CHICAGO, July 2 -- A research of Washington University School of Medicine in St. Louis links outdoor air pollution, even at levels deemed safe, to an increased risk of diabetes globally.

Researchers at the university, in collaboration with scientists at the Veterans Affairs' Clinical Epidemiology Center, examined the relationship between particulate matter and the risk of diabetes by first analyzing data from 1.7 million U.S. veterans, who did not have histories of diabetes and were followed for a median of 8.5 years.

The researchers linked the patient data with the Environmental Protection Agency's (EPA) land-based air monitoring systems as well as space-borne satellites operated by the National Aeronautics and Space Administration (NASA).

They used several statistical models and tested the validity against controls such as ambient air sodium concentrations, and lower limb fractures, as well as the risk of developing diabetes. This exercise helped the researchers weed out spurious associations.

Then they sifted through all research related to diabetes and outdoor air pollution and devised a model to evaluate diabetes risk across various pollution levels.

Finally, the researchers analyzed data from the Global Burden of Disease study, which is conducted annually with contributions from researchers worldwide.

"Our research shows a significant link between air pollution and diabetes globally," said Ziyad Al-Aly, the study's senior author and an assistant professor of medicine at Washington University.

"We found an increased risk, even at low levels of air pollution currently considered safe by the U.S. EPA and the World Health Organization (WHO). Evidence shows that current levels are still not sufficiently safe and need to be tightened," the researcher added.

Among a sample of veterans exposed to pollution at a level between 5 to 10 micrograms per cubic meter of air, about 21 percent developed diabetes. When that exposure increased to 11.9 to 13.6 micrograms per cubic meter of air, about 24 percent of the group developed diabetes.

The researchers also found that the overall risk of pollution-related diabetes is tilted more toward lower-income countries such as India that lack the resources for environmental mitigation systems and clean-air policies.

Diabetes affects more than 420 million people worldwide and 30 million Americans. In the United States, the study attributed 150,000 new cases of diabetes per year to air pollution and 350,000 years of healthy life lost annually.

The findings were published June 29 in the Lancet Planetary Health.