It's certainly not the sexiest topic, but armpit rash and armpit irritation are very real things—very real, very irksome, and very uncomfortable things, at that. "The skin in this area is thinner compared to most parts of the body, making it more sensitive and susceptible to environmental irritants," explains board-certified dermatologist Teresa Song, MD. All these factors make this spot more prone to irritation (or intertrigo, a rash or inflammation in a skin fold).
This darker skin, also known as hyperpigmentation, is common and can have various causes, such as a rash, sun exposure, a medical condition, blue light, or even hormonal changes. Knee hyperpigmentation usually isn't harmful, but many people still may want to get rid of it. With the help of expert advice from board-certified dermatologists Jennifer Baron, MD, and Azadeh Shirazi, MD, we've broken down everything you need to know about dark skin on your knees—from why it happens to how to treat it.