Thousands of workers become sick from exposure to heat every year, and some even die. Heat illnesses and deaths are preventable, and employers must take responsibility for protecting their employees while they are working under conditions of excessive heat.

Workers exposed to hot and humid conditions are at risk of heat illness, especially those doing heavy work tasks or using bulky protective clothing and equipment. New and temporary workers, as well as those returning to work after time off, are especially susceptible to heat-related illness if they have not built up a tolerance to hot conditions. Industries most affected by heat-related illness are construction, trades, transportation and utilities, agriculture, building and grounds maintenance, landscaping services and support activities for oil and gas operations.

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Using the Heat Index to Protect Outdoor Workers from Heat-Related Illness

HEAT INDEX CHART

Outdoor workers who are exposed to hot and humid conditions are at risk of heat-related illness, and both air temperature and humidity affect how hot they feel. The heat index system, developed by the National Oceanographic and Atmospheric Administration (NOAA), combines air temperature and relative humidity into a single value that indicates how hot the weather will feel. The higher the heat index, the hotter the weather will feel, since sweat does not readily evaporate and cool the skin. The heat index is a better measure than air temperature alone for estimating the risk to workers from environmental heat sources.

The heat index can be used to help determine the risk of heat-related illness for outdoor workers, what actions are needed to protect workers, and when those actions are triggered. Depending on the heat index value, the risk for heat-related illness can range from lower to very high to extreme. As the heat index value goes up, more preventive measures are needed to protect workers. Heat index values are divided into four bands associated with four risk levels (see Heat Index Chart).

OSHA's Heat Index Guide outlines protective measures to be taken at each risk level, and employers are urged to train their workers so that everyone is prepared to work safely as the heat index rises.

OSHA has also released a free application for both Android and iPhone mobile devices that enables workers and supervisors to monitor the heat index at their worksites.  The free mobile app, available in both English and Spanish, displays a risk level for workers based on the heat index, as well as reminders about protective measures that should be taken at that risk level.