Humidity and Exercise

Many environmental factors can have either a positive or a negative impact on exercise. Humidity is a general term for various descriptions of the water vapor in the air and is usually seen as having a negative impact on physical activity. This is due to the effect it has as a barrier to cooling the body. The temperature of air is one factor that determines the amount of water vapor that it can hold. The colder the air, the less water vapor it is able to carry. Absolute humidity is the amount of water vapor per mass of dry air in a given volume at a certain air temperature. Relative humidity is the water vapor content of the air relative to its content at saturation. This is the ratio of the current absolute humidity to the highest possible absolute humidity (due to the air temperature). If the air has 100% relative humidity, sweat evaporation is severely limited if not eliminated.

Humidity also contributes to the wet bulb globe temperature (WBGT), which quantifies the level of heat stress. Therefore, a rise in humidity also increases the WBGT. An increase in heat stress is dangerous because of the linear relationship between rise in heat stress and exertional heat illnesses, such as exertional heat stroke, which may prove deadly. Low humidity in a hot environment may also be deceiving, as athletes do not always recognize that they are losing sweat because it evaporates so quickly. Overall, humidity is an important factor in determining the level of heat stress, the ability of humans to cool themselves appropriately, and the amount of fluids lost due to sweating.

Wet Bulb Globe Temperature

WBGT is an indication of the heat stress (and therefore risk) calculated from the ambient temperature, the wet bulb temperature, and the black globe temperature. For outdoor exercise, this is calculated by the following formula:

When exercise is conducted indoors, it is calculated by the following formula:

From this calculation, the wet bulb temperature makes up 70% of the WBGT. A WBGT above 90±F is considered extremely high, and exercise should [Page 586]not be conducted in this environment. Naturally, an environment with a high temperature and moderate humidity will be stressful to the body; however, the WBGT recognizes the importance of humidity and the extra stress it places on the body. Therefore, an environment that has a moderate temperature but a high humidity may have the same WBGT as the former environment. The main emphasis in this measurement is that a lower temperature environment with a high humidity may be equally stressful on the human body as a higher temperature environment with low humidity.