High levels of heat stress among sugarcane workers in Thailand

With continued global warming, the effects of elevated temperatures on the health of agricultural workers are a particular concern. This study characterized the levels of heat stress in Thai sugarcane workers and investigated whether season and harvesting method were associated with it.

Three hundred sugarcane workers in Nakhon Sawan Province, Thailand, were recruited, and information on demographics, working conditions, and clothing characteristics was collected from participants during the cooler months (n = 152 participants, mid-January to mid-February) and hotter month (n = 148, March). Heat stress was measured using the Wet Bulb Globe Temperature (WBGT) index, and the WBGT instruments were operated for a full work shift in the sugarcane fields where the participants worked. One-hour time weighted average (TWA) effective WBGT (WBGTeff-1hrTWA) estimates were determined for different times of the day based on the measured WBGT and clothing adjustment factor.

The average WBGTeff-1hrTWA in the cooler months ranged from 22.5 °C during the early morning to 31.3 °C during the hottest time of the day, and for the hotter month, it ranged from 25.4 °C to 33.9 °C, respectively. The measured WBGT, natural wet-bulb temperature (Tnwb), dry-bulb temperature (Tdb), globe temperature (Tg), air velocity (Av), and absolute water vapor pressure (ea) were all statistically significantly higher in the hotter month than in the cooler months. Harvesting during the hotter month and harvesting burnt sugarcane were significantly associated with increased effective WBGT. The harvesters' heat stress in both seasons exceeded the American Conference of Governmental Industrial Hygienists - Threshold limit value for 72.7% of the working time in the cooler months and 90.9% in the hotter month.

The heat stress in Thai sugarcane workers was high in both seasons, particularly in the hotter month and when harvesting burnt sugarcane. This results in a very high risk of developing heat-related health effects, and measures are needed to reduce heat stress. Heat stress in agricultural and other outdoor work in tropical climates is an immediate and growing problem.