Introduction The current occupational exposure to heat and hot environment standard of Thai law was issued in 2006. The wet bulb globe temperature (WBGT) index was defined for 3 workload levels without a work-rest regimen. To assess heat exposure according to the law, workload and WBGT is monitored for 2 hours during the hottest period of the day in summer. As has been predicted, global warming could significantly impact labour capacity and productivity in Southeast Asian countries. Thus, this study aimed to explore whether the occupational standard could protect outside workers focusing on construction workers in Thailand.

Methods This cross-sectional study included 18 medium and small construction sites and involved 90 heat acclimatised construction workers. Most wore cotton long sleeve shirts and pants. Heart rate (HR) and Aural (tympanic) temperature of the participants were recorded continuously for 2 hours. Exposure data comprised relative humidity, wind velocity and WBGT, including dry bulb, wet bulb and globe temperatures, were monitored and the participants’ workloads were estimated, simultaneously with the collection of physiological and environmental data, i.e. March to June. In addition a questionnaire was used to collect data of the participants.

Result WBGT ranged from 24.35–34.18°C, and 47 participants were exposed to WBGT exceeding the standard. Average air velocity and RH were 1.11 m/s and 35.77%, respectively. The range of average core body temperature and HR for 3 levels of workload were 36.60–39.4°C, 70–97 bpm, 36.62–39.58°C, 80–126 bpm and 37.04–40.08°C, 82–127 bpm respectively. Core body temperature of 29 of 47 participants exposed to heat above the standard exceeded 38°C, among these 11 had symptoms of heat related illness. In all, 18 participants were exposed to heat below the standard but their core body temperatures were higher than 38°C.

Discussion The weather was hot and dry with occasional good air movement. However, 32% of construction workers worked in high risk conditions (WBGT above the standard). Furthermore, 18 (20%) participants worked in an environment below the standard but their body core temperatures exceeded 38°C. Thus, this group of workers was not protected by the standard.