Compared to U.S. citizens, immigrants have more than three times the risk of dying from heat-related illness — with the majority of deaths occurring in just three states, according to a recent study.

The risk is greatest among Hispanic immigrants and those between ages 18 and 24, a group of researchers at the Centers for Disease Control and Prevention report online October 26 in the American Journal of Public Health.

“Three states (Arizona, California and Texas) accounted for 94.5 percent of non-citizen heat deaths,” senior study author Rebecca Noe, of the CDC’s Division of Environmental Hazards and Health Effects at the National Center for Environmental Health in Atlanta, said in an email.

“States where heat deaths are commonly occurring should consider tailoring heat prevention messages,” she told Reuters Health.

The study team looked at 2005-2014 mortality data and residence in the National Vital Statistics System to understand differences in heat-related deaths based on citizenship.

In the United States, an average of 658 people died annually from exposure to excessive natural heat between 1999 and 2009, Noe and her colleagues write in their report. But while heat-related deaths accounted for 0.02 percent of all deaths among U.S. citizens, they represented 2.2 percent among non-citizens, the researchers found.

While immigrants overall had 3.4 times the risk of citizens for dying of heat-related causes, among Hispanics that figure was 3.6 times. For all non-U.S. citizens in the 18-24 age group, the risk was 20.6 times that of citizens, the study also found

Overall, 87 percent of the heat-related deaths occurred among Hispanics, and farms were most often cited as the place of excessive heat exposure.

A limitation of the study is that no occupational information was available to further explain the reason for death. However, it’s likely related to outdoor working conditions on farms, said Tarik Benmarhnia of the University of California at San Diego who wasn’t involved in the study.

A 2012 report by the Arizona Department of Health Services found that more than 1,500 heat-related deaths occurred in the state between 2000 and 2012, and 48 percent of these were among migrants from Mexico, Central America or South America, he noted. These heat-related deaths included adults between ages 20 and 44, Benmarhnia added.

“When workers are young, they don’t feel the symptoms of heat stroke until it’s too late,” he told Reuters Health in a phone interview. “This is a problem for all workers in general, especially for young people and illegal immigrants working on farms and (in) agricultural activities.”

In the current study, the 5-17 age group among non-citizens also had an increased risk for heat-related death, which Benmarhnia found unusual, but he said it could include agricultural workers between 15 and 17.

“I cannot understand why younger children between ages 5 and 10 would die from heat exposure,” he said. “It’s likely teens who are exposed to heat on the job but less experienced. They may take more risks.”

Future studies should identify specific risk factors for heat-related deaths, such as differences in health, lifestyle, occupation or cultural behaviors, the study authors write.

Benmarhnia is studying spatial and geographical patterns linked to heat-related death. In New York City and certain California cities, for example, neighborhoods with more construction work and high afternoon temperatures may have the highest risk. In a study of Paris, Benmarhnia and colleagues found that heat-related deaths occurred in a handful of neighborhoods. Green spaces decreased the risk of death and high air pollution increased the effects of extreme heat.

“When a heat wave occurs, policymakers don’t have to focus on everywhere,” he said. “They can focus their energy on promoting messages in these certain areas where people are dying.