Longitudinal Metal(loid) Exposure Studies in Sonora, Mexico

Climate change poses a major threat to agricultural populations by driving up global temperatures and intensifying extreme heat events. Previous studies in the U.S. demonstrated that agricultural workers experience disproportionately high rates of heat-related illness, exhibiting a heat-related mortality rate nearly 20 times greater than that of other workers. Young outdoor laborers have shown high rates of chronic kidney disease (CKD) that cannot be explained by traditional risk factors such as age, hypertension, or diabetes. Rising core body temperatures, combined with high work intensity and dehydration, can lead to acute kidney injury (AKI). Occupational factors such as heat stress, strain, and dehydration are thought to be major contributors to CKD. In addition, agricultural workers are often exposed to elevated levels of toxic metals and metalloids, which are present in industrial materials as well as common pesticides and fertilizers. Several of these metalloids, including arsenic, cadmium, chromium, lead, mercury, nickel, and uranium, act as nephrotoxins (substances that cause damage to the kidneys) and may contribute to the development of both AKI and CKD. 

The goal of this study was to assess how simultaneous exposure to metal(loid)s and heat impacts kidney function over time. Agricultural workers at a table grape farm along the Arizona-Sonora border region in Northern Mexico participated in this study. Urine and blood samples were analyzed for metalloid content, and environmental conditions were recorded to measure heat strain. All urine samples contained measurable amounts of arsenic, chromium, lead, nickel, and uranium. Arsenic’s negative effects on kidney function and cadmium’s effects on uNGAL (a kidney injury marker) were stronger during the hotter summer season, suggesting that heat exacerbates the impact of toxic metalloids on kidney health. This study highlights the importance of detecting the routes of metalloid exposure in vulnerable populations and developing prevention strategies to keep these communities safe. Dr. Rietta Wagoner, a postdoctoral fellow at the Southwest Environmental Health Science Center’s (SWEHSC) NIEHS T32 Program, explains not only the independent effects of metalloids on kidney health but also their impact when coupled with heat exposure.   

What were the methods of data collection and analysis? 

Participants were recruited through collaboration with field supervisors and farm physicians. The research team obtained biological samples and administered questionnaires to a cohort of 77 adult males employed at a table grape farm along the Arizona-Sonora border region in Northern Mexico in 2019. Another group of 21 adult males engaged in non-agricultural office professions was recruited in Hermosillo, a neighboring city with similar climate conditions, during the same period. Eligible farmworker participants included those 18 years or older and planning to work on the farm until July, encompassing the grape harvest season. After the completion of the baseline workday, participants were interviewed in Spanish by trained research staff. The survey covered demographic details such as age, place of birth, education level, and primary language spoken. 

Environmental conditions were recorded during each sampling period. Air temperature, globe temperature, and wet-bulb temperature were logged at 1-min intervals using a continuous recording monitor. Globe temperature and wet-bulb temperature are both components of the combined Wet Bulb Globe Temperature (WBGT), which is a more comprehensive heat stress index that also includes dry bulb (ambient air) temperature to better represent human heat risk. To measure acute heat strain, the research team used the Physiological Strain Index (PSI). To calculate PSI, inner ear temperature and heart rate were measured and recorded at the beginning and end of the workday and combined to estimate PSI.  

Urine samples were analyzed for metalloid content using inductively coupled plasma-mass spectrometry (ICP-MS), an analytical technique that uses high-temperature plasma to separate and detect the ions in a sample to identify and quantify elements. In addition, they measured urinary neutrophil gelatinase-associated lipocalin (uNGAL), which is a biomarker produced by renal cells and immune cells in response to injury or inflammation that can also indicate AKI. Blood samples were also drawn via venipuncture by an on-site physician and transported to a local laboratory for same-day processing. Blood serum was used to calculate the estimated glomerular filtration rate (eGFR), which is a blood test that measures how well the kidneys are filtering waste and toxins from the blood. Higher uNGAL indicates more kidney injury, while lower eGFR indicates decreased kidney function.  

What were the results of this study? 

The participants were all males, averaged 29 years old, mostly from the states of Puebla and Chiapas, and 35% spoke an Indigenous language as their primary language. Over half (56 %) reported smoking and nearly all (97 %) reported consuming alcohol within the two days preceding the questionnaire. 

Kidney function declined from spring to summer, reflected in a decrease in mean eGFR over the four-month period, while uNGAL levels increased from spring to summer. Both of these results indicate a decrease in kidney function and increased prevalence of AKI. All urine samples contained measurable amounts of arsenic, chromium, lead, nickel, and uranium, with cadmium detected in 98.3% of the samples. Significant differences between baseline and follow-up measurements were observed for urinary lead, nickel, and uranium. A positive relationship was observed between urinary arsenic, cadmium, and chromium with kidney injury, meaning that with increased levels of these metalloids, markers of kidney damage also increased. In addition, an inverse relationship occurred between urinary uranium and kidney function, meaning that kidney function decreased as uranium levels increased.  

Unexpectedly, concentrations of arsenic, lead, and nickel decreased over time, suggesting reduced exposure as the season progressed. Many agricultural workers originated from southern states such as Chiapas, suggesting that exposure sources from their home states should be considered alongside those in their workplace of Sonora. Arsenic exposure can arise from both natural sources and anthropogenic activities such as mining, pesticides, and building materials. In contrast to other metals, urinary uranium concentrations increased over time, indicating potential exposure linked to residence or work activities in Sonora. Notably, the adverse effects of arsenic on kidney function and the associations between cadmium and kidney injury markers were more pronounced during the summer months.  

What is the importance of detecting metalloid exposure in agricultural settings? 

This study highlights the importance of monitoring metalloid exposure and evaluating the health impacts of heat in agricultural settings. Heat mortality rates in agricultural workers are 20 times higher than mortality rates among other working populations. When combined with exposure to toxic metalloids commonly present in pesticides and fertilizers, this population faces increased vulnerability to adverse health outcomes.  

This is the first study to simultaneously assess individual-level heat exposure and metalloid concentrations within a migrant agricultural workforce. The findings highlight not only the widespread presence of metalloids in biological samples but also the patterns of exposure that may inform how agricultural workers encounter these contaminants. Primary exposure pathways include ingestion of contaminated food or water and inhalation of particulates such as dust or smoke.  

Given these risks, several prevention strategies are recommended. Home-based cooling interventions and culturally tailored educational campaigns may help to promote awareness of kidney health and heat-related illness. These strategies support individual behavior change while addressing systemic vulnerabilities by empowering farmworkers and their families to advocate for safer living and working conditions. Additionally, the study supports the need for clinical screening initiatives that prioritize younger, at-risk populations, as well as intervention studies grounded in the industrial hygiene hierarchy of controls to reduce heat strain, improve hydration, and prevent exposure to nephrotoxic chemicals. Recognizing early signs of heat stress and kidney injury in combination with preventative measures may help to strengthen community resilience while improving long-term health outcomes in this essential but underserved population.