Use wet bulb globe temperature, not heat index, to protect workers

A 30-year-old farm worker in North Carolina died last September on a day that was fairly typical for the state. Temperatures were in the mid-90s and the heat index, which includes humidity, was 96 degrees F. This index is often referred to as the “feels like” temperature, and is commonly used to gauge heat stress on the body.

But use of the heat index to gauge whether José Arturo González Mendoza and his fellow sweet potato harvesters could work safety grossly underestimated the lethal risks at play. Mendoza’s death, for which an eastern North Carolina farm was fined $187,509 in March for labor code violations, highlights the inadequacy of current occupational heat risk measures.

The industry practice needs to move away from the heat index and begin using the wet bulb globe temperature (WBGT), which assesses heat risk by factoring in temperature, humidity, wind speed, and solar radiation.

If supervisors at Barnes Farming had used the wet bulb globe temperature where Mendoza had been working, it would have indicated conditions well into “black flag” territory, which signals extreme danger and outdoor activities should immediately cease.

We aren’t the only ones who believe that the wet bulb globe temperature should be the gold standard metric for exertional heat stress. It was developed by the U.S. military, which currently uses it. Organizers of marathons, and many of the governing bodies that oversee sports, including the North Carolina High School Athletic Association, have also adopted it to determine if it’s safe to exercise outside on a hot day. But it’s not widely used in farming and other industries whose workers labor under the hot sun in the hottest times of the year.

The same standards used for soldiers and student athletes should also be used for workers.

The heat index is a measure of air temperature — measured in the shade — plus humidity. The wet bulb globe temperature is determined by measuring the temperature from a thermometer — in the sun — covered in a wet cloth. In addition to air temperature and humidity, this measure also incorporates wind speed and solar radiation. These two additional components are key factors in heat stress on the body. Increased wind speed can counteract the effect of high temperature and humidity, while low or no wind can heighten the effects of both. Solar radiation, either from little cloud cover or radiating from ground surfaces, affects how much heat is directed toward the body.

Adopting the wet bulb globe temperature standard for all occupational settings would involve federal agencies like the Occupational Safety and Health Administration (OSHA), which recommends its use but doesn’t mandate it, and state agencies with the power to enforce these recommendations. Given OSHA’s recent National Emphasis Program, which aims to improve heat safety, a large onus is on states and agricultural associations to unify efforts for consistent, effective heat exposure standards.

On the day Mendoza died, there had been warning signs all day if anyone had been looking for them using the right tool. According to a nearby weather station, the WBGT readings had been in both the yellow- and red-flag zones, signaling a need for longer rest periods and more hydration, before hitting the black-flag stage, in which all outdoor activities should stop. Following WBGT warnings could have prevented Mendoza’s death.

Despite its accuracy and utility, the wet bulb globe temperature has not become the default tool in occupational settings due to a combination of factors, including lack of awareness among industry leaders, the cost of the monitoring equipment, communication challenges about what wet globe bulb temperatures mean, and a resistance to changing existing practices. But the undeniable benefits it offers for worker safety make a compelling case for its widespread adoption.

While adopting advanced metrics like the WBGT for assessing outdoor working conditions is essential, it’s equally important to reinforce existing regulations that ensure safe work environments. A gap currently exists in worker protection and safety guidelines: specific measures for heat-related safety are notably absent. This means protection against heat exposure often falls under the broad General Duty Clause in Section 5(a)(1) of the Occupational Safety and Health Act. The vague language of this clause, while mandating a safe working environment “free from recognized hazards,” fails to explicitly address the nuances of heat exposure. This gap spans both federal oversight, such as OSHA, and state-level directives, contributing to a patchwork of protections that vary significantly by location, underscoring the need for unified, comprehensive guidelines that specifically address heat exposure risks.

A dual approach — enhancing environmental assessment with more sophisticated tools, such as the wet bulb globe temperature, and refining regulatory guidelines — would form the cornerstone of a comprehensive strategy to safeguard workers from the escalating risks of heat stress.

These measures must be complemented with robust education programs for employers and workers on the dangers of heat exposure and the importance of safety practices. Educating workers empowers them to proactively recognize and respond to heat stress. But such an effort faces a significant barrier: fear of retribution for reporting unsafe conditions. Many workers, wary of being reprimanded or losing their jobs, stay silent, allowing unsafe practices to persist unchecked. This underscores the need for a culture shift that encourages open dialogue about workplace safety.

Practical strategies to mitigate heat exposure, such as adjusting the timing of crop harvests to allow for evening or nighttime harvesting, face their own set of challenges. While shifting work hours could reduce heat-related risks, environmental conditions, such as the presence of dew — which increases the rate of spoilage, mold, and spread of disease — limit this flexibility, presenting yet another layer of complexity in safeguarding agriculture workers from the heat.

Mendoza’s death is not an isolated incident. It’s a warning sign of the growing dangers posed by climate change. As global temperatures rise, so does the frequency of extreme heat events, putting more workers at risk. Adopting the wet globe bulb temperature as the standard for occupational heat stress assessment is not only about preventing tragedies, but is also about adapting labor practices to the realities of a warming world.

Ashley Ward directs the Heat Policy Innovation Hub at Duke University. Jordan Clark is a senior policy associate at the Nicholas Institute at Duke University.

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