In the first revision in 30 years of its criteria document on workers’ exposure to heat and hot environments, the National Institute for Occupational Safety and Health of the Centers for Disease Control and Prevention removed ceiling limit recommendations for acclimatized and un-acclimated workers, but has left untouched its recommended exposure and alert limits.

“It was determined that the current RALs [Recommended Alert Limits] for un-acclimatized workers and RELs [Recommended Exposure Limits] for acclimatized workers are still protective for most workers,” NIOSH said in the document released in March. “Most healthy workers exposed to environmental and metabolic heat below the appropriate NIOSH RALs or RELs will be protected from developing adverse health effects.”

Occupational exposure to heat can result in injuries, disease, reduced productivity, and death, NIOSH said. In hot environments and extreme heat, workers run the risk of experiencing heat stress. Other non-fatal illnesses include heat stroke, heat exhaustion, heat syncope, heat cramps, and heat rashes. According to the Institute, heat also increases the risk of workplace injuries, such as those caused by sweaty palms, fogged-up safety glasses, and dizziness. At risk individuals include outdoor workers and workers in hot environments, such as firefighters, bakery employees, farmers, construction workers, miners (particularly surface miners), boiler room workers, and factory workers.

Although the research agency left the RALs and RELs intact, it did away with ceiling limit recommendations for acclimatized and un-acclimated workers. “In fact, many acclimatized workers live and work in temperatures above the ceiling limits without adverse health effects,” NIOSH stated as justification for its decision. Acclimatization refers to physiological changes that occur in response to a succession of days of exposure to environmental heat stress, which reduce the strain caused by this stress and enable a person to work both with greater effectiveness and with less chance of heat injury.

This revision includes additional information about the physiological changes that result from heat stress; updated information from relevant studies, such as those on caffeine use; evidence to redefine heat stroke and associated symptoms; and updated information on physiological monitoring and personal protective equipment and clothing that can be used to control heat stress. The document also lists more resources than before on worker and employer training. The Institute noted that more research is needed, and added that two newer research areas are the effects of climate change on workers and how heat stress affects the toxic response to chemicals.

As the days grow warmer, outdoor workers who are exposed to hot and humid conditions are at greater risk of heat-related illness. While the recommendations in the NIOSH document are not mandatory, some states, including California, require employers to take steps to prevent heat illness in employees. California’s Heat Illness Prevention Regulations require that employers develop and implement written procedures for addressing heat illness prevention, train employees and supervisors, and provide adequate water and shade.