Air pollution associated with increased brain shrinkage

By Sarah MooreReviewed by Emily Henderson, B.Sc.Nov 18 2020

A new study has revealed a link between high levels of air pollution and brain shrinkage, a characteristic of Alzheimer's disease. Scientists published their findings this month in the medical journal, the American Academy of Neurology, demonstrating that older women living in areas where they are exposed to higher levels of air pollution are more likely to develop brain shrinkage in areas of the brain known to be vulnerable to Alzheimer's disease.

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The study will contribute to a growing body of evidence that supports the role of fine particle pollution in the etiology of several serious human health issues.

Increasing concern over the dangers of air pollution

Numerous studies conducted over recent years have elucidated the link between air pollution and various illnesses. It is considered to be the world’s largest environmental threat, with nine out of ten people worldwide breathing air that exceeds WHO guideline limits.

Statistics show that each year around 4.2 million people die due to exposure to ambient air pollution, and a further 3.8 million die as a result of exposure to household air pollution such as smoke from stoves. Given the significant impact on human health, research has continued to explore how air pollution contributes to illness.

The current study focused on fine particle pollution, known as PM2.5 (referring to particles measuring 2.5 micrometers or smaller), chronic exposure to which is known to increase a person’s risk of cardiopulmonary mortality. Additionally, data shows that the elderly and children, as well as those with pre-existing lung or heart disease, are at an elevated risk of suffering the effects of PM2.5.

Researchers looked at long-term PM2.5 exposure in relation to brain changes characteristic of dementia and Alzheimer's disease, specifically, they were looking at changes in brain volume as a smaller volume is a known risk factor of these diseases.

PM2.5 exposure and brain shrinkage

The study recruited 712 women who began with no diagnosis of dementia. MRI scans were taken of the brains of all individuals at the start of the study, and again five years later. To determine the level of air pollution the participants were exposed to the researchers used data relating to their residential addresses. Data were gathered from three years before the first scan, which was used to allocate participants into four groups based on their level of exposure.

A machine learning tool that had been developed to identify patterns of brain shrinkage characteristic of Alzheimer's disease was used to read the brain scans. Scores were assigned from 0-1 to participants’ brain scans at the start and end of the study relating to how much they matched patterns of Alzheimer's disease. Higher scores demonstrated more brain changes.

Results showed that as air pollution increased, so did the differences between the before and after scans, indicating increased brain changes correlating with increased air pollution. For each increase of 3 μg/m3 in air pollution that the women were exposed to, their risk of developing Alzheimer's disease increased by 24%.

Pressure to tighten regulations on air pollution

Study author, Diana Younan, believes that these findings will have a significant impact on public health implications, “not only did we find brain shrinkage in women exposed to higher levels of air pollution, we also found it in women exposed to air pollution levels lower than those the EPA considers safe”.

The study’s results further highlight the real danger that air pollution poses to human health. It is hoped that these findings, along with those of similar studies, will encourage governments to tighten regulations and implement strategies to reduce the level of air pollution and protect public health.