 This study examined the effects of environmental heat on outdoor workers in Thailand. It compared the levels of heat exposure in three different seasons and assessed the relationship between heat exposure and dehydration. The results showed that the average wet bulb globe temperature, WBGT, was highest during the summer months, followed by the rainy season and then winter. Additionally, the study found that the farm workers experienced higher body temperatures, heart rates, and respiratory rates during the summer months than during the other two seasons. Furthermore, the study also found that the farm workers experienced more skin rash slash itching, dizziness, muscle cramps, dyspnea, and weakness during the summer months than during the other two seasons. Finally, the study found no correlation between WBGT and urine-specific gravity, SP.GR, in any of the three seasons. Overall, these results suggest that outdoor workers in Thailand are exposed to high levels of environmental heat and experience increased symptoms of heat-related illnesses during the summer months.