 When sweat evaporates, it cools the body down. Also, circulation improves and blood volume increases, so the heart doesn't have to work as hard to pump blood. These healthy changes come in response to careful, gradual exposure to heat stress and can make hot days feel a little more bearable. But heat can be particularly dangerous for people in poor health or those who aren't used to it. Health experts recommend staying hydrated, informed, and self-aware, and staying cool as much as possible on really hot days.