Excessive workplace heat exposures create well-known risks of heat stroke, and it limits the workers’ capacity to sustain physical activity. There is very limited evidence available on how these effects reduce work productivity, while the quantitative relationship between heat and work productivity is an essential basis for climate change impact assessments. We measured hourly heat exposure in rice fields in West Bengal and recorded perceived health problems via interviews of 124 rice harvesters. In a sub-group (n=48) heart rate was recorded every minute in a standard work situation. Work productivity was recorded as hourly rice bundle collection output. The hourly heat levels (WBGT=Wet Bulb Globe Temperature) were 26–32°C (at air temperatures of 30–38°C), exceeding international standards. Most workers reported exhaustion and pain during work on hot days. Heart rate recovered quickly at low heat, but more slowly at high heat, indicating cardiovascular strain. The hourly number of rice bundles collected was significantly reduced at WBGT>26°C (approximately 5% per °C of increased WBGT). We conclude that high heat exposure in agriculture caused heat strain and reduced work productivity. This reduction will be exacerbated by climate change and may undermine the local economy.