First, try exercising in the cooler hours of morning and evening. But when that heat index climbs, take health precautions for your safety. The heat index is figured by combining the dry bulb temperature and the relative humidity, and humidity is the key factor. Because runners' health varies from day to day, these recommendations do not guarantee protection from environmentally induced illnesses.
Dehydration is common during prolonged endurance events in hot weather conditions.
The average participant loses between a pint and more than a quart of fluid, and usually doesn't drink enough to replace it. Runners may experience hyperthermia depending on environmental conditions, caloric intake, fluid consumption, and clothing. On extremely hot days, up to half the participants in an event may require treatment for heat exhaustion or heatstroke. Heat exhaustion, typically the most common heat illness among athletes, is the inability to exercise in the heat. Heat exhaustion means that it's hard to breathe, and that you're hot and dehydrated.