The surprising link between air pollution and Alzheimer’s disease

With environmental regulations expected to come under heavy fire from the Trump administration, new research offers powerful evidence of a link between air pollution and dementia risk.

For older women, breathing air that is heavily polluted by vehicle exhaust and other sources of fine particulates nearly doubles the likelihood of developing dementia, finds a study published Tuesday. And the cognitive effects of air pollution are dramatically more pronounced in women who carry a genetic variant, known as APOE-e4, which puts them at higher risk for developing Alzheimer’s disease.

In a nationwide study that tracked the cognitive health of women between the ages of 65 and 79 for 10 years, those who had the APOE-e4 variant were nearly three times more likely to develop dementia if they were exposed to high levels of air pollution than APOE-e4 carriers who were not.

Among carriers of that gene, older women exposed to heavy air pollution were close to four times likelier than those who breathed mostly clean air to develop “global cognitive decline” — a measurable loss of memory and reasoning skills short of dementia.

While scientists have long tallied the health costs of air pollution in asthma, lung disease and cardiovascular disease, the impact of air pollutants on brain health has only begun to come to light. This study gleans new insights into how, and how powerfully, a key component of urban smog scrambles the aging brain.

Published Tuesday in the journal Translational Psychiatry, the research looks at a large population of American women, at lab mice, and at brain tissue in petri dishes to establish a link between serious cognitive decline and the very fine particles of pollution emitted by motor vehicles, power plants and the burning of biomass products such as wood.

All three of these biomedical research methods suggest that exposure to high levels of fine air pollutants increases both dementia’s classic behavioral signs of disorientation and memory loss as well as its less obvious hallmarks. These include amyloid beta protein clumps in the brain and the die-off of cells in the brain’s hippocampus, a key center for memory formation.

Using air pollution standards set by the U.S. Environmental Protection Agency, researchers found significant differences on all those measures between those who breathed clean air and those exposed to pollution levels deemed unsafe.

In lab mice, breathing air collected over the 10 Freeway in Los Angeles led to brain concentrations of amyloid protein that were more dense and more likely to form dangerous clumps than breathing air that satisfied EPA standards before 2012. When lab mice were bred with a strong predisposition to develop dementia and its hallmarks, the brain differences between pollution-breathing animals and those that breathed clean air were starker.

In 2011, a study in the journal Lancet found that those who lived close to densely trafficked roads were at a far higher risk of stroke and dementia than those who lived farther away. A year later, a team led by Alzheimer’s disease researcher Dr. Samuel Gandy at Mt. Sinai in New York first established that air pollutants induced inflammation, cell death and the buildup of amyloid protein in the brains of mice.

The new study extends those findings.

Authored by geriatric and environmental health specialists at USC, the new study estimates that before the EPA set new air pollution standards in 2012, some 21% of new cases of dementia and of accelerated cognitive decline could likely have been attributed to air pollution.

There is potential legal significance to the researchers’ finding that women (and mice) who carried a genetic predisposition to developing Alzheimer’s disease were far more sensitive to air pollution’s effects. In devising pollution standards, the EPA is currently required to consider their health impact on “vulnerable populations.” The agency is also required to use its regulatory authority to take steps to protect those populations.

Air pollution has been declining steadily since the EPA promulgated new standards in 2012. But Dr. Jiu-Chiuan Chen, an environmental health specialist at USC’s Keck School of Medicine and the study’s senior author, said it’s not clear that even current standards are safe for aging brains, or for brains that are genetically vulnerable to Alzheimer’s.

The Trump administration has signaled it will look to scrap or substantially rewrite Obama administration regulations that tightened emissions from power plants and established tougher fuel efficiency standards for cars in an effort to curb climate change and reduce air pollution.

“If people in the current administration are trying to reduce the cost of treating diseases, including dementia, then they should know that relaxing the Clean Air Act regulations will do the opposite,” Chen said.

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