Air pollution may cause about 21 percent of cases of dementia worldwide, including Alzheimer's disease, if a study of older women can be extended to the general population.
Women aged 65 to 79 are 81 percent more at risk for general cognitive decline and 92 percent more likely to develop dementia if they live where fine particle matter exceeds the U.S. Environmental Agency's standards, according to the study, led by University of Southern California scientists.
The study didn't examine whether inhaling these particles also puts men at elevated risk.
These extremely small particles are just 2.5 millionths of a meter or smaller. They mainly come from power plants and automobiles and have long been a significant health concern. They escape the body's defenses more easily than larger particles, said Robert Kard, director of the San Diego County Air Pollution Control District. And when they lodge in the body, they tend to stay there.
Natural gas plants produce very low levels of these particles, known as PM2.5, Kard said. Oil-burning plants produce considerably more of them.
The study examined 3,647 women aged 65 to 79 in 48 states. It was adjusted for biases such as ethnic background, education and medical conditions.
The correlation is consistent with an observation others have made, the researchers said: Reducing air pollution coincides with a lower rate of dementia per age group in recent years. (Because the elderly population keeps growing, the total number of cases is expected to keep increasing.)
Researchers also found a plausible cause in an experiment on female mice exposed to these particles. These mice have a variant of a gene called APOE4, known to be an Alzheimer's risk factor. Compared to a control group, they accumulated as much as 60 percent more beta amyloid plaque, the toxic clusters of protein fragments that further the progression of Alzheimer's.
"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," said Caleb Finch, a study author, in a USC statement.
The study was published Tuesday in the journal Translational Psychiatry. Finch and Jiu-Chiuan Chen are co-senior authors. The study can be found at j.mp/poldem.
Two thirds of those with Alzheimer's and other dementias are women, said Mary Ball, president and CEO of Alzheimer's San Diego.
And while Alzheimer's is the sixth leading cause of death nationwide, it's the third leading cause of death in San Diego County, Ball said. That is probably due in part to the county's commitment to accurate reporting on cases, and secondly to having a older-skewing population. Age is the main risk factor for developing Alzheimer's.
Air pollution doesn't appear to be a factor in elevating risk locally. San Diego County is well within the limit for these PM2.5 particles, said Kard, of the San Diego County Air Pollution Control District. That limit is 35 micrograms per cubic meter.
According to data supplied by the district, during 2016 the level never reached 20 micrograms per cubic meter, and levels very rarely exceeded the federal limit in records going back to 1999.
The levels were greatly exceeded during the 2003 and 2007 wildfires, but since the agency had no control over those events, it doesn't count against the limit.
Huaxi Xu, a top Alzheimer's researcher at Sanford Burnham Medical Discovery Institute in La Jolla, said the study provides important evidence advancing understanding of air pollution as a risk factor for these diseases.
"Since particles of this size are found in polluted air, the findings confirm recent evidence linking environmental factors, such as automotive-derived air pollution, to higher levels of dementia among people living near heavy traffic roads," Xu said. He cited one new study, published Jan. 5 in The Lancet, as evidence.
"However, as it is often the case, the study leaves a number of unanswered questions that are worth pursuing," Xu said.
These include the cellular and molecular causes of degeneration of a specific region of the brain called the hippocampus, how efficiently beta amyloid is cleared from the brain and whether changes in the brain's blood vessels contribute to dementia.
Xu also said followup studies should look at other potential effects of air pollution on the brain, such as the involvement of other genes or sex-dependent factors.
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