Extreme heat causes many times more injuries in the workplace than official records are recorded, and those injuries are concentrated in the poorest workers, new research shows, the latest evidence of how climate change is worsening inequality.

Hotter days not only mean more cases of heat stroke, but injuries from falls, collisions with vehicles, or improper use of machines, which, according to data, lead to an additional 20,000 work-related accidents per year in California alone. The data suggest that heat increases workplace injuries by making it difficult to focus.

“Most people still associate climate risks with sea level rise, hurricanes and forest fires,” said R. Jisung Park, professor of public policy at the University of California at Los Angeles and lead author of the study. “Heat is only just beginning to creep into consciousness as something directly harmful.”

The results follow record-breaking heat waves in the western United States and British Columbia in recent weeks that killed an estimated 800 people, aggravated forest fires, triggered power outages, and killed hundreds of millions of marine animals.

But the new data, described in a testimony to Congress on Thursday, underscores how heat waves can hurt people in unexpected ways too.

For example, extreme heat is a threat not only to outdoor workers but also to those who work indoors such as manufacturing facilities and warehouses. These additional injuries mean lower wages and higher medical bills for low-income workers in a variety of industries, widening the wage gap as the temperature rises.

In order to understand the connection between extreme heat and accidents at work, Dr. Park, together with his co-authors Nora Pankratz and A. Patrick Behrer, compiled California industrial accident reports from 2001 to 2018 and created a database of more than 11 million injuries with date and zip code.

The authors combined these reports with the maximum temperature values ​​for each day and location. It was then examined whether and how much the number of injuries had increased on days with higher temperatures.

This strategy offers a new way of estimating the number of heat-related injuries, rather than just relying on the causes of injury listed in workers’ compensation insurance reports. Those reports showed an average of about 850 injuries per year officially classified as being caused by temperature extremes, but the new data suggests the number is way too low.

On days when the temperature was between 85 degrees and 90 degrees Fahrenheit, the researchers found that the overall risk of workplace injuries, regardless of the official cause, was 5 to 7 percent higher than days with temperatures in the 60s . At temperatures above 100 degrees, the overall risk of injury was 10 to 15 percent higher.

That indicates a high number of heat-related injuries that are listed in other categories. The researchers found that extreme heat likely caused about 20,000 additional injuries per year, or 360,000 additional injuries during the 18-year study period.

“This is roughly eleven times the number of concussions in the workplace and at least nineteen times the annual number of accidents at work recorded by the temperature extremes compensation microdata,” the authors write.

The results are to be published as a working paper on Monday. Dr. Park previewed his findings Thursday during a House committee hearing on the climate crisis.

The additional risks of injury in the workplace that arise from high temperatures are not evenly distributed. The worst-paid 20 percent of workers suffer five times as many heat-related injuries as the highest-paid 20 percent of workers, the researchers found.

That difference could reflect the type of work that low-paid workers do compared to their higher-paid counterparts, said Dr. Park. In manufacturing, for example, high temperatures increase the injury rate by around 10 percent and for workers in wholesale professions by 15 percent. People in these industries tend to be exposed to dangerous conditions in the first place, and difficulty concentrating can lead to injuries.

By comparison, finance, insurance, or healthcare workers did not see a strong association between temperature and injury. This could reflect the increased air conditioning at these workplaces and the freedom from hazards: If someone who sits at a desk all day has difficulty concentrating because of the heat, “it has no real safety implications,” says Dr. Park said.

The gap in heat-related injuries between low-paid and high-paid workers could also be due to living conditions.

Researchers at the University of California at San Diego reported this week that low-income neighborhoods in the United States tend to be significantly warmer in the summer than more affluent neighborhoods. The vulnerability of low-income workers to heat-related injuries could be due to a lack of air conditioning and higher home temperatures, said Dr. Park.

Income is not the only way heat-related injuries are unevenly distributed among American workers. Hot days are three times as dangerous for men as they are for women, the data shows, perhaps because men are more likely to work in places with dangerous conditions. And for workers in their 20s and 30s, the additional risk from higher temperatures is about twice as great as for workers in the 50s and 60s.

The results also contain good news.

The link between extreme heat and workplace injuries weakened after 2005, the researchers found. This is also the year California began requiring employers to take measures to protect workers from excessive heat, such as: B. Water, shade and rest breaks for outdoor workers on days with over 95 degrees.

While this doesn’t prove that California regulations have reduced heat-related injuries, there is a chance employers and governments can reduce the impact of extreme heat on worker safety, the authors said.

But only so much. After 2005, the association between temperature and injuries did not disappear – it decreased by about a third.

A message for lawmakers, said Dr. Park, is that governments should do more to reduce emissions of greenhouse gases like carbon dioxide that are warming the planet to curb future temperature increases. In the meantime, however, workers need more protection from the effects of high temperatures, he said.

“We shouldn’t just advocate aggressive climate protection – that means moving away from fossil fuels,” said Dr. Park in front of the committee Thursday. “Politicians might also want to think proactively about climate adaptation.”