From the CDC: Several factors affect the body's ability to cool itself during extremely hot weather. When the humidity is high, sweat will not evaporate as quickly, preventing the body from releasing heat quickly. Other conditions related to risk include age, obesity, dehydration, heart disease, poor circulation, sunburn, and prescription drug and alcohol use.
Tips:
1. Drink plenty of fluids
2. Replace salt and minerals lost to sweat
3. Wear appropriate clothing and sunscreen
4. Schedule outdoor activities carefully