demonstrated for larger droplets and by contact, said Dr. Benedetta Allegranzi, an infectious disease expert and technical lead for the WHO.

While studies have shown that viral RNA is present in the air in some health care settings, “transmission is different, and it has not been demonstrated,” she said.

“It is disappointing that the WHO is dismissing that latest evidence that N95s are far more effective than surgical masks in protecting health care workers from COVID-19 exposure,” said David Michaels, an epidemiologist at George Washington University who headed the Occupational Safety and Health Administration during President Barack Obama’s administration.

“If the problem is the shortage of N95s, the WHO should acknowledge that and not pretend that medical masks are equally effective.”

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