单项选择题

Heat and Health

Extremely hot weather is common in many parts of the world. Although hot weather just makes most people hot, it can cause medical problems--and death.
Health experts say that since the year 1900, extremely hot weather has killed more people in the United States than any other natural event. One year--the unusually hot summer of 1980--heat caused about 1,700 deaths in the United States. In 1995, more than 600 people died in a similar heat wave in one city--Chicago.
To measure extreme heat, government weather experts have developed the Mean Heat Index. It measures the average of how hot it is felt all day on an extremely hot day. Experts say it is the total heat of a hot day or several hot days that can affect health. Several hot days are considered a heat wave. Experts say heat waves often become deadly when the nighttime temperature does not drop much from the highest daytime temperature.
The most common medical problem caused by hot weather is heat stress. Usually, it also is the least severe. For most people, the only result of heat stress is muscle pain. The pain is a" warning that the body is becoming too hot. Doctors say drinking water will help the pain disappear after the body again has the right amounts of water and salt. For some people, however, the result is much more serious.
For example, doctors say some people face a greatly increased danger from heat stress. These people have a weak or damaged heart, high blood pressure, or other problems of the blood system. Severe heat can help cause a heart attack or stroke. Health experts say this is the most common cause of death linked to hot weather.
Muscle pain in hot weather means that your body needs______.