Health Effects: Working Heat Stress

The heat coming from the surrounding environment is not the only source of heat for the body. If a person is carrying out heavy work in a hot place additional heat can be generated by the persons (Parsons, 2003). As little as 20% of muscle energy contributes to external “work” (Parsons, 2003), and the rest becomes “waste heat” inside the body that needs to be released to the external environment. At high air temperatures (above 34-37 °C), the only method of heat loss to counteract bodily heat gain caused by work, is by evaporation of sweat.  When there is high humidity, sweat evaporation is insufficient and other physiological changes cannot prevent the core body temperature from rising to dangerous levels (Bridger, 2003).  

There are a number of different tools for measuring occupational heat exposure. Published evidence concerning the impact on health and work capacity has used WBGT (Wet Bulb Globe Temperature) as the main heat exposure variable (Parsons, 2003), which makes it possible to use exposure-response relationships from such analysis in our impact assessments. However, it must be emphasized that any method produces approximate estimates (Epstein and Moran, 2006), and that individual variation is great. As long as the four climate variables and the two additional variables are considered and available for calculations, any heat stress index can be calculated (McLaurin et al., 1962; Budd, 2001, 2008; Brotherhoood, 2008).

We use WBGT as the key indicator of occupational heat exposure as it is based on thermal physiology mechanisms (Yaglou and Minard, 1957; Parsons, 2003) and is the most common human heat exposure index used in situations of occupational heat stress. It is used in the international standard for occupational heat exposure (ISO, 1989), in the recommendations from the American Conference of Governmental Industrial Hygienists (ACGIH, 2009), and in guidelines or regulations from government departments in a number of countries (e.g. New Zealand: DoLNZ, 1997).

In a many working situation without air conditioning the only way to protect people from the resulting heat effects of heat stress is for them to slow down and to take longer rest times to slow down the heat generated by their muscles. This is effective, but does have outcomes such as loss of productivity and potentially loss of income and if it is widespread could have effects on the local economy. So it is important to note that rising temperatures resulting in increase heat stress risk will not just have an impact on public health but could also have an impact on the economy (Kjellstrom et al., 2009, other more recent refs?).