Excessive Heat’s Impact on Workers and Employers

By Hon. Susan V. Hamilton, Former Assistant Secretary and Deputy Commissioner, California Workers’ Compensation Appeals Board

No matter the source of your media consumption, it seems that the topic of heat is everywhere. Stories about excessive heat have become ubiquitous. For example, on December 5, 2023, US News & World Report released an article calling 2023 the hottest year on record. (Smith-Shoenwelder, C., 2023 Tops Hottest Year on Record, US News & World Report, 12/5/23). The account, which was based on findings from the European Space Agency’s Copernicus Climate Change Service (Copernicus), reported that 2023 was the warmest year in recorded history with average temperatures exceeding preindustrial levels by more than 2.5 degrees Fahrenheit. But just six months later, Copernicus released new data showing that May 2024 was the 12^th consecutive month during which average global temperatures continued to surpass all observations since 1850, with average global temperatures rising 2.9 degrees Fahrenheit above preindustrial levels. (Dance, S. A Year of Record Global Heat Has Pushed Earth Closer to Dangerous Threshold, Washington Post, 6/5/24).

But that’s not all. On August 19. 2023, NBC News (Silva, D, Hot Classrooms are Impairing Student Learning and Health Amid Record Hot Year, Teachers Say) reported that longer periods of hot weather, particularly in regions of the United States that have not historically experienced multiday heat waves, coupled with aging buildings without adequate cooling systems, are making it difficult for teachers to teach and putting students at even more risk. A recent headline in The Washington Post read, “Where Temperatures Will Be the Highest During Next Week’s Brutal Heat Wave: Record breaking heat could affect millions with heat indexes over 100 reaching as far north as Canada’s Hudson Bay.” (Cappucci, M., The Washington Post, June 14, 2024)

Since the 1970s in the United States, record setting daily temperatures across the country have become more frequent than low temperatures, and climate models predict an increase in the frequency, severity, and length of excessive heat over the coming decades. (Extreme Heat | U.S. Climate Resilience Toolkit.pdf, www.heat.gov). Extreme heat events are one of the leading weather-related causes of death in the United States. (Id.) In 2023, heat was responsible for more weather-related fatalities than hurricanes, tornados, and flooding combined. (https://www.weather.gov/hazstat/) Outdoor workers, including those in agriculture, landscaping, construction, public safety, mail and delivery services, are especially vulnerable, but they aren’t the only ones. Workers in manufacturing, warehousing, and, yes, even teachers in older school buildings that lack adequate air-conditioning systems are equally at risk.

A recent study conducted by the Workers’ Compensation Research Institute (WCRI) examines how excessive heat impacts work injuries. That study, The Impact of Excessive Heat on The Frequency of Work-Related Injuries (study) (Negrusa, S., Fomenko, O, and Thumula, V., WCIRB, 24-18, 5/2024, https://www.wcrinet.org/reports/impact-of-excessive-heat-on-the-frequency-of-work-related-injuries) finds strong and robust evidence that excessive heat increases the frequency of work injuries. The study’s findings are important because they underscore the importance of work safety, protection, and prevention against the increasingly frequent occurrences of excessive heat throughout our country. Of significance, the study reports that between 1981 and 2021, there has been a clear increase in the number of days with temperatures above 90°F and 100°F, not just in the South and Midwest, but also in the Northeast. Assuming this trend continues, the US workforce will be exposed to more days of excessive heat.

A Brief Word About Heat’s Impact Upon the Body

Before diving into the study and its findings, a basic understanding of how excessive heat can impact the human body will add important context. Normally the human body regulates its temperature and cools itself by sweating. But exposure to heat above normal levels can reduce the body’s ability to regulate physiological processes. That is, sweating may not be enough to cool the body during periods of excessive heat. When body temperature increases above normal levels, it can have detrimental effects on heart rates, respiratory rates, and blood pressure. Such effects can lead to fatigue, cramps, heat exhaustion, heat stroke, syncope, and even death. Excessive heat exposure can also include the risk of cardiovascular, respiratory, or kidney failure. Additionally, excessive heat can affect a worker’s ability to conduct cognitive, perceptual, or psychomotor tasks by reducing a worker’s attention and speed of reaction, which can lead to a higher probability of work-related injuries such as falling off a ladder. The study emphasizes that these types of injuries are attributable to excessive heat but are often recorded in data as having other causes. Further, excessive heat is known to exacerbate existing health problems such as asthma, kidney failure and heart disease.

It is possible to adapt to excessive heat, and through adaption (often referred to as “acclimatization”) the effects of excessive heat can be mitigated. Over time, the body’s repeated exposure to heat can lead to physiological changes that allow the body to better endure activities performed in hot conditions. Some people are more vulnerable to excessive heat than others. Factors such as age and pre-existing conditions can make a person more in peril from excessive heat.

The Objective of the Study

The main goal of the study was to measure the extent to which excessive heat increases the incidence of work-related injuries by considering both direct (i.e., heat exhaustion) and indirect (i.e., impaired cognitive function) heat-related injuries in different regions within the United States. The inclusion of indirect heat-related injuries, such as falling off a ladder due to impaired motor function, is significant because past studies have largely focused only on direct heat-related injuries.

One secondary goal of the study was to assess whether there is a variation in how excessive heat increases the frequency of work-related accidents in the different regions of the country. Another goal was to ascertain whether the effects of excessive heat on the frequency of work injuries is more pervasive in certain industries and on certain injury types.

The Data and Methods Used

First, the study limited its focus to 24 states: Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Massachusetts, Minnesota, Missouri, New Jersey, New York, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, and Wisconsin. These 24 states represent about half of the workers’ compensation benefits paid in the United States.

Second, the study period was identified as the years 2016-2021; the months of May-October; and the typical working days of Monday through Friday.

Next two main data sources were used. The first data source was WCRI’s Detailed Benchmark Evaluation (DBE), which includes all aspects of the workers’ compensation insurance market (i.e., voluntary insurance, self-insurance, state fund, and residual market). The DBE is recognized as among the most complete workers’ compensation claims databases. For example, it contains information on the day and zip code when a given work-related injury occurs. The other data source used contains information collected by the National Oceanic and Atmospheric Administration (NOAA) that reflects the daily maximum temperature by zip code. The study’s authors collapsed the zip-code day-level temperature to the county level and factored in the employed population in each zip code from data maintained by the U.S. Census Bureau.

Maximum daily temperatures were classified into nine categories: below 65°F, 65-70°F, 70-75°F, 75-80°F, 80-85°F, 85-90°F, 90-95°F, 95-100°F, and above 100°F. Based on OSHA recommendations, the study used the range of temperatures between 65-70°F as the reference category.

Finally, the study merged the daily maximum temperatures to the daily injury counts and injury rates per 100,000 workers at the county day-level for the 24 states.

The Findings

The NOAA daily temperature data predictably revealed that peaks in temperature occur primarily in the months of July and August. Similarly, that data showed a higher concentration of colder days (below 65°F) in the Midwest and Northeast relative to the South, and a higher concentration of hotter days (above 90°F) in the South than in the Northeast and Midwest.

Finding 1: Impact of Maximum Daily Temperature on Work Injuries

Using the reference point of 65°F-70°F, as temperatures increase beyond that value, the injury counts also increase. When the daily maximum temperature exceeds 90°F, for example, the incidence of work-related injuries increases by 5-8 percent. This finding is consistent with the findings from previous studies.

Finding 2: Impact of Maximum Daily Temperature by Region on Work Injuries

Next, the researchers considered how the effect of excessive heat might very by region. It compared the findings of the 12 Southern states (Arkansas, Delaware, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas, and Virginia), with the findings of the 7 Midwest states (Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, and Wisconsin), and with the findings of the 5 Northeastern states (Connecticut, Massachusetts, New Jersey, New York, and Pennsylvania). Not surprisingly, the comparison showed the largest effects of excessive heat in the South. When the maximum daily temperature increases above 90°F, the work-injury frequency rate increased by 9-11 percent. This finding is consistent with an important finding from an earlier study that hot days appear to be more harmful in a warmer climate. This observation suggests that there may be limitations on a person’s ability to adapt to excessive heat; that is, even acclimatization may have its limits during periods that are hotter than usual. The study’s authors characterize the finding of a larger impact of excessive heat in the South as noteworthy since the number of days with excessive heat in the South increased substantially over the past decades and are expected to increase in the future as well.

While the South was the leader in the number of days with excessive heat precipitating an increase in work injuries, it was not the outlier. When the maximum daily temperature in the Northeast reached 90°F to 100°F, the number of work injuries increased by 8 percent. In the Midwest when the maximum daily temperature reached 80°F to 95°F, the incidence of work injuries increased by 2 percent.

Finding 3: Impact of Maximum Daily Temperature by Industry on Work Injuries

The study looked at the effects of excessive heat on construction workers. Like other workers who perform their jobs primarily outdoors (e.g., agriculture, landscaping, mail delivery, public safety), the data showed that the effect of excessive heat on construction workers is substantially larger than the overall effect on all workers in general. For construction workers, when the daily temperature exceeds 90°F, the likelihood of a work-related injury increases to 14-20 percent relative to days when the temperature is in the 65°F-70°F range.

While the findings regarding the effects of excessive heat on construction workers are not surprising, another finding was. The study also looked at clerical and professional workers. These workers work indoors, often in air-conditioned environments. Nonetheless, when temperatures exceeded the range of 70°F to 90°F, the work injury incidence rate was 5 percent. The study offered several hypotheses. First, that in regions where excessive heat is rare, office buildings and other indoor facilities may not have air conditioning. Another hypothesis is that the cooling systems in office and similar buildings are not robust enough to keep pace with the outdoor temperature. Further, these primarily indoor workers likely have less opportunity to adapt to excessive heat than those workers who routinely work outdoors. The NBC news story on the effects of excessive heat on school classrooms referenced previously pointed out that many school buildings in the Northeast lack air-conditioning systems altogether or have older systems that can’t provide the cooling required by higher-than-normal temperatures.

Indoor work other than clerical and professional can expose workers to excessive heat. For example, workers who work near a furnace or bakers who work near ovens routinely deal with exposure to heat. When outdoor temperatures become increasingly hot, the effects on these workers can be deleterious.

The study also considered the effects of excessive heat on workers in industries such as healthcare, manufacturing, service, trade, and transportation. When temperatures were in the range of 80°F to 100°F relative to a day in the range of 65°F to 70°F, the likelihood of work-related injuries increased to the range of 4-7 percent. The study did note that jobs in manufacturing, service, trade, and transportation might involve combinations of both indoor and outdoor work, but the study data did not allow for drilling down to the occupational level to determine the relative amount of direct versus indirect heat exposure.

Finding 4: Impact of Maximum Daily Temperature by Injury Type

The study used two categories of injuries: traumatic injuries and soft tissue injuries. Traumatic injuries encompass fractures, dislocations, contusions, and lacerations. Soft tissue injuries included sprains and strains. As temperatures increase, the frequency of traumatic injuries increases relative to soft tissue injuries. This finding is said to be consistent with prior studies.

In the Southern region, the effect of excessive heat was greater than in other regions. On days with maximum daily temperature above 80°F, the number of traumatic work-related injuries was 6-10 percent. The number of sprains and strains was in the same range. In the Midwest, excessive heat’s effect on the number of traumatic injuries was 3 percent on a day with maximum temperature in the range of 75°F to 95°F but was statistically insignificant for sprains and strains. Finally, in the Northeast, the study observed increases in the incidence of both traumatic injuries and sprains and strains, with the larger increases for traumatic injuries during periods of excessive heat.

The Study’s Shortcomings and Contributions

As with any research, the study had certain limitations. For one, while the claim level data sample used in the study was large, it was a convenience sample, not a representative sample of all work-related injuries, so the outcome variables employed for the analysis may not always be representative for the risk of injury in the local areas. Overall, however, the study’s authors were confident that such limitation would not drive the study’s overall results, and that the analysis employed provides a valid approximation of the impact of heat on the overall number of work-related injuries.

Another limitation was that the study did not include more populous Western states in the analysis. A further limitation was that a thorough review of direct heat-related injuries, such as heat stroke, was beyond the study’s scope.

Nonetheless, the study produced strong evidence that excessive heat increases the frequency of work-related injuries by 5-8 percent on a day when the maximum daily temperature exceeds 90°F relative to a day when the maximum daily temperature is between 65°F-70°F. Notably, the effect of excessive heat on the frequency of work-related injuries increased to 9-11 percent in the Southern region when the maximum daily temperature exceeds 90°F compared to a day when the maximum daily temperature is in the range of 65°F-70°F. Of particular significance, the study found that construction workers have a work-related injury frequency rate of between 14-20 percent when the maximum daily temperature surpasses 90°F relative to a day when the maximum daily temperature is in the range of 65°F to 70°F. Another important finding is that the impact of excessive heat is larger on traumatic injuries as opposed to sprains and strains.

What’s Next

The study is important because it documents the overall effect of excessive heat on both direct and indirect heat-related injuries based on more recent data as well as data from a large swath of the United States that was not previously studied. But the study’s authors admit that more research is needed. For example, often excessive heat and wildfires go hand in hand. Wildfires and the smoke they cause may have impacts on occupational health similar to excessive heat. This is one area that is ripe for further study. Other areas include the impact of excessive heat on other outcomes such as medical payments, indemnity benefits, and disability duration. Additionally, further investigation of direct heat-related injuries such as heat stroke could provide invaluable information on how workers are affected by or adapt to excessive heat.

Heat standards are another area for further study. Under the federal Occupational Health and Safety Act (OSHA) employers have a general duty to provide their workers with a workplace that is free from recognized hazards that are causing or likely to cause death or serious harm to their employees. This general duty includes heat-related hazards that are likely to cause death or serious bodily harm. On July 1, 2024, the U.S. Department of Labor issued a proposed rule that aims to protect millions of workers from the health risks of excessive heat. If the rule is approved, it will protect workers in indoor and outdoor workplaces. The rule would require employers to develop an injury and prevention plan to control heat hazards in workplaces affected by excessive heat. Employers would be required to evaluate heat risks and when heat increases risks to workers, employers would be required provide drinking water, rest breaks, and control of indoor heat. It would also require employers to provide a plan to protect new or returning workers unaccustomed to working in high heat conditions as well as to have procedures to respond and take immediate action to help a worker exhibiting signs and symptoms of a heat-related emergency. Once the proposal is published in the Federal Register, the public is encouraged to submit written comments. (https://www.osha.gov/heat-exposure/rulemaking)

If the proposed regulation is adopted, it would be the first major federal regulation to protect workers in the United States from the effect of excessive heat on the job. (Davenport, C. and Weiland, N., New Territory for Americans: Deadly Heat in the Workplace, New York Times, May 25, 2024).

Five states currently have heat standards in place under their state-based OSHA programs. Those states are California, Colorado, Minnesota, Oregon, and Washington. The study references a 2021 study that evaluated California’s heat standard for outdoor work and found evidence consistent with the possibility that it reduced the incidence of work-related injuries.

California’s heat standard is triggered at 80°F, and it requires employers to provide outdoor workers with shade, potable water, training to employees, and planning to both foster adaptions to heat and allow rescheduling of work to cooler periods of the day. (Cal. Code Reg., tit. 8, § 3395). On June 20, 2024, the California OSHA Standard Board approved a heat standard for indoor work. It applies to most indoor workplaces and is triggered when the temperature reaches 82°F. The standard has been submitted to the Office of Administrative Law, which has 30 days to review the proposal. If approved, the California OSHA Standards Board has requested that the standard become effective immediately. (See, Cal. Code Reg., tit.8, § 3996). Proposed legislation that would create a presumption of work-related injury to agricultural worker heat-related injury claims is currently pending in the California Legislature. Senate Bill 1299 (Cortese) creates a rebuttable presumption that a heat-related injury arose out of and occurred in the course of employment where an employer in the agricultural industry failed to comply with California’s OSHA heat standard.

Finally, the study endorses research as to whether the effects of humidity in conjunction with high temperatures might further increase the risk of work-related injuries.

Heat-Related Information for Employers and Employees

While the scientific evidence is clear that temperatures across the United States have been on the increase over the past several decades, our understanding of the impact of excessive heat on workers and their employers is only now coming to light, thanks in large part to research projects such as the WCRI study that is the subject of this article. Employers and their employees alike are alarmed by the challenge of how to work safely, both indoors and outdoors, during periods of excessive heat. Can a particular industry withstand modifications that may be necessary to ensure worker safety? What are the economic implications for employers? What might the health consequences be for their employees? How will compensation insurers respond to these challenges? All these important issues are fertile ground for additional research.

In the meantime, however, there are some tools that can be used by both employers and their employees. The National Integrated Heat Health Information System has put together a US Climate Resilience Toolkit on Extreme Heat. The toolkit provides information and tools to reduce heat-related illnesses and mitigate the effects of excessive heat. One of the tools is a mobile application for use on Android and iPhones. The application is a heat safety tool that can be used by employers and workers to calculate the heat index for a specific worksite. The application indicates the risk to workers based on the heat index and suggests actions to protect workers from heat-related injuries and illness. It is available in English and Spanish. (https://toolkit.climate.gov). Another important tool is a Heat Illness Prevention Guide published by OSHA. This resource includes information on employer responsibilities, information for employees, and links to a wide variety of resources on heat-related concerns, including heat illnesses. (https://www.osha.gov/heat)

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