After spending a warm day walking or being active outdoors, you might notice a red or purplish rash on your legs. Many people mistake it for sunburn, bug bites, or even an allergic reaction. But in some cases, it could be your body signaling something more serious.

This rash may actually be exercise-induced vasculitis—a condition where small blood vessels, usually in the lower legs, become inflamed after extended physical activity in hot weather. Though not widely recognized, it’s more common than you’d think, particularly among active adults over 50, with women being affected most often.

So, what exactly is exercise-induced vasculitis? It happens when exertion in the heat sends extra blood to surface capillaries, sometimes causing blood to pool in the lower limbs. This leads to inflammation and the appearance of a rash, most commonly on the ankles, calves, or shins, and occasionally higher up the legs.

Cruise passengers are frequent sufferers, often developing the rash after long, hot walks ashore—only to later discover that rest and elevating the legs are usually the best remedies.