A modified physiological strain index for workplace-based assessment of heat strain experienced by agricultural workers

Background: As global temperatures rise, increasing numbers of individuals will work in hot environments. Interventions to protect their health are critical, as are reliable methods to measure the physiological strain experienced from heat exposure. The physiological strain index (PSI) is a measure of heat strain that relies on heart rate and core temperature but is challenging to calculate in a real-world occupational setting. Methods: We modified the PSI for use in field settings where resting temperature and heart rate are not available and used the modified physiological strain index (mPSI) to describe risk factors for high heat strain (mPSI ≥ 7) experienced by agricultural workers in Florida during the summers of 2015 through 2017. mPSI was calculated for 221 workers, yielding 465 days of data. Results: A higher heat index (β = 0.185; 95% CI: 0.064, 0.307) and higher levels of physical activity at work (0.033; 95% CI: 0.017, 0.050) were associated with a higher maximum mPSI. More years worked in US agriculture (−0.041; 95% CI: −0.061, −0.020) were protective against a higher maximum mPSI. Out of 23 workdays that a participant experienced a maximum mPSI ≥ 7, 22 were also classified as strained by at least one other measure of high heat strain (core temperature [Tc] >38.5°C, sustained heart rate >(180 − age), and mean heart rate > 115 bpm). Conclusions: This study provides critical information on risk factors for elevated heat strain for agricultural workers and suggests a practical approach for using PSI in field-based settings.