Heat stress and heat strain among outdoor workers in El Salvador and Nicaragua

Occupational heat stress poses significant risks to outdoor workers performing physically demanding labor, with climate change expected to increase ambient heat. In Central America, particularly among sugarcane workers, a high burden of chronic kidney disease of non-traditional origin has heightened concern about heat exposure. Prior research has largely relied on WBGT and estimated metabolic rates, showing frequent exceedances of protective thresholds, but there are limited data on physiological heat strain (core temperature and heart rate) in this region. This study aimed to characterize both environmental heat stress and physiological heat strain across multiple industries and to evaluate the roles of job task, breaks, hydration, and baseline kidney function in relation to heat strain in workers in El Salvador and Nicaragua.

Previous Central American studies frequently reported high WBGTs in sugarcane work, often exceeding protective thresholds (e.g., maximum WBGTs >28–35 °C; mean WBGTs >30 °C), and high heat index risk days. Salvadoran sugarcane cutters spent much of the workday above 50% of estimated maximum HR while exposed to WBGTs >26 °C. Studies in the U.S. and Mexico using ingestible core sensors have shown workers' core temperatures occasionally or regularly exceeding 38 °C and increasing ≥1 °C across shifts, though differences in tasks, protections, hydration, comorbidities, and climate limit extrapolation to Central America. This study addresses gaps by directly measuring both heat stress (WBGT) and heat strain (Tc, HR) in multiple industries within the MANOS cohort.

Design and population: Data were from the MANOS longitudinal occupational cohort of 569 male outdoor workers in El Salvador and Nicaragua, monitored across three (usually consecutive) workdays between January–May 2018. Industries included sugarcane, corn, plantain, brickmaking, and construction. Sugar workers were recruited from two Salvadoran and three Nicaraguan companies. Environmental monitoring: WBGT was recorded every minute during shifts using TSI QUESTemp 46 meters with wind speed attachment, mounted ~1 m above ground near participants and moved as needed. Two thermometers were used simultaneously when available, and readings were averaged. The first 10 minutes were removed for device stabilization; data were smoothed with a 20-minute rolling average. Effective WBGT (WBGTeff) included a 0.5 °C clothing adjustment for Nicaraguan agrichemical applicators wearing coveralls. Heat index was calculated from dry temperature and humidity using NWS methods. Personal monitoring: Core body temperature (Tc) was measured continuously during shifts using ingestible CorTemp sensors with a wearable data recorder collecting every 10 seconds. Due to variable ingestion timing relative to shifts, Tc data were extensively quality-checked, with implausible values removed and LOESS smoothing (25% span) applied; algorithms flagged and removed portions exhibiting the “bouncing ball” effect or unrealistic slopes. Heart rate (HR) was recorded beat-to-beat using Polar H7 chest straps, transmitted to ActiGraph devices; HR data were LOESS-smoothed (10% span). Maximum HR (HRmx) was estimated as 220 − age; %HRmx was computed per minute. Physical activity was measured at ≥30 Hz via ActiGraph hip-worn accelerometers; vector magnitude (VM) counts were used to classify breaks/low activity (VM <150 counts per minute) and estimate energy expenditure (kcal/min) using the 2011 Freedson VM3 equation combined with the 1998 Williams Work-Energy Equation; metabolic rate (kcal/hour) was averaged across shift minutes. Height and weights (pre- and post-shift each day) were measured; average baseline weight was used in metabolic rate and NIOSH REL calculations; pre/post weight differences were not used for sweat loss assessment. Biological samples: Blood was collected pre- and post-shift on Day 3 (or Day 1/2 for some brick workers). Serum creatinine (IDMS-traceable) was analyzed in national labs (Nicaragua) and at Quest Diagnostics (USA) for eGFR estimation using CKD-EPI; a subset retested at Quest confirmed minimal inter-lab differences. Questionnaires: Administered at enrollment and post-shift to capture workday characteristics (timing, breaks, hydration including water and electrolyte solution consumption, medications, PPE, symptoms) and job tasks (later categorized, e.g., harvesting). Data processing and exclusions: Device data were cropped to shift times; implausible values (e.g., HR <30 bpm; Tc <32 °C) set missing. Person-days with >50% missing for a metric were excluded for that metric (HR n=123, physical activity n=40, WBGT n=141, Tc n=60). Overnight shifts (n=48 person-days, 2.8%) and person-days with <50% of shift monitored were excluded from models. Heat stress thresholds: NIOSH Recommended Exposure Limits (RELs) for WBGT were computed per participant using average body weight and estimated average metabolic rate. Minutes and percent of shift above REL were calculated. Statistical analyses: Associations between job task, hydration (water, electrolyte solution), break duration (percent shift VM<150), and baseline kidney function (pre-shift eGFR categories: <60, 60–90, >90 mL/min/1.73 m2) with maximum Tc were examined using multivariable linear regression; participants with eGFR <60 were excluded from most models except kidney function effect models. Mixed-effects models (random intercept and random slope for day) assessed determinants of median %HRmx during shifts. Covariate adjustment sets included industry/company, job task, shift duration, age (for kidney function models), consumption of other beverages, median WBGT, mean metabolic rate, and percent shift VM<150, as specified. Models for electrolyte solution were restricted to Nicaraguan sugar workers. Analyses used SAS 9.4 and R 3.6.1.

- Environmental heat: Median WBGTs were high at most sites (>27 °C), notably plantain (29.2 °C) and construction (28.9 °C), and sugar sites SUGAR-E2 (28.9 °C) and SUGAR-N1 (28.3 °C). Maximum WBGTs reached 32.4–34.6 °C at several sites. - Heat stress relative to REL: Person-days with WBGTeff above REL for ≥25% of shift were most frequent at SUGAR-N1 (79%), SUGAR-E2 (35%), and SUGAR-N3 (28%); 0% among brick workers. - Work intensity: Estimated median metabolic rates were highest among Nicaraguan sugarcane workers (e.g., SUGAR-N1 median ~402 kcal/hr; SUGAR-N2 and N3 ~291–292 kcal/hr) compared with other industries (e.g., construction 175 kcal/hr; plantain 103 kcal/hr; corn 102 kcal/hr; brick 175 kcal/hr). - Break time: Most workers spent little time on break; median percent of shift VM<150 was ~5–7% at most sugar companies; higher in construction (~19%) and plantain (~21%). - Core temperature (Tc): Sugarcane workers—especially in Nicaragua—had the highest Tc measures. Tc >38 °C for ≥5 minutes occurred on 73–81% of person-days at Nicaraguan sugar companies and 52% at SUGAR-E2; extreme Tc >39 °C occurred in all industries except corn. After excluding sensors swallowed <3 h pre-shift, maximum Tc up to 40.1 °C (SUGAR-E1). Nicaraguan sugar workers spent substantial portions of shifts with Tc >38 °C (medians 16–29%) and had the largest cross-shift Tc increases (0.98–1.3 °C). - Heart rate: Median %HRmax was highest among Nicaraguan sugarcane workers (61–66%) and Salvadoran harvesters (~60%), with over a third of person-days at SUGAR-N1 having ≥25% of the shift above 75% HRmax. Cross-shift HR increases were largest at Nicaraguan sugar sites (e.g., +62 bpm at SUGAR-N1). - Hydration: Reported water intake was generally 3–4 L/shift; higher at SUGAR-E2 (median 5.5 L) and SUGAR-N3 (8.0 L). Electrolyte solution consumption occurred mainly among Nicaraguan sugar workers (median ~0.6–2.0 L/shift). - Kidney function: Lower baseline eGFR was associated with higher heat strain. In adjusted models, eGFR <60 and 60–90 mL/min/1.73 m2 were associated with higher maximum Tc (+0.20 °C and +0.11 °C vs >90, respectively) and higher median %HRmax (+1.7 and +2.9 percentage points vs >90). - Determinants of heat strain: Increased percent of shift on break (per +10%) was associated with lower median %HRmax (−1.5 percentage points; 95% CI: −2.1, −0.85). Water and electrolyte solution consumption were positively associated with median %HRmax in adjusted models (+0.46 and +1.2 percentage points, respectively), and electrolyte solution was positively associated with maximum Tc (+0.15 °C per liter; 95% CI: 0.04, 0.26). Water consumption and percent time at low activity were not significantly associated with maximum Tc after full adjustment. - Scope: This is the largest study to date of heat stress and strain among Central American outdoor workers; sugar company workers frequently experienced Tc >38 °C (76.9% of monitored person-days in Nicaragua; 46.5% in El Salvador).

Findings demonstrate substantial occupational heat stress and physiological heat strain among outdoor workers, with the greatest burden among sugarcane workers—particularly Nicaraguan cane harvesters and agrichemical applicators who exhibited the highest metabolic rates, high WBGT exposures relative to individualized RELs, and the highest Tc and %HRmax. Despite some workplace protections (hydration policies and electrolyte provision), workers spent limited time on breaks, and many experienced Tc exceedances and high cardiovascular strain. The association between lower baseline eGFR and higher Tc and HR suggests impaired kidney function may exacerbate thermoregulatory challenges, potentially creating a cycle where impaired renal function increases heat strain and risk of acute kidney injury. Break time was associated with reduced HR strain, supporting rest as a protective measure. Positive associations of electrolyte solution and water intake with HR and of electrolyte solution with Tc likely reflect confounding by workload/work intensity and possibly by urine concentrating ability or other unmeasured factors, rather than harmful effects of hydration. Results underscore the importance of task-specific risk (e.g., harvesting and agrichemical application) and support the hypothesis that chronic occupational heat exposure contributes to health risks, including kidney injury, in this region.

This study provides comprehensive characterization of heat stress (WBGT, REL exceedance) and heat strain (core temperature and heart rate) among outdoor workers in multiple industries in El Salvador and Nicaragua. Sugarcane workers—especially Nicaraguan harvesters and agrichemical applicators—performed the most strenuous work under hot conditions and experienced frequent, sometimes extreme, elevations in Tc and HR while spending relatively little time on break. Workers with impaired kidney function exhibited higher heat strain. These findings identify worker groups at highest risk and support strengthening heat illness prevention measures (e.g., task-specific pacing, shaded rest breaks, engineering controls) and monitoring. Future research should link measured heat strain to health outcomes (e.g., acute kidney injury), refine protective WBGT thresholds and screening guidance for this population, and evaluate multi-component interventions (hydration, rest, shade, work-rest cycles) on both heat strain and kidney function.

- Monitoring covered only three days (fewer for Tc) per worker, and industries were monitored sequentially, not concurrently, introducing potential temporal and climatic variability. - The presence of the study team may have altered shift duration or tasks; observed sugarcane cutter shifts were likely shorter than typical, making estimates conservative. - WBGT monitoring was limited or distant during brick oven shifts due to equipment constraints and many oven shifts were overnight, potentially underestimating environmental heat during peak periods. - Energy expenditure was estimated from hip-worn accelerometry, which may misclassify workload; arm-intensive tasks (e.g., cane cutting) may have been underestimated. - Ingestible Tc sensors were not always swallowed sufficiently early; data required extensive cleaning, and ingestion timing varied by site. - Hydration measures were self-reported, introducing misclassification. - Overnight shifts (2.8% of person-days) and person-days with <50% monitored data were excluded from models, which may affect generalizability.