Two research collaborations—one in Florida, the other in California—are illuminating a potential impact of climate change on occupational health. The two groups of researchers published papers this month on the relationship between heat strain, dehydration, and acute kidney injury among farmworkers. It’s a topic that has been explored among agricultural workers in Brazil (here), El Salvador (e.g., here, here) and Nicaragua (e.g., here, here). These two new papers focus on farmworkers in the U.S.

Jacqueline Mix and colleagues at Emory University and the Farmworker Association of Florida recruited 192 agricultural workers from Pierson, Apopka, and Immokalee, Florida for their study. The workers were employed during 2015 and 2016 at ferneries, in nurseries, and in field crops. Over the study period, the average heat index was 91°F (range 78°F to 99°F).

Workers provided baseline urine and blood samples, as well as pre-shift and post-shift samples of both. The urine samples were used to calculate “urine specific gravity” (i.e., a measure of hydration status used in outdoor settings such as with athletes and military personnel.) The blood samples provided data on creatinine and blood urea nitrogen (BUN) levels to assess each worker’s kidney function. The researchers defined acute kidney injury (AKI) using criteria developed by the International Society of Nephrology. Stage 1 AKI is defined as an abrupt (within 48 hours) reduction in kidney function.

Their findings, which are published ahead of print in the Journal of Occupational and Environmental Medicine, include:

• Each five degree increase in the heat index was associated with a 37 percent increase in the likelihood of acute kidney injury.
• About 3 percent of the workers had pre-shift USG values that indicated a clinically dehydrated state. In the post-shift period, 13 percent of workers met that threshold.
• Using the USG values, on a typical work day about 50 percent of workers were dehydrated before their shift, and more than 80 percent after their shift.

This last finding sticks in my head and raised lots of questions: How would a farmworker know if they are mildly or even moderately dehydrated? Using urine and blood samples in a study is one thing, but otherwise are the symptoms obvious? At the end of a workshift, would a typical worker know they were dehydrated?

What does it take to re-hydrate adequately before the next day’s workshift? Reading these studies, it seems that some farmworkers may go for days in some state of dehydration because safe recovery from it may not be obvious or undestood.

Mix and colleagues note that the prevalence of pre-shift dehydration is consistent with research involving workers employed in other hot conditions.

“Our findings, and that of other studies indicate that many workers start their work shift dehydrated, which warrants efforts to increase prevention in this population. Future research and outreach efforts should include educating workers about the importance of hydration post-work shift, to facilitate recovery after working in hot conditions.”

Antonio Tovar-Aguilar with the Farmworker Association of Florida (FWAF) pointed me to tools, such as urine color diagrams. FWAF uses this kind of tool in some of the safety training classes they offer to farmworkers. Tovar-Aguilar and I recognize that many farmworkers never receive safety training.

As part of the California Heat Illness Prevention Study at UC-Davis, Sally Moyce and colleagues examined kidney function and heat strain among 300 agricultural workers in California. Their latest paper was published this month in BMJ’s Occupational and Environmental Medicine. 

The California farmworkers were recruited in 2014 from 15 farms in the state’s Central Valley. The researchers fitted each worker with a heart rate monitor, measured changes in their core body temperature using ingestible probes, and used pre-shift and post-shift blood samples to assess each worker’s kidney function. The researchers, like those involved in the Florida farmworker study, defined acute kidney injury (AKI) using criteria developed by the International Society of Nephrology.

Among the findings reported by Moyce et al, 11 percent of the California farmworkers experienced an abrupt reduction in kidney function from pre-shift to post shift (i.e. Stage 1 AKI). They also examined the relationship between AKI and heat strain (calculated using core body temperature and heart rate.) Among the 182 male farmworkers in the study (64% of the participants), heat strain was associated with a 31 percent increase in the likelihood of AKI.

Moyce and her colleagues make an important observation that bears repeating:

“Agricultural workers are a vulnerable population, often undocumented, living in poverty, culturally and linguistically isolated, with reduced worker protections.

The development of AKI over the course of a work shift in this population may lead to further kidney damage, including chronic kidney disease, particularly because it is unlikely that workers know they are damaging their kidneys while working in the fields.”

Some growers or others may look at the results of these studies and blame farmworkers for failing to stay hydrated — “They should just drink more.”  Both research teams briefly address that possible reaction. Obstacles to staying hydrated include: proximity of fresh water to their work area; piece-rate pay rather than an hourly wage; the need to urinate more frequently; and reluctance to use toilets that are not close-by or are not considered safe from sexual assault.

The occupational health and safety community has a growing list of concerns related to the adverse consequences of climate change. Extreme weather events, including heat waves, is certainly on the list. The research described above helps illuminate a current health hazard for farmworkers that will only get worse with the changing climate.