A researcher has linked certain water-related skin rashes to an allergy to potassium peroxymonosulfate, commonly used as a “shock.” In those who are sensitive to the chemical, an itchy skin irritation develops between four and 48 hours after exposure, and can last up to two weeks, according to Dr. Matthew

Zirwas, director of Ohio State University Medical Center’s Dermatitis Center.

“It’s not something that says something bad about the pool or the chemical,” Zirwas said. “There’s just this group of people who are sensitive.”