International occupational health and safety standards for heat stress

Introduction:  The international standards system for assessing heat stress to avoid casualties at work involves three levels of approach in three separate ISO standards. This is supported by standards for screening workers in terms of health and fitness for work in the heat (ISO 12894); the estimation of the thermal properties of clothing (ISO 9920); an estimation of the heat produced in the body by work (ISO 8996); and a standard for risk assessment (ISO 15265) as well as others.    

Methods:  The first level is described in ISO 7243 and is a simple heat stress monitoring method based upon the WBGT (Wet Bulb Globe Temperature) index. The second level is termed the Predicted Heat Strain method and is an analytical method based upon a calculation of sweating required to provide sufficient heat loss from the worker by evaporation to ensure that thermal strain is within acceptable limits. The third level uses physiological measurements on individual workers to directly monitor their health and thermal strain. ISO 9886 provides guidance on how to measure heart rate, mass loss, skin temperature and internal body temperature on workers and interpret the values.    

Results:  The current system of standards has been developed mainly in industrialised countries, for healthy male adult workers and largely for indoor work. Climate change will provide more extremes of weather which are atypical of a geographical region. The system of standards can inform practices for work in the heat. Extreme hot weather may increase heat stress exposure indoors. It will also greatly increase load for outdoor work. This paper describes international standards for the assessment of hot working conditions and considers their applicability and validity for protecting people from a range of diverse populations across the world as they face the challenges of more extreme hot weather due to climate change.