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Heat Stress Index

Heat Stress Index

Knowing the heat stress index is critical for those who work outside, exposed to harsh sun and high humidity.  The heat stress index is defined as the relation of the amount of evaporation (or perspiration) required as related to the maximum ability of the average person to perspire (or evaporate fluids from the body in order to cool themselves).  When the heat stress index is high, humans can experience heat stress, which can lead to particularly dangerous conditions in which people can actually die from being too warm and unable to cool themselves properly.  Severe dehydration and even death can result from overexposure when the heat stress index is high.

Every year, athletes, construction workers and others who regularly train or work outdoors are at risk of heat stroke or other forms of heat stress thanks to a high heat stress index.  Athletes have a particularly hard time with this when training for sports during the summertime when both humidity and heat are at its peak.  It has become a rather commonplace occurrence to hear about cases where young athletes die as a result of overtraining in a high heat stress index environment.  This has become a concern for parents and coaches who want to ensure that their children and students will be safe.

Understanding the heat stress index and having a portable meter to measure it while you are outdoors is the key to preventing these types of deaths and overexposure.  Knowing the heat stress index can help to prevent fatigue, heat cramps, exhaustion and in some cases, even death.  In addition to careful monitoring of the heat stress index, staying hydrated and taking frequent breaks in the shade or in a cooler indoor area can help to prevent injuries and fatalities related to the sun.  Look for signs of heat emergencies in yourself and in others when working or training outdoors during times when the heat stress index is high.  Excessive flushing of the skin, dizziness, confusion and fainting are all signs that a person needs to immediately seek shade and hydration.  A heat stress monitor like the Kestrel 5400 can help you determine when the heat stress index is at dangerous levels.

It’s important for those who work with athletes and those who manage outdoor work sites to track the heat index every day, throughout the day, in order to ensure the safety of others who are onsite.  When working outdoors can’t be avoided, set up a cooling area where individuals can take breaks and rehydrate.  Possible risk factors for heat stress due to a high heat stress index include obesity, high blood pressure, dehydration and alcohol consumption.  For those who fall into any of those categories, increased hydration and more frequent breaks may be needed in order to avoid heat stress issues.

By allowing plenty of breaks and fluids throughout the day, and by avoiding work when both the sun and heat stress index are at their highest, site managers, coaches, parents, and supervisors can ensure that they are taking the proper precautions when it comes to being outdoors in hot weather.   

Thermal Work Limit

Thermal Work Limit (TWL) is defined as the limiting (or maximum) sustainable metabolic rate that well-hydrated, acclimatized individuals can maintain in a specific thermal environment, within a safe deep body core temperature (< 38.2 °C or 100.8 °F) and sweat rate (< 1.2 kg or 2.6 lb per hour).[1] The index is designed for self-paced workers and does not rely on the estimation of actual metabolic rates, a process that is difficult and subject to considerable error. The index has been introduced into the United Arab Emirates and Australia resulting in a substantial and sustained fall in the incidence of heat illness in the latter.

Wet-bulb globe temperature (WBGT)

The wet-bulb globe temperature (WBGT) is a type of apparent temperature used to estimate the effect of temperature, humidity, wind speed (wind chill), and visible and infrared radiation (usually sunlight) on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures. 

Heat Stress Index Reference Guide and Thermal Work Limits Conversion Booklet

Download Heat Index Reference Guide (Same as charts below) - Download Now (PDF)

Download Thermal Work Limits Conversion Booklet - Download Now (PDF)