Investigating the Impact of Heat Exposure Among Agricultural Communities in a CKDu Hotspot

Introduction

Climate change has emerged as a pressing global challenge, with far-reaching implications for human societies and ecosystems. Among its numerous consequences, increasing temperatures and heat waves have drawn attention due to their adverse effects on human health. As anthropogenic greenhouse gas emissions continue to escalate, the warming trend shows no signs of abating, leading to concerns regarding the impacts of ambient heat, especially on vulnerable populations. Particularly, agricultural workers, who are highly exposed to the changing environmental conditions, are susceptible to detrimental consequences.

Exposure to high ambient heat can lead to several health problems, including dehydration, heat stress, exhaustion, and stroke. These detrimental health effects are particularly worrisome in individuals with pre-existing health conditions.

Dehydration reduces blood flow to the kidneys, causing concentrated urine with reduced volume, darkened color, and a burning sensation during micturition.¹ Severe hyperthermia-induced volume depletion can lead to acute kidney injury (AKI). Repeated episodes of AKI can lead to irreversible, progressive kidney function loss or chronic kidney disease (CKD). CKD is one of the fastest-growing non-communicable diseases worldwide, with a particularly rapid increase in low and lower-middle-income countries.

Population clusters with higher than usual CKD prevalence have been reported in many parts of the world. This CKD of unknown origin (CKDu) is mostly encountered in rural agricultural communities. One such region is the coastal district of Srikakulam, Andhra Pradesh, India, known as Uddanam. The age and sex-adjusted CKD prevalence in this area is estimated at 18-22%, which is 2.5 to 3.3 times higher than the population CKD prevalence reported from other Indian regions.

Although the exact reason for the CKDu development is unknown, prolonged exposure to excessive heat stress combined with a heavy workload has been postulated as one risk factor,² especially among those working long hours in hot temperatures.³ Other postulated mechanisms include exposure to environmental contaminants like pesticides and silica.

Given that Andhra Pradesh has experienced repeated heat waves for 4-5 years, evaluating its effect on outdoor workers, with an emphasis on kidney function, provides a unique opportunity to understand if heat exposure is responsible for the high CKD burden in this region.

We conducted this preliminary study to assess ambient heat exposure’s impact on the working population’s health. We examined seasonal variations in heat exposure and their impact on kidney health among various Uddanam communities.

Materials and Methods

The cross-sectional study was conducted in 36 villages across six mandals (administrative units) in Uddanam after obtaining approval from the Institutional Ethics Committee of the George Institute for Global Health (IEC Ref no: P01009). The study was conducted per the guidelines of the Declaration of Helsinki for human studies. Prior approval was taken from the workplaces. Data were collected during the cooler (November 2020 - January 2021) and hotter seasons (September 2021 - October 2021 and May 2022 - June 2022) during the regular workday (8:00 am to 5:00 pm) [Supplementary File A].

The study included 18–75-year-olds who had worked at least six months. Patients with diabetes or hypertension were excluded. The workers’ heat exposures, perceived heat stress, measured heat strain symptoms, and self-reported health impacts were collected with informed consent. Blood test for kidney function was assessed in 30% of consenting subjects. HSI and renal function were measured (twice) in 73 workers to identify seasonal variation.

Results

A total of 596 participants, 358 and 238, during the hotter and cooler seasons, respectively, were evaluated. Of them, 60.1% were males and 39.9% were females. The age range was 18-75, with a median age of 42 years [Table 1]. The occupation distribution among males and females has been mentioned in Supplementary Figure 1. Illiteracy was higher among females (60.9%) than males. Males had significantly higher alcohol use (68.4%) and smoking (43.6%) [Table 1].

A total of 208 (58%) and 150 (42%) male participants had heavy and moderate workloads, respectively. About 81% and 19% of females had heavy and moderate workloads, respectively. Based on all observations (n=669), the TLV data categorized 455 (68%) as heat-exposed and the remaining (n=214, 32%) as unexposed.

Heat stress profiles

Figure 1 depicts WBGT (°C) exposures, ranging from 28.0°C in Kondavoru (poultry farm) to 41.6°C in Iddivanipalem (agriculture with direct sun exposures), with an average of 31.7°C ± 2.2°C during the hotter season. Cooler season WBGT varied from 19.6°C in Kusumpuram (agriculture) to 31.7°C in Haripuram (coir production exposed to ambient heat), with a 25.5°C ± 2.5°C average. Cashew boiling (3.0%; n=18/596), agriculture (14.0%; n=84/596), and construction (12.2%; n=73/596) had the most workers crossing the safe limit, followed by the coir industry 9.6%; n=57/596), fishing (9%; n=53/596), animal husbandry (5.9%; n=35/596), and stone quarry (5.7%; n=34/596). In the hotter season, nearly all workers were subjected to excessive heat and exceeded the safe TLV limit, whereas, in the cooler season, 16.7% of workers (n=43) were exposed to heat levels that exceeded the TLVs. 64.1% of participants experienced heat exposure above TLV, with 65.4% of males (n=234) and 62.2% of females (n=148) exposed. On the other hand, 34.6% of males (n=124) and 37.8% of females (n=90) remained unexposed, accounting for 35.9% of the participants. Based on the number of participants recruited, males had slightly higher exposure rates [Table 1 and Supplementary File C].

Supplementary File C

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Figure 1:

Attributed Wet Bulb Globe Temperature (WBGT °C) profiles across various Occupational sectors (2020-2022) in Uddanam, Andhra Pradesh. Note: A: Agriculture, B: Brick kiln, C: Construction, D: Animal Husbandary, E: Coir industry, F: Cashew boiling, G: Stone Quarry, H:Welding, I: Mahatma Gandhi Rural work, J: Kirana shop, K: Cashew cutting, L: Poultry farm, M: Primary Health Centre, N: Carpenting, O: Anganwadi, P: Fishing, TLV: Threshold limit value.

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Discussion

This is the first study assessing heat exposure in Uddanam, Srikakulam district, Andhra Pradesh, a heat wave and CKDu hotspot.¹⁹ According to literature from other countries with high CKDu prevalence, exposure to excessive ambient heat is a main explanation. CKD is common in Uddanam farmers, agricultural laborers, and outdoor occupations.²⁰ No systematic heat exposure assessment has been done.

WBGTs exceeded safe working TLVs for most outdoor workers. Season significantly affected HRIs and HSIs. The post-shift urine-specific gravity showed moderate to severe dehydration, as observed in hot job environments. Agricultural workers were more exposed to direct sunlight during the hottest months, while coir workers were at high heat-related symptom risk year-round due to intense workloads and poorly ventilated workspaces. Heavy manual workers exceeded safe TLV limit more often than moderate workers, regardless of season.²¹ Male participants had a higher water intake compared to females during the hotter season. This difference may be due to heavier physical workload. High heat, poor fluid intake, and heavy perspiration cause common HRI and dehydration symptoms.¹¹ Even the unexposed group and records from the cooler season showed excessive thirst, urine-specific gravity, and eGFR, which is not surprising considering that workload, less water intake, and inadequate workplace welfare facilities also affect the physiological strain. Heat exposure and exertion cause whole-body water changes,²² and body mass loss, which dehydrates and elevates urine-specific gravity,²³ which can compromise renal function with repeated exposures.² Swelling in the extremities, particularly during heat exposure, can be a fluid retention indicator, signaling dehydration, electrolyte imbalance, or early signs of heat-related illness. Rhabdomyolysis, hypotension, and renal damage can ensue from chronic dehydration.²⁴ Inaccessible potable water and inadequate sanitation in the workplace could increase heat-related health risks, including excessive thirst, by preventing female workers from drinking enough water to replace sweat lost due to heat and exertion and reducing urination.²⁵

Heat exposure is linked to reduced eGFR in other groups.²⁶ Heat exposure doubled the chance of kidney damage, and nearly half experienced HSI symptoms beyond the permissible range.²⁷ This study had many participants with eGFRs < 90 mL/min and < 60 mL/min. This could be due to constant heat exposure, frequent dehydration, or hyperosmolar stress, all of which could affect kidney function over time. In addition, socio-economic factors, poor hydration practices, and limited healthcare access could exacerbate underlying vulnerabilities.

Like this study, heat stress, heavy labor, and warmer seasons increased the renal function decline probability.^28-30 SwR, higher body core temperatures, and longer work hours can aggravate heat-induced tubular kidney injury.³¹ Increased proteinuria could be attributed to the physiological stress induced by heat exposure, which may lead to transient renal dysfunction that alters renal hemodynamics, including reduced renal blood flow and GFR, potentially leading to protein leakage in the urine. Hydration and work-rest balance can reduce renal disease risk.³² In-depth studies on occupational heat stress and kidney health risk reduction can help design safe work practices like water-rest-shade and improve workers’ welfare facilities.

Our study is notable for being the first to examine occupational heat exposures and renal health in high-CKDu endemic India. The systematic extensive WBGT monitoring, adjusted with TLV, season and physiological monitoring, and HOTHAPS questionnaire evaluation of heat-related symptoms, allowed a comparison between the jobs’ physical demands and heat exposure levels.

This study has many limitations. Preliminary screening was based on self-reported symptoms, which may not be accurate. Although the population was broadly representative, the number of participants varied between different sectors. We measured tympanic membrane temperature, which is not the most reliable method to monitor CBT. We measured the tympanic temperature three to five times to overcome this and averaged the results. This study primarily aimed to assess the extent of heat exposure and its physiological burden on workers. In other studies, CKDu has been suggested as a possible result of repeated heat stress. However, we only showed a high perceived symptom burden and physiological stress, which may make people more likely to develop kidney dysfunction over time, not proving any definitive causality claims. Another limitation is that this study emphasized “eGFR <90” to primarily be an indicator of physiological kidney response to heat rather than a standalone diagnostic criterion for kidney disease, which is exploratory at this stage. This study did not exclude individuals with pre-existing renal dysfunction, as the focus was on capturing occupational heat exposure’s broader impact across the working population. Future studies could consider stratifying or excluding such individuals based on their renal history to ensure more targeted analyses. Finally, this study is largely cross-sectional; repeat measurements (longitudinal study design) are needed to assess heat stress’s impact on overall and kidney health. These limitations notwithstanding, the study highlights important new findings and has preventive policy implications for the working population that is exposed to high-heat environments daily.

In conclusion, heat exposure, self-reported heat strain symptoms, and HSI measurements in the study population have close association. Heat stress and strain were stronger among exposed participants. Dehydration was common, indicating poor heat stress management and hydration practices. Heat stress and strenuous work in hotter seasons can lead to kidney function deterioration. This requires a holistic response to occupational heat stress involving home, local, and technical approaches. Larger epidemiological studies using kidney biomarkers are planned to understand CKDu etiology better. Comprehensive protective labor laws require more research and evidence to protect the health of those exposed to excessive heat.