The Heat Strain Index indicates how hard your body is working to cool itself, and is a function of both core temperature and skin temperature (see the articles CORE’s Heat Strain Index and CORE’s Heat Zones for details). Any given value can be achieved in numerous combinations of core and skin temperature. The chart below shows approximate temperatures needed to reach each Heat Zone.

To understand how to raise or lower your Heat Strain Index, it is important to know what causes core and skin temperatures to change. They are mainly influenced by metabolic heat production, physiological responses, and environmental conditions.

The human body functions by converting carbohydrates and body fat into both muscular energy and energy that allows the brain, liver, and other organs to function. This conversion is very inefficient, however – between 70% and 95% of the fuel is generated as heat. This is how, even at rest, the body can create enough warmth to keep core temperature in the relatively constant range around 37°C/98.6°F.

When exercising, large amounts of fuel are needed to power skeletal muscles and consequently huge quantities of heat are generated internally. Blood circulation disperses this heat from the muscles throughout the body, raising core temperature.

When core temperature rises, the body acts to shed that heat, primarily by sweating and by widening blood vessels in the skin. The evaporation of sweat removes great amounts of heat energy from the skin, causing a cooling of both the skin and the blood flowing beneath it.

At the same time, the body is widening the blood vessels in the skin (vasodilation), allowing more blood to reach the skin and lose heat.

Heat training both increases the sweat rate and improves vasodilation, allowing for quicker and more efficient cooling.

The temperature of the skin is critical in regulating core temperature. The greater the difference between core and skin temperatures, the faster the transfer of heat from the body to the environment. In other words, a cool skin temperature results in a cooler core temperature.

In general, the body heats itself from within, and environmental conditions dictate whether or not that internal heat is trapped or dissipated. In other words, the environment acts a blanket – either a very thin one trapping little of the body’s generated heat, or many very thick blankets trapping a great deal of the body’s generated heat.

Dry air, almost no matter its temperature, allows substantial sweat evaporation from the skin. This greatly reduces skin temperature and Heat Strain Index. Humid air, however, allows little sweat evaporation and little reduction in skin temperature. In extremely humid conditions, the body relies primarily on skin vasodilation to lose heat. This is much less effective than sweat evaporation, and skin temperature and Heat Strain Index will likely be elevated.

For more information about the interplay of temperature and humidity and their effects on thermoregulation, see the article How Hot is it Really?

Clothing also prevents heat from leaving the body. Fabrics that allow sweat evaporation contribute to cooling, while those that trap sweat (ie. a rain jacket) limit evaporation and increase heat strain. Likewise, heavy thermal layers prevent convective heat loss, warming the skin and making heat loss through skin vasodilation less effective.

In extreme situations, the body can gain heat from the environment (very hot air, strong sunshine, or a sauna/hot tub). Actions to shield the body from those sources (ie. seeking shade, covering the skin to block sun exposure, etc) can prevent the rise in Heat Strain Index.

This understanding of heat production and heat loss lets us apply effective strategies to either raise or lower our Heat Strain Index. In summary: