Agricultural work entails long work hours, physical exertion, and exposure to an array of biomechanical, physical, and chemical hazards, including direct sunlight, high temperatures, and humidity (Nguyen et al. 2018). Due to these and other factors, farmworkers are one of the highest risk groups for heat-related illness (HRI). Mortality from HRI among agricultural workers in the United States was nearly 35 times more likely compared to other workers (Gubernot et al. 2015). The majority of farmworkers in the U.S. (61%) are Latino migrants, and adverse effects, like HRIs, can be more likely due to concurrent factors they experience (“National Agricultural Worker Survey” 2023; Castillo et al. 2021). These concurrent factors could include, but are not limited to; documentation status, fear of deportation, language/cultural barriers, and piecemeal pay (Castillo et al. 2021).

With increasing outdoor temperatures due to climate change (Ebi et al. 2018), accurate measures of workplace environmental exposures are needed to determine appropriate interventions to prevent HRI. Wet Bulb Globe Temperature (WBGT) serves as the preferred environmental heat metric for HRI prevention of all occupational groups (NIOSH 2016). WBGT measures dry air temperature, humidity, and radiant energy to calculate what thermal load a person is experiencing in direct sunlight, whereas heat index measures the thermal load in shaded areas (NIOSH 2016). Dally et al. (2020) observed the correlation of increasing WBGT to occupational injury; for every 1 °C increase in WBGT above 30°C (86°F), the mean count of daily recorded occupational injuries increased by 3%.

Crop fields uniquely contribute to environmental heat stress. Crops can transpire or lose water to the atmosphere from small openings on the leaf surfaces. Transpiration can increase humidity in the air, increasing the WBGT experienced by those working there (Souri et al. 2020). North Carolina farmworkers reported microclimates while laboring in tobacco fields, perceiving higher heat and humidity, which increased their thirst and need for water (Mizelle et al. 2022). Tall, dense crops may block the wind; thereby, increasing heat stress by inhibiting body cooling through sweat evaporation (California DIR 2024; NIOSH 2016; Osilla et al. 2022; Papadavid G and Toulios 2017).

While WBGT measurements have been collected in several farmworker HRI studies (Dally et al. 2020; Messeri et al. 2019; Mitchell et al. 2017; Vega-Arroyo et al. 2019; Wagoner et al. 2020), these studies were limited to one WBGT monitor or used data from local weather stations. Upon a literature review, no studies were found to have compared WBGT measurements from the middle of tall, dense crop fields to the field perimeter. Understanding more about the environments within crop rows that farmworkers are laboring in is an important step to effective prevention strategies and interventions against HRIs. The purpose of this study was to assess the heat stress exposure of workers within sweet corn and tobacco crops and to determine the differences of environmental heat stress within crop rows compared to field perimeter.

This observational study compared the WBGT measurements and corresponding heat stress levels inside fields of sweet corn and tobacco to field perimeters. Data were collected using heat stress monitors in fields where farmworkers could have labored. The study did not involve human subjects or animals and, therefore, was approved by the East Carolina University Institutional Review Board (IRB) as exempt.

Data were collected over 28 days for each crop, totaling 192 hours for tobacco and 225 hours for sweet corn. The results of paired sample t-tests are presented in Table 1 for corn and Table 2 for tobacco. These tests support that WBGT measurements were significantly higher within the crop row compared to the field perimeter for peak (11AM-2PM) and afternoon (2PM-5PM) times in tobacco. Figures 1 and 2 compare all WBGT measurements per monitor location and time of day for sweet corn and tobacco, respectively. The morning (8AM-11AM) and peak humidities in both tobacco and corn were also significantly higher at the field center. There was also a greater amount of unsafe risk levels and TLV^® exceedances within the crop row compared to the crop perimeter for most times in both crops.

This was the first study found to compare WBGT measurements from the within the crop rows of crop fields to the field perimeter, in fields of tobacco and corn. The WBGT was higher within the crop rows of sweet corn and significantly higher at the field center of tobacco. With only a few degrees difference in WBGT, the microclimate within the crop rows made a notable difference in the risk levels for HRI. If farmworkers labored within the crop rows during this time and in these locations of NC, 6 to 14% more work hours were deemed unsafe.

Significant differences were found in relative humidity in the field centers of corn and tobacco. High humidity could inhibit physiological processes the body uses to cool, leading to an unsafe rise in core body temperature. Crop transpiration and evapotranspiration could explain the high relative humidity at the field center. The study findings align with the perceptions of North Carolina tobacco workers of warmer and more humid microclimates within the crop rows (Mizelle et al. 2022).

Recently published was a similar study conducted in Washington State in 2022 (Flunker et al. 2024). These researchers also measured environmental heat stress indexes using QUESTemp°34 monitors in the center of crop rows and compared them to open field locations. Measuring among cherry trees about 14 ft (4.3 m) tall and among grape trellises 6 ft (1.8 m) tall, no significant difference in the WBGT was found within the crop rows versus open locations. Flunker et al. was limited to three days of data collection. The authors reported that the cherry and grape crops had row spacing of around 9 feet (2.74 m). The tobacco and corn crop rows in this study were spaced much closer (3 ft/0.9 m) which could reduce airflow leading to higher WBGT readings (Vanderwende and Lundquist 2016).

Several farmworker health studies have collected or reported WBGT (Dally et al. 2020; Messeri et al. 2019; Mitchell et al. 2017; Vega-Arroyo et al. 2019; Wagoner et al. 2020), which is considered more accurate for assessing heat stress than outdoor temperature or heat index (NIOSH 2016). WBGT data were either measured using one monitor outside the crop field or from a local weather station and most studies only collected data over a few days. Mitchell et al. (2017) and Vega-Arroyo et al. (2019) focused on low-lying crops such as squash, peppers, melons, etc., which are likely not tall enough to block wind. Dally et al. (2020) included WBGT measurements near sugarcane fields over several harvest seasons in Guatemala. Sugarcane grows 6 to 20 ft (1.8 to 6.1 m) tall. Although North Carolina has a subtropical climate (warm and humid) and Guatemala is in the tropics (very warm and humid), the average WBGT recorded in the Dally et al. (2020) study and the current study were almost identical, at 30ºC. While harvesting sugarcane occurs in the cooler months (November-April) and crops in North Carolina are harvested in the hottest months (June-September), crop harvesters in these areas could work in environments with similar HRI risk.

Personal monitors or sensors are an emerging technology. While most WBGT monitors are too large for individual use, there are a few handheld devices that can provide real-time measurement and reporting of WBGT (Carter et al. 2020). Individual heat and humidity sensors can be used to estimate WBGT and corresponding heat stress levels, like in Wang et al. (2019). Calculated WBGT from personal monitors clipped to worker shoes estimated 5.7% more work hours that exceeded ACGIH TLVs^® than data from the local weather stations; suggesting that using personal WBGT monitors may provide more accurate measurements than data from local weather stations. While many sensors on the market today are designed for athletes (Verdel et al. 2021), research is currently underway to develop sensors that are tailored to farm work (Kim et al. 2022).

While there are no occupational health and safety regulations that specifically address HRI prevention, five states currently have standards for occupational HRI protection. As of October 2021, OSHA initiated the rulemaking process to develop a federal heat-specific workplace standard; (“Heat Injury/Illness Prevention in Work Settings” 2024) however, the current administration has issued a freeze on any rules from executive departments and agencies (U.S. Office of the Federal Register, 2025). The federal standard would require employers to provide employees with HRI training, drinking water located close to the worksite, shaded rest breaks, and other HRI prevention interventions. As written in the Benefits section of the proposed law, OSHA projects compliance with this standard will substantially improve worker health and safety, “primarily through the reduction of occupational non-fatal heat-related injuries and illnesses (HRIs) and heat-related fatalities.” The proposed standard emphasizes that currently, employers may not take adequate steps to protect workers from hazardous heat, such as not providing breaks. Passage of the proposed standard may help combat the current likelihood of inadequate protection.

Overall, data collection was limited to one month for each crop and only over one agricultural season and therefore not generalizable to other crops or other locations. Farmworkers in NC commonly work 6 AM to 6 PM shifts, so data collection did not completely match their average daily exposure. Matched data were limited on some mornings, as data collection started at 9 AM instead of 8 AM. Rainy conditions, particularly in the afternoon, limited data collection on seven of the days. The monitors were fixed in place so may not represent farmworkers’ true environmental exposures. The sweet corn data were likely not reflective of an average June in NC. The month of June 2023 was mild for NC, with an average temperature of 74.6ºF, ranking the lowest temperature for June in the past 10 years (NOAA 2024). Because July is the hottest average month in NC, it was expected the number of hours above the TLVs to be greater in tobacco. The current study design makes it difficult to compare the two crops because WBGT measurements were not collected over the same period, but instead the data collection period likely reflected the highest risk periods for workers in these specific crops due to peak crop harvesting requirements.

Through several weeks of precise measurements of environmental heat metrics, this study supports that microclimates within crop fields could increase farmworker risk for HRI. This adds awareness of yet another health risk from farming and crop work. Farmworkers in North Carolina, particularly in tobacco crops, could spend the majority of their shifts working in environments deemed high or extremely high risk for HRI. Working in the morning hours could reflect lower WBGT and HRI risks. Workplace heat prevention plans should specify additional protections for work in dense, tall-growing crops.

Swift passage of the proposed OSHA heat standard could lead to improvements in worker health and safety. Legislators in states without occupational heat standards could pass temporary heat standards as the federal proposal could take several years to become regulation. Workplace heat prevention plans should specify additional protections while laboring in dense, tall-growing crops. Those additional protections could include fewer afternoon work hours inside the crop rows, longer or additional rest and water breaks, and wearing of loose-fitting clothing. Workplace heat stress monitoring to implement additional protections should be taken within the crop rows rather than outside of the crop rows. Further study of worker microclimate and worker heat strain could result in recommendations for additional rest time to accommodate the higher heat and humidity when working within the crop rows. Future microclimate studies could also provide more accurate HRI risk estimates by capturing breaks, movement, and changes in location. Education of employers and employees on WBGT readings should include reference ranges to emphasize that even small changes in the readout can denote substantial changes in HRI risk.