Research led by USC scientists provides new evidence that exposure to diesel soot and other kinds of fine-particle air pollution may increase our risk for dementia and Alzheimer’s disease.

The study, published Tuesday in the journal Translational Psychiatry, found that elderly women living in areas of the United States where fine-particle air pollution exceeded federal health standards were nearly twice as likely to develop dementia, including Alzheimer’s disease.

Women who already had high genetic predisposition for Alzheimer’s faced an even greater risk, the study found. These women had a 263 percent increased risk for the disease.

If these findings hold up in the general population, air pollution could be responsible for about 21 percent of dementia cases, the study says.

Dr. Jiu-Chiuan Chen, a senior co-author of the study and a USC associate professor of preventive medicine, said by telephone that these results concern him because they suggest the federal health standard for particle pollution may not be tough enough to protect the 16- to 20-percent of people with higher genetic risk for Alzheimer’s.

“We really don’t know if the health goal is providing a safe margin for those with greater risk,” said Chen of USC’s Keck School of Medicine. “This is something that the policymakers need to be aware of.”

The researchers analyzed health data from 3,647 elderly women, ages 65 to 79, gathered in 48 states between 1999 and 2010. The data were originally collected for a federally funded study examining the outcomes of hormonal therapies, Chen said. None of the women had dementia when they enrolled in the research.

Chen’s colleagues then followed up in the laboratory by examining the brains of female mice that carry the genes associated with Alzheimer’s. The mice were exposed to controlled amounts of fine-particle air pollution for 15 weeks.

The exposed mice were 60 percent more likely to have amyloid plaques associated with Alzheimer disease in their brains when compared with mice not exposed to the pollution. These sticky plaques are toxic and cause the progressive decline of brain function that eventually ends in death.

The study has implications for smoggy Southern California.

In 2015, the region failed to meet the U.S. Environmental Protection Agency deadline to bring fine-particle pollution down to the federal health standard for daily exposure. Unhealthful levels of such pollution were recorded during 17 days last year within our ocean-to-mountains air basin. That, however, was an improvement from 2015, when 26 unhealthful days were logged.

Officials at the South Coast Air Quality Management District now expect to bring daily levels of fine-particle pollution down to the federal health standard by the end of 2019, district spokeswoman Tina Cox said in an email.

Fine-particle pollution is a toxic stew of soot, chemical compounds and other airborne specks no bigger than 2.5 microns in diameter or about 1/ 28th the width of a human hair. People living near freeways, ports and warehouse complexes are exposed to higher amounts because it is a component of diesel exhaust.

Earlier studies have blamed this kind of air pollution for early deaths, heart disease, stroke, and reduced lung function in children.

Jo Kay Ghosh, the air district’s health effects officer, spoke highly of USC’s brain study because it combined epidemiological observations on elderly women with laboratory work on mice.

“It is novel to see the use of both epidemiological and toxicological approaches together in the investigation, which adds to the strength of the study,” she said in an email.

Keith N. Fargo, director of scientific programs and outreach at the Chicago-based Alzheimer’s Association, welcomed the new research, but he cautioned against drawing definitive conclusions from the results.

Fargo said he was not convinced that the new analysis of health data from elderly women fully accounted for other Alzheimer’s risk factors, such as lower education, midlife obesity and lack of exercise.

He also noted that only 20 mice were used in the study’s laboratory analysis.

The study “needs more confirmation,” Fargo said. “But we believe this is an important area and that we need to do more of this kind of research.”

Chen said the analysis factored in the other risks associated with Alzheimer’s, but he agreed more research is needed to confirm the results.

The study follows earlier research that has found that pollution particles are so tiny they can move from the blood stream through cell walls and go into our brains. There, the particles trigger a immune system response that results in the formation of plaques associated with brain disease.

“Our study has global implications as pollution knows no borders,” Caleb Finch, co-senior author of the study and a professor at the USC Leonard Davis School of Gerontology, said in a statement.