Due to direct exposure to high temperatures in summer, sanitation workers can easily experience heat-related illness and even mortality. This paper aims to determine the thresholds of the environmental parameters for sanitation workers in summer. Firstly, a field test of sanitation workers was conducted, the environmental parameters (solar radiation intensity, dry bulb temperature, wind speed and relative humidity) and mean skin temperature were measured, and the thermal comfort and work willingness were investigated via a questionnaire. Then, the mathematical programming method was adopted to obtain the safety limits and danger limits of the environmental parameters. Finally, the thresholds of the outdoor heat stress indexes, namely, the environmental stress index (ESI), discomfort index (DI), heat index (HI) and relative strain index (RSI) were obtained. The results indicate that a high solar radiation intensity, high dry bulb temperature, high wind speed and low humidity will cause more adverse effects on sanitation workers in summer. The safety limits of the solar radiation intensity, dry bulb temperature, wind speed and relative humidity are 182.42 W/m(2), 28.42 degrees C, 0.15 m/s and 71.35%, respectively, and the danger limit value are 876.86 W/m(2), 34.98 degrees C, 1.27 m/s and 36.59%, respectively. In addition, the safety limit values of the ESI, DI, HI and RSI are 27.08 degrees C, 26.23 degrees C, 31.71 degrees C and 0.13, respectively; and the danger limit values are 29.60 degrees C, 27.84 degrees C, 36.21 degrees C and 0.24, respectively. This paper can provide a study method and reference data for the work arrangement and health protection of sanitation workers in summer.