n October, when the Canadian air starts drying out, the men flock to Sandy Skotnicki’s office. The men are itchy. Skotnicki studied microbiology before becoming an assistant professor of dermatology at the University of Toronto. She has been practicing for 23 years, always with an eye to how the environment—including the microbial one on our skin—affects health. “I say to them, ‘How do you shower?’ ” she told me. “They take the squeegee thing and wash their whole body with some sort of men’s body wash. They’re showering twice a day because they’re working out. As soon as I get them to stop doing that and just wash their bits, they’re totally fine.”

Bits?

“Bits would be underarms, groin, feet,” she said. “So when you’re in the shower or the bath, do you need to wash here?” She pointed to her forearm. “No.” Even water alone, especially hot water, slowly strips away the oils in the outer layers of skin that help preserve moisture—and the drier and more porous someone’s skin, the more susceptible it is to irritants and allergens.

Skotnicki believes that this is one way overwashing prompts eczema to flare in people with a genetic predisposition to the disease. While eczema itself can be debilitating, it often does not travel alone. It seems to be part of a constellation of conditions caused by immune-system misfires. Infants with eczema have an increased risk of developing allergic rhinitis or asthma in childhood, part of a cascade of immune-system overreactions known as the “atopic march.”