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Occupational heat strain is a public health threat, and for outdoor industries there is a direct influence from elevated environmental temperatures during heat waves. However, the impact in indoor settings is more complex as industrial heat production and building architecture become factors of importance. Therefore, this study evaluated effects of heat waves on manufacturing productivity. Production halls in a manufacturing company were instrumented with 33 dataloggers to track air temperature and humidity. In addition, outdoor thermal conditions collected from a weather station next to the factory and daily productivity evaluated as overall equipment efficiency (OEE) were obtained, with interaction between productivity and thermal conditions analyzed before, during, and after four documented heat waves (average daily air temperature above 24 degrees C on at least three consecutive days). Outdoor (before: 21.3 degrees +/- 4.6 degrees C, during: 25.5 degrees +/- 4.3 degrees C, and after: 19.8 degrees +/- 3.8 degrees C) and indoor air temperatures (before: 30.4 degrees +/- 1.3 degrees C, during: 32.8 degrees +/- 1.4 degrees C, and after: 30.1 degrees +/- 1.4 degrees C) were significantly elevated during the heat waves (p < 0.05). OEE was not different during the heat waves when compared with control, pre-heat-wave, and post-heat-wave OEE. Reduced OEE was observed in 3-day periods following the second and fourth heat wave (p < 0.05). Indoor workers in settings with high industrial heat production are exposed to a significant thermal stress that may increase during heat waves, but the impact on productivity cannot be directly derived from outdoor factors. The significant decline in productivity immediately following two of the documented heat waves could relate to a cumulative effect of the thermal strain experienced during work combined with high heat stress in the recovery time between work shifts.