Cases have become more common since the coronavirus outbreak began, some experts say. In one Canadian survey of 77 dermatologists published in September, more than a third reported either a large or slight increase in perioral dermatitis instances since the beginning of the pandemic. “In our clinic,” Dr. Chien said, “we are seeing many more perioral dermatitis and other rashes related to masks.” Dr. Jessica Sprague, a dermatologist at the U.C. San Diego School of Medicine, noted a similar phenomenon. “Masks alter the skin environment, and they can also cause a lot of skin irritation,” she said, adding, “I’m definitely seeing it more now in the setting of mask wearing.”