The above chart is a heat index chart. It is determine by the temperature and humidity. By intersecting the 2 values you come up with the heat index. It is defined as the apparent temperature of the atmosphere that you feel...rather than the temperature that is measured in the shade. Heat indexes above 105 degrees can cause some body stress...if proper fluids(non-liquer) are not taken. Heat indexes from 110-120...can cause serious stress on the body. Greater than 120 degrees should be experienced as little as possible and prolong exposure can lead to a heat stroke.

Heat disorders generally have to do with a reduction or collapse of the body's ability to shed heat by circulatory changes and sweating, or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise and heat-related illness may develop.

Ranging in severity, heat disorders share one common feature: the individual has overexposed or overexercised for his age and physical condition in the existing thermal environment.

Sunburn, with its ultraviolet radiation burns can significantly retard the skin's ability to shed excess heat.

Studies indicate that, other things being equal, the severity of heat disorders tends to increase with age--heat cramps in a 17-year old may be heat exhaustion in someone 40, and heat stroke in a person over 60.

Acclimatization has to do with adjusting sweatsalt concentrations, among other things. The idea is to lose enough water to regulate body temperature, with the least possible chemical disturbance.