Policy-Relevant Causal Approach to Assessing the Impact of Occupational Heat and Airborne Particulate Matter Exposure on Acute Kidney Function in Guatemalan Sugarcane Workers

Background: Chronic Kidney Disease of unknown etiology is a growing health concern in low- and middle-income countries. Recurrent acute kidney injury has been implicated in the mechanistic pathway to chronic disease. While occupational heat stress is recognized as a potential contributor to kidney dysfunction among agricultural workers, the causal relationship between heat stress, core body temperature (Tc), and kidney function remains unclear. Methods: This was an observational study conducted in Guatemala following 148 male sugarcane workers across two six month harvests. Heat stress was measured using heat index (HI) and Tc with ingestible telemetric temperature pills. Particulate matter (PM) exposure was measured using personal breathing-zone samplers with a size-selective inlet for particles below 5 m (PM5). Kidney function was measured as the change in estimated glomerular filtration rate (eGFR) from pre- to post-shift. Two doubly-robust analyses were applied: exposure-response curves that describe kidney function across the observed range of each exposure, and modified exposure policies that estimate the change in kidney function under realistic reductions in HI, Tc, and PM. Results: The average daily HI was 37.4 {degrees}C (SD: 2.0) with an average Tc increase of 1.16 {degrees}C (SD: 0.48) per shift. Both HI and Tc were associated with declines in eGFR across the work shift. At an HI of 34 {degrees}C, the estimated average eGFR decline was about 2.5 mL/min/1.73 m^2 and was not statistically significant, while at 40 {degrees}C the decline was about 10 mL/min/1.73 m^2 and was statistically significant. A modeled policy reducing HI exposure by 5% improved eGFR change by 2.94 mL/min/1.73 m^2 (95% CI: 0.96, 4.91). No statistically significant association between PM exposure and eGFR decline was detected. Conclusion: Reducing workday heat exposure may mitigate acute kidney function decline. These findings support the development of policy interventions aimed at reducing external heat exposure and internal heat strain to protect kidney health. More research is needed to investigate the potential contribution of other environmental factors, including PM exposure.