Farmworker drinking water in the field
Photo credit: Joe Proudman, UC Davis

California Heat Illness Prevention Study Findings

Avoidable deaths and heat-related illnesses still occur among California farmworkers despite regulations from the California Occupational Safety and Health Administration (Cal/OSHA) and a campaign to encourage drinking more water and taking more rests in the shade. The California Heat Illness Prevention Study (CHIPS) aimed to provide new data to quantify the physiological and behavioral risk factors for heat related illness (HRI) and develop the findings to produce more effective workplace strategies to reduce risk.

Physical Indicators of Heat Illness

CHIPS was the first study to collect comprehensive physiological data to monitor HRI, including core body temperature and the intensity of work performed during a shift, from 587 individuals working on a wide range of farms throughout California. All the farms studied complied with Cal/OSHA HRI prevention regulations.

Nevertheless, nearly 8% of the workers were at risk of HRI (experienced a core body temperature ≥ 101.3°F) and 6.5% were at risk because their body temperature increased ≥ 2.7°F over a work shift. Of the workers who participated in the study: nearly 12% were dehydrated at the end of the day (lost water weight of ≥ 1.5% of body weight), over 12% suffered reversible acute kidney injury over the work day, and 50% said they had at some point experienced a heat illness symptom while working.

HRI symptoms are avoidable if risk reduction protocols are followed. When we accounted for both personal and environmental/work characteristics, the most important factors associated with higher core body temperature were the ambient environmental temperature and the intensity of the work performed during a shift or “work rate”.

Men were more likely to suffer elevated core body temperature, especially those working piece rate (paid by units of production rather than by the hour), irrigating, or working multiple outdoor tasks per shift.

The workers who performed tasks that were slower-paced and less physically demanding were at less risk of HRI. Risk increases with age as well as with the level of dehydration, while high body mass index (high body weight for height), clothing choice, and other modifiable factors did not affect the risk of HRI in the workers.

These findings provide scientific data to help answer the question about the severity of HRI risk and can be used to develop better recommendations for workplace practices in hot weather.

Preventative Behavior and the Effects on Economic Earnings

Focus groups held throughout the California Central Valley indicated that workers mostly know they should be drinking more and resting frequently in hot weather, but they are not following through on these protective measures. There are some cultural beliefs that need to be addressed (such as the possible negative effect of drinking cold water when a person is hot), but the main reason workers risk their heath is economic.

If workers stop work to drink more or visit the bathroom, they reduce their productivity. On the one hand, this will materially affect their earnings if they are being paid by the piece.

On the other, there is concern for their job security as workers are very aware their employer wants to have workers who can maximize productivity, no matter how hot the weather. These two factors suggest that though employers may encourage workers to drink and rest, their employees may not respond to the offer.

This explains why the current regulations have not been as effective as hoped, and point to improvements in both worker training and management that will reduce the risk of heat illness. The main audience for the new information will be occupational safety and health professionals, employer and worker organizations, insurers, and regulators.

Training Methods Focus Groups

Training workers to reduce the risk of HRI can still be effective, but currently workers mostly receive HRI prevention training in ways that are not optimal for their understanding or encourage compliance. Most workers have a 6th grade education or less, so text-based trainings may not be effective.

During focus groups, workers called for HRI prevention training to take place at the actual work site (not in a lecture setting), in an interactive fashion so that specific practical examples and strategies can be demonstrated, and consequently more memorable.

Analysis of the information gathered in the focus group sessions indicate that workers need encouragement to adopt protective behaviors from both the employer and their direct supervisor; and incentives to follow good health practices, not directives that will cause them loss of earnings.

New Training Methods

Utilizing findings from both components of the study, new comprehensive and interactive heat illness prevention training programs and manuals for farmworkers and supervisors were produced.

Farmworker trainings employ demonstrations relevant to the workers to both teach them how their bodies respond to the heat, and how drinking frequently helps them maintain their work rate.

Trainings include information to encourage workers to look after themselves and each other and checks to ensure workers retain critical information. These new trainings were piloted at a variety of farms, and positively evaluated by follow-up discussions after a few months.

A new train-the-trainer (ToT) program was developed to increase the capacity of the HRI prevention trainings. To date, WCAHS has trained 53 supervisors on delivering heat illness trainings to their workers. Each supervisor trained will return to their workplace and train on average over 166 workers using the interactive materials.

The ToT program, which includes interactive presentations and participatory group and individual activities, helps trainers evaluate whether their workers understand and retain the messages, how to reinforce the themes throughout the summer, and how to further encourage workers to practice safety in the heat. The participants received a workbook with the ‘lesson plans’, exercises, games, information, and other resources.


The findings from the CHIPS may be used to make significant reduction of the risk of heat illness. As summers are predicted to become increasingly hot, the conclusions and translation of findings from this study will only become more important to the 800,000 farmworkers in California.

Study results, both physiological and behavioral, have been published in peer-reviewed journals and presented at professional meetings and continuing education programs. Materials produced will be shared with Cal/OSHA and the rest of the agricultural community.

The translational aim of the study produced innovative, interactive trainings and material which will be published in Spanish and English on the Western Center for Agricultural Health and Safety website. Materials will be made available to the wider agricultural community through grower groups, the agricultural insurance industry, and worker groups. Parts of the material are specific to California, but the majority may be adapted for other agricultural areas across the USA where there are large numbers of immigrant Latino workers.

Click here for more information about upcoming Heat Illness Prevention Trainings.

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