In hot environments, your body may have trouble keeping its temperature at a safe level. As a result, your body temperature rises to unsafe levels. This is heat stress. Heat stress can develop quickly. It can also be very dangerous to your health.
To function, your body requires your core temperature stay very close to normal. Your body has a normal core temperature of 98.6°F (37°C). If the environment is hot or you are very active, your body heats up. To keep your core temperature stable, your body releases excess heat into the air. The heat leaves your body in two ways: from the blood vessels near the skin's surface and through sweat.
When your body needs to release heat, the blood vessels near the surface of your skin widen. Extra blood flows through them. This extra blood brings more body heat to the surface, which releases it into the air. Your body needs enough water and minerals, such as sodium, to keep blood flowing smoothly to these vessels and to the rest of the body.
If increased blood flow alone isn't enough, your body also increases the amount that you sweat. As sweat dries (evaporates), it cools the skin. As you sweat, your body loses water (and some minerals such as sodium and potassium). This water (and minerals) must be replaced to keep you feeling well and healthy, and to allow further sweating.
Too much activity: The more active you are, the more heat your muscles generate. Heavy physical activity also sets up competition between your muscles and skin for the blood supply.
High environmental temperature: As the temperature in your environment goes up, so does your body temperature. When it's hot from the sun or another heat source, such as a furnace, your body can't move heat into the air as effectively.
Too little air movement: Air moving across your skin carries away heat brought to the surface by blood vessels. It also helps sweat evaporate. Too little air movement means these processes don't work as well.
High humidity: Humidity is the amount of moisture in the air. The higher the humidity, the less sweat evaporates. That's because the air is too wet to absorb more moisture.
Medical problems: Some medical conditions, including diabetes and heart failure, can increase your susceptibility to heat stress.
Medicine: If you take medicine for heart rate, asthma, kidney disease, or to manage fluid retention, you may be more sensitive to heat.