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Heat Stress for Outdoor Workers

Outdoor workers, including landscaping, construction, and maintenance personnel are at a great risk of heat-related illnesses when environmental conditions are conducive to high heat and humidity. Heat stress can manifest itself in a number of physiological effects, ranging from heat cramps to heat stroke. OSHA requires employers to assess and eliminate hazards on job sites, including thermal strain disorders; workers must be adequately trained in recognition, assessment, and control of heat stress.

Physiological Response


The Human body maintains its temperature at about 98.6 degrees Fahrenheit. Extreme variations from this average can affect biological functions, and this thermal stress is manifested as physiological responses such as heat cramps, heat exhaustion, and heat stroke. Heat exhaustion and heat cramps are less serious, but may be precursors to heat stroke. Heat exhaustion symptoms include fatigue, nausea, headache, pale skin, and muscle cramp; it can be treated with frequent rest breaks and hydration. Heat stroke is characterized by a lack of sweating and disorientation, and requires immediate medical attention.

Hazard Assessment

The body is cooled by evaporation of sweat, which is dependent on the ambient temperature and humidity. The factors that influence an individual worker’s tolerance for heat stress include body type, age, cardiovascular health, physical fitness, and alcohol use. Assessing environmental conditions is essential for determining the potential for heat-related stress. A method for estimating thermal hazards is the Corrected Effective Temperature (CET), which accounts for the four factors that affect heat stress: ambient air temperature, humidity, air velocity, and radiant sources. Wet Bulb Globe Temperature (WBGT), measures the combination of temperature and relative humidity. Instruments are available for directly measuring WBGT and estimating the CET for immediate assessment of ambient conditions.


Control of heat stress includes training, hygiene, medical surveillance, and a heat-alert program. Training should follow both a pre-placement and periodic schedule. It should include a description of heat stress responses, first aid, acclimatization, and hygiene practices. Heat stress hygiene practices are personal actions taken by workers to avoid heat stress, including fluid replacement, lifestyle, and self-determination. It is recommended that workers drink at least quart of water per hour during exertive work. A healthy lifestyle, including a balanced diet and limited alcohol intake, can reduce the risk of heat-related disorders. Light, loose-fitting clothing will assist in evaporation of sweat. Frequent rest breaks will reduce fatigue. Acclimation to work in a hot environment can be achieved in workers according to the following schedule recommended by NIOSH: Over a six-day period, reduce the workload,with the first day starting at 50 percent workload, each day thereafter add 10 percent. The seventh day will reflect a full workload with full acclimation. Workers should be trained and alert to symptoms of heat stress in themselves and coworkers.

Heat-related disorders, including heat cramps, heat exhaustion, and heat stroke, are serious medical conditions common in outdoors workers. Training, including recognition, evaluation, and control of thermal hazards, first aid, and physiological self-awareness should be a requirement for starting any work in hot outdoor environments.

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