Most people feel comfortable when the air temperature is between 20°C and 27°C and the when relative humidity ranges from 35 to 60%. When air temperature or humidity is higher, people feel uncomfortable. Such situations do not cause harm as long as the body can adjust and cope with the additional heat. Very hot environments can overwhelm the body's coping mechanisms leading to a variety of serious and possibly fatal conditions.

How does the body work?
The healthy human body maintains its internal temperature around 37°C. Variations, usually of less than 1°C, occur with the time of the day, level of physical activity or emotional state. A change of body temperature exceeding 1°C occurs only during illness or when environmental conditions surpass the body's ability to cope with extreme temperatures.

As the environment warms-up, the body tends to warm-up as well. The body's internal "thermostat" maintains a constant inner body temperature by pumping more blood to the skin and by increasing sweat production. In this way, the body increases the rate of heat loss to balance the heat burden created by the environment. In a very hot environment, the rate of "heat gain" exceeds the rate of "heat loss" and the body temperature begins to rise. A rise in the body temperature results in heat illnesses.

Consequences 
Mental problems:
- Increased irritability
- Loss of concentration
- Loss of efficiency in mental task

Psycho-physiological problems:
- Loss of efficiency in skilled tasks
- More incidents

Physiological problems:
- Disturbed water and electrolyte balance
- Heavy load on heart and circulation
- Fatigue and threat of exhaustion

At 35 - 40°C:
- Limit of high temperature tolerance 



Source: Canadian Centre for Occupational Health and Safety