Humans and many other mammals have unusually efficient internal temperature regulating systems that automatically maintain stable core body temperatures in cold winters and warm summers. In addition, people have developed cultural patterns and technologies that help them adjust to extremes of temperature and humidity . In very cold climates, there is a constant danger of developing hypothermia , which is a life threatening drop in core body temperature to below normal levels. The normal temperature for humans is about 37.0degC. However, differences in persons and even the time of day can cause it to be as much as 6degC higher or lower in healthy individuals. It is also normal for core body temperature to be lower in elderly people. Hypothermia begins to occur when the core body temperature drops to 34.4degC. Below 29.4degC, the body cools more rapidly because its natural temperature regulating system usually fails. The rapid decline in core body temperature is likely to result in death. However, there have been rare cases in which people have been saved after their temperatures had dropped to 13.9-15.6degC. This happened in 1999 to a Swedish woman who was trapped under an ice sheet in freezing water for 80 minutes. She was found unconscious, not breathing, and her heart had stopped beating, yet she was eventually saved despite the fact that her temperature had dropped to 13.7degC. In extremely hot climates or as a result of uncontrollable infections, core body temperatures can rise to equally dangerous levels. This is hyperthermia. Life threatening hyperthermia typically starts in humans when their temperatures rise to 40.6-41.7degC. Only a few days at this extraordinarily high temperature level is likely to result in the worsening of internal organs and death. People are unlikely to survive under the body temperature  _  .
higher than 34.4degC lower than 29.4degC between 40.6-41.7degC between 34.4-37degC