Heat exposure and heat stress in outdoor workers: a review of measures in published studies

BACKGROUND: Increased ambient air temperatures resulting from climate change led to higher heat levels for outdoor workers. Heat exposure refers to ambient environmental heat exposures; while heat stress also considers the clothing type and the metabolic rate resulting from physical activity. We reviewed the types of heat exposure/stress measures used in published studies. METHODS: A literature review was conducted to identify and examine measures of heat exposure and heat stress in outdoor workers. RESULTS: The most common indices measuring heat exposure/stress in occupational health include the Wet Bulb Globe Temperature (WBGT), Environmental Stress Index (ESI), Heat Index (HI), Humidex (HD), and Discomfort Index (DI). HI, HD and DI account for ambient air temperature and relative humidity surrounding workers. WBGT accounts for air temperature, radiant temperature, humidity, speed of air movement, and adjusts for clothing and metabolic rate. ESI considers factors like WBGT except for speed of air and does not account for clothing and metabolic rate. Both ESI and WBGT fail to consider the risk of dehydration resulting from excessive sweating in hot work environments. HI, HD and DI measure heat exposure but not heat stress as they do not account for the metabolic rate; also, they do not consider radiation and may underestimate the potential danger of working in excessive heat. These indices are considered simpler to use than WBGT. However, WBGT is more precise and covers environmental factors influencing heat exposure/stress. Questionnaires are also used to assess workers’ perceptions of heat exposure and self-reported health impacts. Their findings, in general, have not been comparable due to the heterogeneity of questions used. CONCLUSIONS: Although WBGT is recommended for accurate measures of heat exposure/stress, its high cost has been a deterrent for workplaces in low-income countries. Projects with low financial resources opt for simpler and cheaper measures of heat. KEY MESSAGES: • The use of reliable devices is necessary to measure and monitor heat exposure in outdoor work. • It is desirable to generate homogenous results for the comparability of study findings in occupational health research.