The human body maintains a constant internal temperature despite varying environmental temperatures. This process, called thermoregulation, is essential for optimal organ function and survival. The body relies on various mechanisms to achieve thermoregulation, including sweating, vasodilation, and behaviour adaptations such as seeking shade or drinking cool fluids.

In a desert environment, the air is typically hot and dry. The high temperature poses a challenge for the body to dissipate heat effectively. However, the low humidity facilitates efficient evaporation of sweat from the skin. This cooling effect, combined with the body's natural thermoregulatory mechanisms, allows humans to endure temperatures as high as 40°C (104°F) in the desert.

On the other hand, the tropics are characterized by both high temperatures and high humidity. While the temperature may be similar to that of a desert, the high humidity hinders the evaporation of sweat. This impaired sweat evaporation reduces the body's ability to cool down, resulting in heat stress. The body may struggle to maintain its internal temperature, leading to potential heat-related illnesses and even death in extreme cases.

Therefore, while 40°C may be bearable in a desert due to efficient sweat evaporation, it can become lethal in the tropics due to high humidity, which impedes cooling mechanisms and poses greater risks of heat-related illnesses.