Seasonal Comparisons of Acute Kidney Injury, Heat Exposure, and Heavy Metal Exposure in a Sonoran Agricultural Population

Abstract

Background: Agricultural workers in Mexico endure physically demanding work in a hot environment, increasing the risk for kidney injury and impaired kidney function. They are also exposed to nephrotoxic metal(loid)s such as arsenic, cadmium, chromium, lead, mercury, nickel, and uranium from natural and anthropogenic sources. However, few studies have investigated how combined exposures of heat and metal(loid)s impact kidney injury and functioning. Objectives: To fill this research gap, we assessed the combined effects of exposure to metal(loid)s and heat on acute kidney injury (AKI) and kidney function over time. Methods: Biological samples, anthropometric measurements, and questionnaires were collected from a cohort of adult males working at a Sonoran grape farm and a control group of non-farmworkers in a nearby city. Acute heat strain was assessed using inner ear temperature and heart rate to calculate the physiological strain index (PSI). Urine samples were analyzed for toxic metal(loid)s, specific gravity, and neutrophil gelatinase-associated lipocalin (uNGAL), while blood samples were analyzed for creatinine levels to calculate estimated glomerular filtration rate (eGFR). Descriptive statistics were computed, linear regression models were used to assess the relationship between socioeconomic variables and metal exposure, and linear mixed effect (LME) and linear regression models were used to evaluate the impact of metal(loid)s and PSI on kidney injury and function. Additionally, we examined an interaction term between each metal(loid) and sampling period as well as each metal(loid) and acute heat (PSI) in each of the LME models. Results: Agricultural workers reported lower education rates (93.5% < high school vs. 19%) and pay (69% less than $100USD/month vs. 0%), higher rates of migration for work (100% vs. 24%), and a greater proportion of Indigenous language speakers (27% vs. 4.8%) compared to non-agricultural workers. Creatinine-adjusted uNGAL levels among agricultural workers rose in summer compared to spring (4.47 μg/g creatinine vs. 1.98 μg/g creatinine, p<0.01), and eGFR declined during this period (108.41 vs. 122.38 mL/min/1.73m², p<0.01). In mixed models, PSI was inversely associated with eGFR (β-coefficient: -2.87, 95% CI: -4.02, -1.72), though this effect was attenuated when time was included. Significant differences were found between non-farmworkers and farmworkers in unadjusted and specific gravity-adjusted geometric means (GM) of urinary arsenic, lead, nickel, and uranium. Among farmworkers, urinary uranium increased over time (GM: 0.05 vs 0.22 µg/L), while urinary arsenic (103.91 vs 83.33 µg/L), lead (2.37 vs 1.65 µg/L), and nickel (8.96 vs 5.81 µg/L) decreased. Metal(loid) concentrations in farmworker urine were significantly higher than those reported for Mexican Americans in the 2017-2018 United States National Health and Nutrition Examination Survey. We observed positive associations between urinary arsenic (β=0.35, 95%CI: 0.15, 0.55), cadmium (β=0.27, 95%CI: 0.14, 0.40), and chromium (β=0.54, 95%CI: 0.20, 0.88) with uNGAL, and a negative association between urinary uranium and eGFR (β=-2.45, 95%CI: -4.81, -0.08). These associations were attenuated when models were stratified by season and PSI tertile. Interactions between arsenic, cadmium, and chronic heat exposure exacerbated the effects on kidney functioning during the summer, but no interactions with acute heat stress were observed. Discussion: As climate warming intensifies, vulnerable populations, especially agricultural workers in hot climates, face increased heat-related morbidity. This study highlights the role of season and PSI in predicting kidney function, emphasizing the importance of considering heat as an exposure modifier in future environmental and occupational toxicity research. The findings contribute to the characterization of metal(loid) exposure in northern Mexico and may inform exposure modeling, mitigation strategies, and policy development in rural and agricultural settings. Ensuring rest, hydration, and recovery during high temperatures is essential to protect farmworker health and advance our understanding of potential etiological agents of CKD and AKI.