Heat stress is not a subject that laboratories generally pay much attention to. For the most part, lab staff work in comfortable air-conditioned spaces. But there are potential operations in these facilities where heat stress could become a concern for workers—these include cage wash areas for labs involved in animal research, autoclave rooms, labs with a large number of furnaces or reactors in a small space, and similar areas. In addition, field situations should not be discounted during the summer months, especially in the south. This month’s column presents the basics of heat stress: signs, symptoms, and prevention.
Risks of heat stress
Workers who are exposed to extreme heat for extended periods or work in hot environments are at risk for heat stress. Those that are at greater risk of heat stress include older workers (65 years of age or older), overweight workers, and those with underlying health conditions such as heart disease, high blood pressure, or those taking medications that are affected by extreme heat.
A recent analysis of 25 heat-related illnesses by the Centers for Disease Control and Prevention (CDC) and OSHA showed 14 fatalities. Six of the fatal cases occurred when the heat index was less than 91°F and four others occurred when the heat index was between 85°F and 90°F. This study led OSHA to lower its recommendation for heat stress screening from 91°F to 85°F.
Heat stress stages
Dehydration is the primary cause of heat stress. Heat stress symptoms range in severity from reduced strength, to nausea, to unconsciousness, and even death if not dealt with immediately. In as little as two hours of moderate work activity, workers can lose up to 1.5 liters of fluid, and may begin to experience the initial stages of heat stress. Add, humidity, work intensity, or protective clothing, and the risk due to heat stress is magnified. Replacing fluids lost through sweating is a must for proper heat stress control. Occupational illnesses and injuries resulting from heat stress are generally placed in three main categories. In order of seriousness, these are:
This is the first stage of heat stress, the most common, and the least troublesome. Profuse sweating while performing strenuous work depletes the body’s salt and water levels, which causes cramps, usually in the muscles of the arms, legs, or abdomen. If ignored, the heat stress will progress to the next levels rather quickly. If cramps develop, workers should stop their activity and rest in a cool place, drink water or a sports beverage (at ambient temperature for fastest absorption), and avoid strenuous tasks until cramps have completely subsided.
The second stage of heat stress is heat exhaustion and results from excessive loss of water and salt. Symptoms include heavy sweating, weakness or fatigue, nausea, clammy or moist skin, flushed complexion, and elevated body temperature. Fast, shallow breathing, muscle cramps, and confusion may also develop. To counteract heat exhaustion, remove the worker to a cool, shady, air-conditioned space. Make sure they drink plenty of water or a good sports beverage. In addition, a cool shower or bath can be used to lower body temperature if necessary.
Heat stroke is the most serious stage of heat stress, rapidly reached if the first two stages are ignored. After extreme water and salt loss, the body’s sweating mechanism shuts down, rendering the body incapable of regulating temperature and cooling down. The internal core temperature rises quickly to 106°F or higher, with hot, dry skin, chills, confusion, slurred speech, hallucinations, and intense headache. If a worker reaches this stage, you must call emergency services immediately. If not stopped and reversed, death is a possibility. Remove the worker to a cool area, soak their clothes with water, and continue spraying, showering, and fanning until help arrives.
Heat stress regulations
OSHA does not have a standard governing heat stress prevention for occupational settings. The General Duty clause (§ 654 5(a)(1) of the Occupational Safety and Health Act) requires that the employer furnish each employee with employment and places of employment free from recognized hazards causing or likely to cause death or serious physical harm.
Heat stress guidelines have been published by the American Conference of Governmental Industrial Hygienists (ACGIH). These threshold limit values, or TLVs, establish work-rest regimens based on temperature and work stresses. Careful consideration regarding the strict application of work/rest regimens is recommended. For example, moderate work in an 88°F Wet Bulb Globe Temperature Index (WBGT) area triggers a 25 percent work, 75 percent rest regimen under current ACGIH TLVs. Each possible heat stress situation needs a detailed evaluation prior to development of realistic controls.
Start by measuring the full range of temperature and humidity conditions of hot work areas. Measurement is often performed of environmental factors that most nearly correlate with deep body temperature and other physiological responses to heat. At the present time, the WBGT is the technique typically used to assess these environmental factors. Conduct a heat stress survey using a calibrated heat stress datalogger. The WBGT readings are useful in establishing the relative degree in which work areas are hotter than ambient.
OSHA’s web page, Using the Heat Index to Protect Workers, indicates an increased risk when the ambient heat index is greater than 91°F. However, this should be lowered to 85°F. Strenuous work combined with use of protective clothing creates an additive effect and can increase risk. Managers need to understand that there are limitations to human endurance and unique individual responses to situations.
Use of the Heat Index and WBGT are recommended in developing a site-specific heat stress policy. These measurements combined with the level of work effort are needed for good decision making. The NIOSH-CDC website has a Heat Safety Tool App, publications, posters, fact sheets, and podcasts to guide you and help train your workers.