In a California farming region, researchers are mapping rural heat to protect farmworkers

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In the summers, the sky is jet black when Raul Cruz arrives at this Imperial Valley sugarcane field to start his day. He chops, cleans and bundles the crop, taking heed as the sun rises. It's hard work, but so is starting at 4 a.m., even though he knows it's the safest thing when temperatures in this California desert frequently soar into the triple digits.

“We just have to because we need to beat the heat," said Cruz, who’s worked here for 15 years. They finish work by 9 or 10 a.m. to avoid the risk of heat stroke, he added, but when heat starts creeping up around 8 a.m., "mentally, it's stressful."

The hot climate that makes this Southern California region a farming powerhouse is also what makes it dangerous for farmworkers, who are increasingly vulnerable to rising temperatures due to greenhouse gas emissions from burning coal, oil and natural gas. Researchers from San Diego State University are working to understand the health consequences of heat stress on farmworkers and where heat is most extreme in this rural landscape. They hope their findings can lead to a better understanding of rural heat islands, identify gaps in research and help develop interventions that better protect them in the face of climate change.

“Workers could potentially be dying or having some serious issues," said project leader Nicolas Lopez-Galvez, assistant professor in the School of Public Health at SDSU. “It’s better to start acting sooner."

Since the start of the 20th century, California temperatures have increased almost 3 F (about 1.7 C), according to state and federal data. Warming has accelerated, and seven of the state’s last eight years through 2024 were the warmest on record. While all areas of the state have warmed, Southern California is heating up about twice as fast as Northern California.

Ana Solorio, an organizer with the farmworker advocacy group Líderes Campesinas that is working with researchers, remembered feeling “suffocated” in the Coachella Valley summer heat when she was a farmworker. "With the humidity, it felt awful," said Solorio, who's lived in the Imperial Valley for more than 30 years. The heat was so intense she didn’t return for another season, preferring instead the cooler winter harvesting months of lettuce in the Imperial Valley.

“This (heat) can cause a lot of harm to their health," she said.

Researchers are trying to understand how farmworkers' heat stress might vary depending on the crops, the season and the number of breaks they take.

Over the past two years, they've collected year-round data from some 300 farmworkers. Body sensors measure things like core body temperature and heart rate while they work. Elsewhere in the fields, environmental monitors measure the day's temperature, humidity, wind speed, sun angle and cloud cover, also known as the wet-bulb globe temperature, considered the best metric to understanding heat stress. Using satellite imagery along with historical and current wet-bulb globe temperature data, researchers are mapping areas of extreme heat, particularly in the Imperial and Coachella valleys.

Researchers are learning that ground level crops can expose workers to higher heat levels compared to tree crops, for example, but it also depends on their harvesting months. In the summers, farmworkers who prepare fields for planting or help maintain irrigation systems are also more exposed.

Rural heat can vary based on things like tree cover, proximity to a body of water and empty fields, which may be hotter. “It creates this island where people might be living or working that are higher in terms of heat stress compared to other places,” said Lopez-Galvez.

Bordered by the Colorado River to the east, the Salton Sea to the northwest and Mexico to the south, the Imperial Valley is home to hundreds of thousands of acres of farmland and produces billions of dollars in agricultural production. It grows two-thirds of winter vegetables consumed nationally and provides thousands of jobs. From 2023 to 2024 alone, about 17,579 migrant and seasonal farmworkers were employed in Imperial County, according to the state.

It's also extremely hot. In a given year, there are about 123 days with temperatures over 95 F (35 C), often exceeding 110 F (43 C) in August and early September, according to calculations by Sagar Parajuli, research scientist and adjunct faculty with SDSU's geography department. The county has one of the largest Latino populations and the highest number of heat-related illnesses among workers than anywhere else in the state.

Some of their data analysis has already been published.

One study found that irrigating crop fields in the Imperial Valley reduced the wet-bulb globe temperature on summer days, thanks to the cooling effect of evaporating water. But on summer nights, the opposite occurred: irrigation increased the wet-bulb globe temperature as humidity spiked. Irrigation also heightened heat in nearby urban and fallow areas adjacent to crop fields due to moisture transport.

“It is a concern because an elevated nighttime temperature restricts the ability of farmworkers to cool down,” said Parajuli, the study's lead author. “So they can’t recover from the heat stress they could be accumulating from the daytime."

Through this research, the authors were able to recommend how frequently farmworkers should take rest breaks to protect themselves from heat stress, based on how often wet-bulb globe temperatures exceed safety thresholds across seasons and work shifts. While California has heat rules, they're not strictly enforced, he added.

“We realized that farmworkers are not getting enough rest breaks, and also there are no clear policy guidelines in terms of heat-related rest breaks," he said.

Lopez-Galvez said they plan to continue their research in California's Central Valley and hope to expand it into Yuma, Ariz. and other parts of the Southwest.

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