Background <p>Agricultural workers are a highly exposed yet understudied group in the context of climate change.</p> Objective <p>This study filled the research gaps in assessing personal heat exposure, its association with immediate changes in heart rate variability (HRV), and intervention choices for agricultural workers.</p> Methods <p>Twenty-five participants were recruited from a pear orchard community, a rice seedling field, and a vegetable greenhouse farm in 2020 summer. Seven-day exposure of wet-bulb globe temperature (WBGT) was assessed for each participant with personal sensors for temperature, relative humidity, and solar radiation, as well as fixed-location monitoring for wind speed. These data were used as inputs for thermodynamic-principle-based Liljegren equations to calculate WBGT. Additionally, participants wore small medically certified HRV biosensors for 96 h.</p> Results <p>WBGT levels were 27.0 ± 1.7, 31.9 ± 3.7, and 32.6 ± 6.3 °C for the participants at work in the pear orchards, rice fields, and greenhouse farm, respectively. The highest percentages above 32.2 °C of the rice-field and greenhouse participants were 60.8% and 66.3%, respectively. Peak WBGT reached 48.2 °C. The ambient WBGT from a fixed-location monitor underestimated the mean exposure, maximum exposure, and percentages above the critical thresholds of 32.2 °C and 36 °C for outdoor unshaded participants. The evident interpersonal variations emphasized the importance/necessity of individual WBGT assessments. A 1 °C WBGT increase was statistically significantly associated with a 1.05–2.44% decrease in all HRV indicators, except low-frequency/high-frequency, and a 0.65% increase in HR. Wearing gloves, masks, and long garments may aggravate HRV responses, whereas short garments may be protective.</p> Significance <p>This is the first study applying personal environmental and biological sensors assessing WBGT and WBGT-HRV relationships of outdoor workers. The findings highlight the need for heat-health adaptation strategies, including personalized protective measures and WBGT-based warnings. The innovative methodology can be widely used for other outdoor workers.</p> <p></p> Impact <p>This study addresses critical knowledge gaps in occupational heat exposure research by employing innovative exposure and epidemiological methodologies to assess personal WBGT exposure and its association with HRV in agricultural workers. The findings highlight substantial discrepancies between ambient and personal WBGT levels. By integrating advanced sensors and the OSHA-approved Liljegren equations to obtain WBGT, this study establishes a scalable framework for researchers and policymakers worldwide to accurately assess heat exposure, exposure–health relationships, and intervention strategies. These insights are particularly valuable for protecting agricultural workers as climate change intensifies occupational heat stress, thus fostering resilience in vulnerable populations and promoting overall health.</p>