Air pollution, especially the fine particulates emitted by autos, trucks and power plants, may be a much bigger contributor to dementia in women than we ever knew.

A large new study provides evidence that this kind of invisible air pollution may be responsible for more than one in five cases of dementia. In particular, older women living in highly polluted parts of the country may be more likely to develop dementia—or to get it earlier than they might have if they breathed cleaner air. Here’s what you need to know…and how to protect yourself…

Background: Microscopic particles generated by the burning of fossil fuels get into our bodies directly—they go from your nose right into your brain. Cells in the brain treat these particles as invaders and react with inflammatory responses, which over time may promote Alzheimer’s disease and other kinds of dementia. But how significant the effects are, and what is going on biologically, hasn’t been well understood.

Study: Researchers at University of Southern California and elsewhere combined two different kinds of studies—a large-scale, long-term observational human study, and a series of lab animal experiments designed to shed light on biological mechanisms.

On the human end, researchers analyzed data from 3,647 women enrolled in the Women’s Health Initiative Memory Study. They ranged in age from 65 to 79 when the study began in the 1990s. The women lived all across the US—in 48 states. None had dementia when the study began, and everyone was screened for cognitive health every year.

As part of regular health screenings, they were also tested for APOE genetic mutations. About one in five people have the APOE “4” gene variant, which affects blood fats and greatly increases the risk of developing Alzheimer’s disease, especially in women. (It also increases heart disease risk). They were followed for about 10 years.

On the lab end, the subjects were mice. About half were exposed to microscopically small fine particulant pollutants for 15 weeks, while those in the control group breathed clean air. Some of the mice in both groups carried the APOE4 gene mutation.

Results: Air pollution was strongly linked to cognitive decline in the older women…

  • Women who lived in areas where fine particulate air pollution exceeded the EPA’s standard were 92% more likely to develop dementia than those who lived in areas that were EPA compliant.
  • Women who inherited the APOE 4 gene variant from both parents who breathed this dirty air were 295% more likely to develop dementia than the average for women in the study.
  • The mouse study helped explain why. Both groups of mice who breathed the polluted air were negatively affected. The fine particulates damaged neurons in the hippocampus, the brain’s memory center. But compared with with mice who did not have the APOE 4 genetic variant, the mice who did accumulated 60% more amyloid plaque, the toxic clusters of protein fragments that further the progression of Alzheimer’s.

Bottom line: The researchers estimated that if this finding were extrapolated to the general population, air pollution could be responsible for 21% of dementia cases in the US. Air pollution, especially the fine particulates from fossil fuels, ages the brain and increases the risk for dementia. While this study didn’t look specifically at men, other research has found that being exposed to this kind of pollution—such as living near a major highway—increases dementia risk in both men and women.

When it comes to air pollution, there are no easy solutions. While the study did not make recommendations, you are not helpless no matter where you live. If you live in an area where there’s likely to be lots of particulate pollution from vehicles or other sources, use air conditioning in the summer and invest in a HEPA filter for year-round reduction of the air pollution inside your home. If you exercise outdoors, stay away from major highways.

If you’re moving, try to avoid relocating near major highways, and look for a home surrounded by green. To truly tackle the problem, though, requires society-wide policy, regulation and technology. Reducing our nation’s fine-particulate pollution, this study suggests, might greatly reduce the number of people who end up with dementia.