The chances of developing dementia increase by around 92 percent for women ages 65 to 79 who are exposed to air pollution consisting of particulate matter up to 2.5 micrometers (PM2.5) in diameter, a level that exceeded US Environmental Protection Agency standards from 2012, according to the study, released this week in the journal Translational Psychiatry.
Applying the study's findings to the population beyond older women, PM2.5 could be the cause of about 21 percent of all dementia cases, according to the University of Southern California researchers responsible for the study. PM2.5 "mainly comes from power plants and automobiles," researchers said in a news release.
"Microscopic particles generated by fossil fuels get into our body directly through the nose into the brain," said Caleb Finch, co-senior author of the study and a professor at the University of Southern California's Leonard Davis School of Gerontology. "Cells in the brain treat these particles as invaders and react with inflammatory responses, which over the course of time, appear to exacerbate and promote Alzheimer’s disease.
"Although the link between air pollution and Alzheimer’s disease is a new scientific frontier, we now have evidence that air pollution, like tobacco, is dangerous to the aging brain."
Researchers initially found that mice bred with the gene APOE-e4, a genetic variation that is linked to increased risk of Alzheimer's disease, were more likely to incur cognitive impairment when exposed to polluted air that exceeded federal standards than mice not exposed to same air.
Researchers captured polluted air near the USC campus, along a Los Angeles freeway, for the study. "We now know that the major Alzheimer's risk gene APOE-e4 has an environmental component," Finch said.
The study is unique in that it found the interaction of APOE-e4 and air pollution may boost brain impairment, researchers said.
"Our study — the first of its kind conducted in the US — provides the inaugural scientific evidence of a critical Alzheimer’s risk gene possibly interacting with air particles to accelerate brain aging," said Jiu-Chiuan Chen, co-senior author of the study and associate professor of preventive medicine at the Keck School of Medicine.
"The experimental data showed that exposure of mice to air particles collected on the edge of USC damaged neurons in the hippocampus, the memory center that is vulnerable to both brain aging and Alzheimer’s disease."
Researchers analyzed data from the Women’s Health Initiative Memory Study regarding nearly 3,650 women ages 65 to 79 living across 48 states who did not have dementia when they enrolled in the study. The study adjusted for each woman's geographic regions, race, ethnicity, medical conditions and other factors.
Comparing women who lived in areas of low PM2.5 concentrations, women living in high areas of PM2.5 were 81 percent more likely to develop global cognitive decline and 92 percent more likely to develop Alzheimer's and other forms of dementia, researchers found.
"We analyzed data of high PM2.5 levels using standards the EPA set in 2012," Chen said. "We don’t know whether the lower PM2.5 levels of recent years have provided a safe margin for older Americans, especially those at risk for dementia."
About 48 million people worldwide suffer from dementia, according to the World Health Organization. "Our study has global implications as pollution knows no borders," Finch said.