Heat Index

Heat index is the ambient or environmental temperature as perceived by the human body. The heat index is based on both raw air temperature values and the [Page 821]relative humidity or moisture that is present in the air. The heat index offers a value that expresses how the climate “feels” on the body in an objective and universal manner. Heat and heat-related injuries across the developing world are a significant cause of occupational injuries and hazards and are a preventable illness. Heat-related injuries affect millions in both mild and severe climates each year.

The body is able to release heat by fluctuating the rate and depth of blood circulation in the periphery, by liberating water through the skin and sweat glands, and through respiration. These described compensatory mechanisms are very important for maintaining normal body temperature. More specifically, sweating cools the body with evaporating water off the skin and taking with it some of the heat the body holds. However, if the humidity in the air is high, it cannot easily release this water and therefore, cannot easily release heat. In very humid climates, such as regions along the equator, it is difficult for the body to compensate and to lose heat in an efficient manner. This leads to heat injury and illness.

These compensatory mechanisms start acting when the environment is cooler or warmer than normal body temperature (37.0 degrees C or 98.6 degrees F). The ability to maintain body temperature within these limits is described as thermoregulation. Deviation too far from these normal limits can cause unconsciousness and death. A high heat index is indicative of high ambient air temperatures and high humidity. A low heat index are low ambient air temperatures and low relative humidity. A high heat index can also exacerbate other underlying disease such as respiratory illness as asthma and chronic obstructive pulmonary disease (COPD).

Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which corresponds a temperature and humidity combination to a higher temperature in dry air. Calculation for heat index in Celsius takes into account vapor pressure, relative humidity, and temperature:

where e = V × [(6.112 × 10∘ ((7.5T / 237.7) + T)] × (R / 100); where V = vapor pressure; R = relative humidity, and T = temperature. A table exists that correlates heat and relative humidity in this heat index for easy observation of dangerous heat conditions.

The heat index is based on both raw air temperature values and the relative humidity or moisture that is present in the air.

For example, if it is 27 degrees C ambient temperature with relative humidity of 95 percent it will “feel” like 31.3 degrees C. Maintaining normal body temperature requires much of the energy we get from food consumption. If the body does not have enough fat stores, calorie consumption, or clean drinkable water, thermoregulation becomes compromised and heat illness and injury may occur. Clean potable water sources and consequent consumption is a major threat to much of the developing world where the heat index remains high for much of the year and heat illness inflicts many.

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