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

Abstract

Background: There is growing attention on occupational heat stress in Central America, as workers in this region are affected by a unique form of chronic kidney disease. Previous studies have examined wet bulb globe temperatures and estimated metabolic rates to assess heat stress, but there are limited data characterizing heat strain among these workers.

Objective: The aims were to characterize heat stress and heat strain and examine whether job task, break duration, hydration practices, and kidney function were associated with heat strain.

Methods: We used data from the MesoAmerican Nephropathy Occupational Study, a cohort of 569 outdoor workers in El Salvador and Nicaragua who underwent workplace exposure monitoring, including continuous measurement of core body temperature (T_c), heart rate (HR), physical activity, and wet bulb globe temperature (WBGT), over the course of three days in January 2018 - May 2018. Participants represented five industries: sugarcane, corn, plantain, brickmaking, and construction.

Results: Median WBGTs were relatively high (>27 °C) at most sites, particularly when work shifts spanned the afternoon hours (e.g., 29.2 °C among plantain workers). Sugarcane workers, especially cane cutters in both countries and Nicaraguan agrichemical applicators, had the highest estimated metabolic rates (medians: 299-318 kcal/hr). Most workers spent little time on break (<10% of the shift), as determined by physical activity data. Overall, sugarcane workers-particularly those in Nicaragua-experienced the highest T_c and HR values. However, a few workers in other industries reached high T_c (>39 °C) as well. Impaired kidney function (estimated glomerular filtration rate <90 mL/min/1.73 m²) was associated with higher T_c and HR values, even after adjustment.

Significance: This is the largest study to-date examining heat stress and strain among outdoor workers in Central America. Workers at sugar companies regularly experienced T_c > 38°C (76.9% of monitored person-days at Nicaraguan companies; 46.5% at Salvadoran companies). Workers with impaired kidney function had higher measures of T_c and HR.

Impact statement: This study examined levels of occupational heat stress and heat strain experienced among outdoor workers in five industries in El Salvador and Nicaragua. We characterized heat stress using wet bulb globe temperatures and estimated metabolic rate and heat strain using core body temperature and heart rate. Sugarcane workers, particularly cane cutters and Nicaraguan agrichemical applicators, performed more strenuous work and experienced greater levels of heat strain. Impaired kidney function was associated with higher heart rates and core body temperatures.

Keywords: exposure assessment; heat strain; heat stress; kidney disease; occupational health.

Fig. 1. Adjusted core temperature, heart rate, and metabolic rate, by job task and industry, relative to reference group.

Parameter estimates and 95% confidence intervals for job task in regression models for A median Tc, B maximum Tc, C median % HRmax, and D median metabolic rate, after adjusting for shift duration, median WBGT, and kidney function.