It is very important to "keep your cool" on a hot day.

An immediate concern is dehydration, or not having enough water in your body. When your body loses too much fluid, it begins to take fluid from the blood, which leads to muscle cramps and may result in shock. Early symptoms of dehydration are dry mouth and sticky saliva, in addition to dark urine and reduced urine output.
To reduce the risk of dehydration, drink plenty of fluids, such as water or sports drinks, especially when working outdoors in the heat. Avoid drinks containing caffeine and alcohol because they increase urine output and make you dehydrate faster. Should you start to notice cramping or feel dizzy or fatigued, stop whatever activity you are doing and rest somewhere out of direct sunlight.

When working outdoors, you should always protect yourself from the harmful ultraviolet (UV) rays of the sun. Over time, these UV rays damage the skin, making it less elastic and contributing heavily to the development of skin cancers. In fact, about 90 percent of all skin cancers occur on sun-exposed skin.

The American Academy of Dermatology suggests wearing a hat, sunglasses, and protective clothing when exposed to the sun. Avoid deliberate sunbathing, and use a sunscreen with a sun protection factor (SPF) of at least 15 while in the sun. Apply sunscreen 20 minutes before going out into the sun and reapply every 2 hours.

For more information, visit, http://www.extension.org/pages/Hot_Weather.