FACE MASKS of all kinds have become iconic in the pandemic. At first, people were told not to wear them so as to save the meager supply for health-care professionals. Now facial coverings are more widely available, and evidence of their usefulness is growing. Masks are a critical roadblock to spreading the virus in public places. If candidates are going to stump in this political season, especially indoors, they should implore their audiences to wear them.

A study published in the Proceedings of the National Academy of Sciences finds that face masks are “the most effective means to prevent interhuman transmission,” an inexpensive bulwark that, when combined with physical distancing, quarantine and contact tracing, is “the most likely fighting opportunity to stop the COVID-19 pandemic” absent an effective vaccine or drug therapy. The study, by Renyi Zhang of Texas A&M University, and colleagues, looked at outbreaks of the coronavirus in Wuhan, China; in northern Italy; and in New York from Jan. 23 to May 9. They concluded that “the difference with and without mandated face covering represents the determinant in shaping the pandemic trends in the three epicenters.” Face masks alone “significantly reduced the number of infections . . . by over 78,000 in Italy from April 6 to May 9 and over 66,000 in New York City from April 17 to May 9.” By studying pandemic trends, they concluded that the other measures — distancing, isolation and contact tracing — must be accompanied by face masks to really make a difference.

The researchers say a big reason the face masks are important is that aerosols containing virus particles, not surface contacts, are “the dominant route for the transmission” of this disease. “Even with normal nasal breathing,” they report, “inhalation of virus-bearing aerosols results in deep and continuous deposition into the human respiratory tract, and this transmission route typically requires a low dose.” The research didn’t delve into different kinds of masks — N95 respirators, surgical masks and cloth coverings — and more research is needed about how effectively they protect against the virus.

Last week, the Centers for Disease Control and Prevention urged organizers of large gatherings that involve shouting, chanting or singing to use cloth face coverings to reduce the risks. The World Health Organization, which had earlier been reluctant to recommend masks in community settings, on June 5 shifted and issued a stronger recommendation for their use. The example of Japan, where wearing them was common practice long before the pandemic, suggests they can be effective at reducing virus spread and death.

Public messaging matters. Unfortunately, President Trump has retweeted messages that mock the use of face masks, and he has studiously avoided wearing one in public. This signal is no doubt influencing millions of people to assume that reopening the economy means they can return to crowded bars and massive campaign rallies without face masks. They are wrong. If he does not insist on face masks as a condition for attending his upcoming Oklahoma campaign rally, Mr. Trump is condemning to illness many of his supporters, and uninvolved bystanders as well.

The face mask grates at human nature, at the desire to breathe freely and see another person’s expressions. But it is a vital line of defense.

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Nearly 6 in 10 Americans who are working outside their homes are concerned that they could be exposed to the virus at work and infect their families. (The Washington Post)