Comparison of Shade Materials for Agricultural Worker Heat Exposure Mitigation

Agricultural workers are at high risk of morbidity and mortality from workplace heat stress. Shade contributes to reduced heat exposure by blocking solar radiation. Though several US states have adopted occupational heat rules that require shade, implementation of specific shade approaches is typically not specified in rules. We compared different shade materials for agricultural worker heat exposure mitigation to inform decision-making.

We conducted a quantitative field comparison study over three summer days in a real-world agricultural setting in Central Washington State. We examined the effect of five different shade materials on solar radiation and heat exposure, measured using Black Globe (BG) and Wet Bulb Globe Temperature (WBGT) sensors, respectively. We used multivariable linear mixed-effects regression to examine the effect of different materials on BG and WBGT while accounting for repeated measures and small cluster sizes. We also performed a qualitative assessment of shade preferences among agricultural partners, including workers and supervisors.

The mean (SD) dry air temperature across all study days was 28.3(1.3)°C [82.9(2.4)]°F. Shade materials reduced BG by 6.1-8.8°C and WBGT by 1.7-2.2°C, on average, depending on the material, compared to no shade. Compared to no shade, the largest reduction in BG (-8.81ºC) was seen for a hay tarp material (HAY: 95% Confidence Interval [CI]: -9.39, -8.23) followed by an 80% monofilament material (M80: -7.68°C; 95%CI -8.27,-7.10). For WBGT, we also found the largest reductions among HAY (-2.22°C; 95%CI -2.44,-2.00) and M80 (-2.07°C; 95%CI -2.29,-1.84). Comparing between shade materials, we did not find substantial differences in BG and WBGT. Ease of use and mobility of shade structures near work locations, in addition to effectiveness in reducing heat exposure, was identified in qualitative analyses as factors that may influence workplace shade implementation decisions.

In this study, we found that all studied shade materials significantly reduced BG and WBGT compared to no shade, but we observed no substantial differences in heat exposure mitigation between the shade materials themselves. For similarly performing shade materials, worker and supervisor preferences and practical considerations should be considered when selecting shade approaches in specific workplaces that are likely to be adopted and sustained.