Government guidelines now encourage wearing a mask in public to slow the spread of Covid-19, but that wasn’t always the case. | Rob Kim/Getty Images

America’s 180 on masks, explained.

While working in Beijing last year, I regularly bought packs of N95 masks at my local 7-Eleven. Across the city, masks were a common accessory: In packed subway cars and crowded bike lanes, commuters wore them for protection from the smog year-round.

When the coronavirus surfaced in China late in 2019, not only were masks on hand, they were also a familiar tool for fighting a virus: Many people had used them during the SARS outbreak in 2003. Based on this experience as well as government advice, masks quickly became even more widely used, not only in China, but in other East Asian countries.

In contrast, the US, which had little experience