Exercising outdoors during monsoon may not be free of risks and can even prove to be life threatening. During exercise, the body’s internal (core) temperature increases by about one degree celcius every five minutes, unless the heat is dissipated. In very hot and humid weather, there can be a marked increase in core temperature which increases the risk of heat injuries such as heat cramps, heat exhaustion and heat stroke.

Even moderate-intensity exercise may cause a ten-fold increase in heat production. At more than 24 degree celcius of environmental temperature, exercise in healthy adults leads to an increase in heart rate of one beat per minute for each one degree celcius rise in temperature.
The body has a variety of defense mechanisms to prevent such an increase in body temperature including an increase in skin blood flow and sweating. Drying (evaporation) of sweat causes cooling. Other mechanisms are radiation, conduction and convection. Increase in heart rate, dilation of blood vessels and flushed red face are manifestations of the body’s cooling mechanisms.

Sweating begins within the first few seconds of exercise. By 30 minutes, the rate of sweating reaches a steady level that is determined by exercise intensity and environmental conditions.
Interestingly, in high humidity conditions, evaporation of sweat is significantly reduced. In this situation, sweating is nothing more than a useless loss of water and electrolytes. Rather, formation of copious sweat may lead to dehydration, which may further increase core body temperature.

Strenuous physical effort in hot and/or humid weather may be particularly dangerous in patients with heart disease, kidney disease or those on medication for high blood pressure. During hot and humid climate, an increase in heart rate during exercise results in increased oxygen demand of the heart muscle, which may not be possible to meet in coronary heart disease with narrowing of arteries. Fluid loss equivalent to one per cent of body weight may cause a significant increase in core temperature. This can precipitate angina or arrhythmias (irregular heart beats) during exercise in hot weather.

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