Introduction

Recent studies document the adverse effects of excessive heat exposure in occupational settings, recognising heat-related illnesses and injuries as a significant public health concern. This paper investigates the impact of California’s outdoor heat illness standard, adopted in 2005 to protect workers from excessive outdoor heat. The authors assess whether this policy led to measurable improvements in safety and reductions in injuries among workers with substantial exposure to outdoor heat, such as those in the construction, agriculture, and transportation industries.

Salient features of California’s heat standard

In response to heat-related fatalities among agricultural workers in 2005, California implemented emergency heat standards for outdoor workers in August 2005, making them permanent in 2006. The standard comprises six core components: access to water, access to shade, high-heat procedures, emergency response and heat illness prevention plans, acclimatisation protocols, and training requirements. Employers must provide at least one quart of free, cool, potable water per worker per hour. Shade must be available upon request and is mandatory when temperatures reach 80°F or higher during rest or meal breaks. High-heat procedures are triggered at 95°F or higher for specific industries, requiring supervisor accessibility and mandatory 10-minute breaks every two hours for agricultural workers.

Data

The study relies on the Workers’ Compensation Research Institute’s (WCRI) Detailed Benchmark/Evaluation (DBE) database, covering all workers’ compensation market segments across 32 states. The analysis focuses on the warmer months of May through October between 2002 and 2022. Maximum daily temperatures were obtained at the ZIP Code Tabulation Area (ZCTA) level from the PRISM Climate Group. The main analysis examines four industries: construction, agriculture, transportation (high-risk), and clerical and professional (control group). On hot days in the pre-policy period, the count of injuries was 0.183 in construction, corresponding to one injury every 5.5 hot days.

Empirical approach

The research utilises a triple-differences approach to isolate the effect of the heat standard from confounding factors, such as broader workers’ compensation reforms adopted in California during the early 2000s. The identification strategy compares injury counts before and after the heat standard, on hot versus cool days, and in high-exposure industries relative to those in clerical and professional occupations, which are typically unaffected by outdoor heat. This approach ensures that industry-wide changes in safety or non-industry-specific changes in the workers’ compensation system do not bias the estimates.

Results

The findings document robust and significant reductions in heat-related injuries. Declines in the number of injuries in high-risk industries ranged from 15–17 percent in construction, 19–25 percent in transportation, and 24–27 percent in agriculture. The effect of the heat standard was found to be more pronounced when outside temperatures were higher, with the likelihood of injury declining with each five-degree increase in temperature. Furthermore, the reduction in injury frequency in construction and agriculture was largest among younger workers aged 35 and below. Statistically significant effects were also observed in manufacturing, with a reduction of 8 percent, likely reflecting the proportion of manufacturing jobs performed outdoors.

Alternative approach: a comparison with Arizona

As a robustness analysis, the authors compared changes over time within each high-risk industry across California and Arizona. Arizona serves as a comparator state as it is adjacent to California with a similar climate and diverse industry mix but did not have a heat standard during the analysis years. This alternative specification reinforces the main findings for the construction and agriculture industries, showing similar percentage changes. Results for the transportation industry were more sensitive to this approach, potentially reflecting industry-wide changes in air-conditioning systems and weight exemptions for idle-reducing equipment adopted in Arizona but not California.

Potential remaining limitations

The study acknowledges several limitations, including the use of residential ZIP codes to approximate injury locations, which may introduce measurement error. Additionally, the estimates may capture both the effect of the heat standard and any reduced risk of injury due to workers reducing their hours in response to heat. The researchers also noted the ongoing issue of misclassifying employees as independent contractors. However, because similar effects were found when considering the rate of workplace injuries per 100,000 workers, the results are unlikely to be merely capturing an increase in worker misclassification.

Conclusion

The study provides clear evidence that targeted interventions, such as California’s heat standard, are an effective tool for improving worker safety in industries vulnerable to temperature extremes. The findings have significant implications for ongoing policy debates as other states, including Florida, New York, and Texas, consider the adoption of similar standards. The effectiveness of the standard in the most populous US state provides timely insights into the benefits of such policies and lessons applicable across diverse economic settings.