Introduction Farmworkers experience intense heat in California’s summer harvest. They are at heightened risk for heat related illness (HRI) especially if their intake of water is insufficient to maintain evaporative cooling and therefore protect core body functions.

Methods 288 Latino farmworkers throughout California were monitored across a work-shift in summer 2015. Weight was recorded before and after the shift in a minimum level of clothing to assess change in hydration status. Questionnaires were administered in Spanish.

Results All farm locations provided water for their workers as required by regulation. 226 (78%) of the participants lost weight and 44 (15%) lost ≥1.5% indicating clinically-significant dehydration. Despite mandatory annual training, only 38% knew ≥3 cups of water should be consumed/hour under hot conditions. No association was found between knowledge of hydration needs and action (drinking sufficient volume or frequency). Men were likely to drink a larger volume of water than women (P = 0.002), but not more frequently. Yet 20.6% of the men lost ≥ 1.5% of their body weight versus only 4.2% of women (P = 0.0003). Despite 209 (72.8%) stating they were thirsty to extremely thirsty when working in hot weather, 276 (96.8%) believed they were drinking enough to keep safe. Using multivariate logistic regression, the risk of losing ≥1.5% weight was associated with being male (OR = 5.4, 95% CI: 1.8–16.6), with a higher knowledge of hydration needs (OR = 2.4, 95% CI: 1.2–„4.7) and of borderline significance with any type of piece work (paid by quantity rather than by time) OR = 2.2 95% CI (0.95–4.8).

Conclusion Latino farm workers are not recalling/using knowledge of hydration needs to prevent HRI at work. The reasons behind this disconnect need further exploration to create strategies to reduce dehydration and HRI risk.