Kidney Disease Among Agricultural Workers

the Synergist

Is Extreme Heat Affecting Kidney Function in Outdoor Workers?

BY JAIME BUTLER-DAWSON, LYNDSAY KRISHER, MIKE VAN DYKE, AND LEE S. NEWMAN

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An unusual form of chronic kidney disease of “unknown” cause has grown in prevalence over the past three decades across agricultural communities in Latin America and South Asia. As described in the New England Journal of Medicine, tens of thousands of premature deaths due to this disease, which has been coined chronic kidney disease of unknown etiology (CKDu) or Mesoamerican nephropathy (MeN), have occurred in and around the pacific coastal areas of Latin America. CKDu is not related to established chronic kidney disease risk factors such as hypertension, diabetes mellitus, obesity, or advanced age. It predominantly affects males of working age, mainly without chronic health conditions, who perform strenuous labor in hot, tropical areas. CKDu has also affected women and children residing in the same agricultural communities.

The disease results in a progressive decline in kidney function, but the natural history of CKDu is still not known. Early stages of the disease often go unrecognized and can progress to end-stage kidney failure requiring dialysis or a kidney transplant, both of which—for affected populations in low-income settings—are often inaccessible and unaffordable, if the treatments are even available. CKDu has adversely impacted already vulnerable communities and has overwhelmed healthcare systems in these countries.

MULTIPLE CONTRIBUTING FACTORS Many hypotheses regarding the disease’s cause have emerged from the available evidence and have been extensively debated. An article published in 2019 in the New England Journal of Medicine suggests that multiple occupational, environmental, and social factors—in particular, heat stress, dehydration, heavy metals, agrochemicals, medications, and infections—may contribute to the incidence of CKDu. Growing evidence indicates that a cycle of heat stress and dehydration during intense labor in hot climates is likely one of the contributors to the epidemic of CKDu occurring in these regions, particularly in Latin America. Repeated episodes of heat stress followed by dehydration can cause repeated kidney injury, which over time may lead to CKDu. Furthermore, heat stress and dehydration can increase the reabsorption of toxicants due to low renal blood flow.

One working population in Latin America that is especially affected by CKDu is sugarcane workers. During the harvesting of sugarcane, workers are exposed to a number of potential causes of kidney injury and CKDu, including dehydration due to heat stress and high physical demand as well as muscle damage and the use of nonsteroidal anti-inflammatory drugs (NSAIDs) during extreme labor (studies published in BMJ Open Sport & Exercise Medicine and the American Journal of Kidney Diseases discuss these exposures). An article published in 2017 in BMJ Open Sport & Exercise Medicine that examines acute kidney injury associated with endurance events states that NSAIDs may have harmful effects on renal blood flow. Rehydrating with sugary beverages, which is common among sugarcane workers, could also contribute to kidney injury, although evidence is lacking in epidemiologic studies.

OUR RESEARCH In our research at the Center for Health, Work and Environment (CHWE) within the Colorado School of Public Health, we work with sugarcane cutters and other field workers (seeders and seed cutters, for example) employed at a large agribusiness based in Guatemala to assess and improve their health, safety, and well-being. Two articles about our research are freely available to read in the International Archives of Occupational and Environmental Health and the Journal of Occupational and Environmental Medicine.

In dusty fields and extremely hot and humid conditions, the sugarcane cutters use machetes to manually cut and stack a daily average of six tons of sugarcane. These workers exert high physical effort in the direct sun and humidity, placing them at risk for heat-related symptoms during the six-month harvest season, which runs from November to May. In a recent study, we measured field wet bulb globe temperatures (WBGT) across the harvest. Commonly used in occupational settings, WBGT is a way to measure the heat stress on the body and incorporates air temperature, humidity, radiant heat, and wind speed. For the majority of the study days, the daily average WBGT was above 30 degrees Celsius. U.S. OSHA guidelines recommend that if WBGT is greater than 30 C, workers performing very heavy labor should work no more than 15 minutes of every hour and should rest the other 45 minutes (see the OSHA Technical Manual chapter on heat stress). These guidelines would be impractical and economically untenable for most agricultural industries in Latin America.

Sugarcane workers in the region could also be exposed to agrochemicals and metals at work and in their surrounding communities. Several agrochemicals such as glyphosate and paraquat and metals like arsenic, cadmium, uranium, lead, and nickel are commonly found in the soil and water sources in CKDu-affected agricultural communities in the region due to human activity or naturally from soil or bedrock. (An article published in 2012 in Experientia Supplementum further discusses heavy metal toxicity and the environment.)

In many countries, sugarcane fields are burned prior to workers manually harvesting the cane, and the burning occurs while workers are cutting cane in neighboring fields. This practice emits particulate matter and other pollutants that can contribute to adverse health effects and may contribute to kidney injury. Two articles published in 2012 address the health effects of burnt sugarcane harvesting: one in PLOS ONE examines the cardiovascular effects among workers in Brazil and the other, which appears in the October 2012 issue of Science of the Total Environment, focuses on particulate matter exposure and its effects on lung function, oxidative stress, and urinary 1-hydroxypyrene.

Amorphous silica is naturally present in sugarcane leaves and can convert to crystalline silica when burned, as noted in studies published in the Journal of Environmental Monitoring and Atmospheric Environment. During a pilot study in Guatemala (see our article in the International Journal of Environmental Research and Public Health), we collected personal air samples from study staff simulating cane cutting and found amorphous silica in the air samples as well as trace amounts of crystalline silica. While the health effects of crystalline silica are widely recognized, the impacts of amorphous silica are not fully understood. (An article published in September 2017 in Archives of Toxicology discusses the limitations of researchers’ understanding of the toxicity of amorphous silica nanoparticles.) Research published in Food and Chemical Toxicology and the Journal of Environmental Pathology, Toxicology, and Oncology, has shown that amorphous silica may elicit or augment an inflammatory response that drives kidney injury.

In addition to conducting research, our team develops and implements practical, actionable interventions to benefit these workers now, even though the cause of CKDu is still “unknown.” We know that during intense labor and exposure to heat stress, renal blood flow decreases, which can put stress on the kidneys and increase the risk of kidney injury. In collaboration with the agribusiness, we have implemented several strategies and interventions to optimize work conditions with the aim of reducing dehydration and heat stress and thereby decreasing kidney injury across the work shift and CKDu among the workers. Our interventions have included encouraging workers to drink electrolytes in addition to promoting water, rest, and shade; educating workers on ways to reduce heat stress and dehydration as well as avoiding anti-inflammatory medications, which are harmful to the kidneys when dehydrated; and performing enhanced kidney health surveillance and clinical monitoring for workers during the season.

CONCERNS FOR U.S. WORKERS U.S. workers in hot and dusty trades have similar occupational and environmental exposures compared to workers at risk for CKDu. They also share similar social and economic characteristics, including poverty and harsh working conditions. As temperatures rise and heat waves become more frequent in the U.S., more workers will encounter extreme environmental heat as an occupational health threat. Researchers have documented kidney injury among agricultural workers in California and Florida in studies of physiological responses to heat exposure (see the articles published in Occupational & Environmental Medicine and the Journal of Occupational and Environmental Medicine in the list of resources below), which suggests an emerging U.S. public health concern.

Sugarcane fields being burned prior to harvesting the cane. Photo courtesy of CHWE Research Staff.

Workers covered in black soot after their work shift. Photo courtesy of Amanda Walker.

Rows of harvested sugarcane at the end of the work shift. Photo courtesy of CHWE Research Staff.

It is unknown whether current NIOSH and ACGIH heat stress and hydration guidelines protect workers from kidney injury as these guidelines were not developed to address concerns related to kidney health. ACGIH has established various Threshold Limit Values (TLVs) and best practice guidelines for working in hot environments. The guidelines provide permissible heat exposure limits for various workloads (light, moderate, or heavy) and work/rest regimens. The current TLVs are intended to protect nearly all healthy heat-acclimatized workers at heat stress levels that do not exceed the TLV. Emerging lines of evidence suggest that many workers in the U.S. already labor under conditions that place them at risk of kidney disease (an article published last year in Environmental Research Letters describes the growing heat risk for U.S. agricultural workers). In the absence of updated guidelines that include consideration of kidney-related health outcomes, employers, employees, labor organizations, and government agencies should step up efforts to ensure kidney health of the workforce.

INCREASED WORKER PROTECTION It is imperative that we understand the effects of working in hot conditions on kidney function so that we can advocate for effective health and safety training and public health policies that prevent kidney function loss and protect workers from the health effects of heat exposure. There is a need for increased interdisciplinary collaboration in order to increase surveillance, research, clinical care, and interventions to protect workers. Heat stress prevention strategies should consider a combination of engineering controls, administrative and work practice controls, and personal protective equipment to manage heat stress. Examples include monitoring the temperature at the work site; providing water, electrolytes, and rest breaks in a cool, shaded area; allowing time for workers to acclimatize to working in hot environments; modifying work schedules as necessary to reduce workers’ exposure to heat; and training workers on and monitoring them for the signs and symptoms of heat illness, including acute and chronic kidney injury.

Industrial hygienists and occupational health professionals can play an important role in advocating for and implementing heat stress prevention strategies and conducting research on exposures, practices, and interventions to decrease heat stress and reduce the risk for CKDu in vulnerable workforces worldwide.

JAIME BUTLER-DAWSON, PHD, MPH, is an epidemiologist in the Department of Environmental and Occupational Health and the Center for Work, Health and Environment at the Colorado School of Public Health.

LYNDSAY KRISHER, MPH, is a researcher and instructor in the Department of Environmental and Occupational Health and the Center for Work, Health and Environment at the Colorado School of Public Health.

MIKE VAN DYKE, PHD, CIH, is an associate professor in the Department of Environmental and Occupational Health and the Center for Work, Health and Environment at the Colorado School of Public Health.

LEE S. NEWMAN, MD, MA,

is a distinguished university professor in the Department of Environmental and Occupational Health and Department of Epidemiology at the Colorado School of Public Health. He is the director of the Center for Health, Work and Environment. He is also distinguished professor of medicine in the Division of Pulmonary Sciences and Critical Care Medicine at the University of Colorado Anschutz Medical Campus.

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RESOURCES

American Journal of Kidney Diseases: “CKD of Unknown Origin in Central America: The Case for a Mesoamerican Nephropathy” (March 2014).

Archives of Toxicology: “Toxicology of Silica Nanoparticles: An Update” (September 2017).

Atmospheric Environment: “Production of Potentially Hazardous Respirable Silica Airborne Particulate from the Burning of Sugarcane” (July 2008).

BMJ Open Sport & Exercise Medicine: “Acute Kidney Injury Associated with Endurance Events—Is It a Cause for Concern? A Systematic Review” (2017).

Environmental Research Letters: “Work Adaptations Insufficient to Address Growing Heat Risk for U.S. Agricultural Workers” (August 2020).

Experientia Supplementum: “Heavy Metal Toxicity and the Environment” (2012).

Food and Chemical Toxicology: “Five-Day Inhalation Toxicity Study of Three Types of Synthetic Amorphous Silicas in Wistar Rats and Post-Exposure Evaluations for Up to 3 Months” (October 2007).

International Archives of Occupational and Environmental Health: “Evaluation of Heat Stress and Cumulative Incidence of Acute Kidney Injury in Sugarcane Workers in Guatemala” (2019).

International Journal of Environmental Research and Public Health: “A Pilot Study to Assess Inhalation Exposures Among Sugarcane Workers in Guatemala: Implications for Chronic Kidney Disease of Unknown Origin” (2020).

Journal of Environmental Monitoring: “Generation of Crystalline Silica from Sugarcane Burning” (July 2010).

Journal of Environmental Pathology, Toxicology, and Oncology: “Inhaled Amorphous Silica Particulates: What Do We Know About Their Toxicological Profiles?” (2001).

Journal of Occupational and Environmental Medicine: “Hydration Status, Kidney Function, and Kidney Injury in Florida Agricultural Workers” (May 2018).

Journal of Occupational and Environmental Medicine: “Risk Factors and Mechanisms Underlying Cross-Shift Decline in Kidney Function in Guatemalan Sugarcane Workers” (March 2019).

New England Journal of Medicine: “Chronic Kidney Disease of Unknown Cause in Agricultural Communities” (May 2019).

Occupational & Environmental Medicine: “Heat Strain, Volume Depletion and Kidney Function in California Agricultural Workers” (2017).

OSHA Technical Manual: Section III: Chapter 4, Heat Stress (2017).

PLOS ONE: “Burnt Sugarcane Harvesting - Cardiovascular Effects on a Group of Healthy Workers, Brazil” (September 2012).

Science of the Total Environment: “Burnt Sugarcane Harvesting: Particulate Matter Exposure and the Effects on Lung Function, Oxidative Stress, and Urinary 1-Hydroxypyrene” (October 2012).