The connection between air pollution and Alzheimer’s disease

Air pollution has been a focus of several studies on cognitive impairment and dementia risk. There is evidence that tiny air pollution particles can enter the brain, but at this time we can’t say if they play a role in the development of dementia. There is a strong case for further research into the effect of air pollution on brain health; that’s Alzheimer’s disease.

What is air pollution?

Air pollution is made up of several different components including gases, chemical compounds, metals, and tiny particles known as particulate matter. Long-term exposure or exposure to high levels of air pollution can be hazardous, leading to health conditions that affect the lungs and heart. Most research has focused on a component of air pollution known as fine particulate matter or PM 2.5 – tiny particles that are 40 times smaller than the width of a human hair. A form of iron called magnetite is often found within fine particulate matter and can be studied in the body due to its magnetic properties.

Does air pollution affect the brain?

Magnetite particles are released into the air by burning fuel, but they are also produced naturally in the brain. A study of brain tissue from people in Mexico City and Manchester conducted in 2016 confirmed that magnetite from air pollution can pass into the brain. Using a special electron microscope, the researchers examined the surface properties of the magnetite particles to prove that they had been generated at the high temperatures found in an engine rather than through natural processes. This study confirmed that fine particulate material can pass into the brain via the bloodstream or directly through the thin lining of the nose.

The particles were seen inside protein deposits called amyloid plaques which are abundant in the Alzheimer’s brain, leading to speculation that magnetite could be involved in the development of Alzheimer’s disease. However, the study did not provide evidence that magnetite is involved in the formation of amyloid plaques or that it can lead to the death of brain cells. Alternatively, magnetite particles could enter the brain from polluted air and end up in amyloid plaques as a consequence of the brain’s waste disposal processes.

Studies in mice have shown some effects of breathing polluted air on the brain. Mice that are exposed to polluted air collected from near busy roads show biological changes that are known to cause damage to the brain, as well an increase in levels of the protein amyloid, which is one of the hallmarks of Alzheimer’s disease. However, we know from human brain imaging studies that an increase in brain amyloid protein alone doesn’t necessarily mean that Alzheimer’s disease will develop.

Could air pollution be a cause of dementia?

A direct link between air pollution and Alzheimer’s disease has not been found, however, there are many questions still unanswered. A growing number of studies looking at exposure to pollution from around the world combined with increasingly sophisticated techniques for seeing the fine particulate matter in the brain and body is creating a case for further research.

Studies of mice and dogs living in polluted areas suggest that air pollution could be associated with cognitive impairment. Exposure of mice and rats to traffic pollution in the lab resulted in symptoms such as poorer learning ability, memory, and motor skills. In people, there are a couple of studies showing that those who are exposed to high levels of pollutants perform poorer on cognitive tests over time, but this does not mean they have or will develop dementia.

The most convincing evidence so far comes from the study of 6.6 million people from Canada published in 2016 that reported a potential link between dementia and living close to very busy roads. The study found that those living within 50 meters of a major road were 7% more likely to develop dementia than people living more than 300 meters away, where fine particulate matter levels can be up to 10 times lower. As there are other factors associated with living on a busy road, such as high noise pollution and stress, this study doesn’t prove that air pollution causes dementia. However, it does suggest that the study of air pollution and dementia should be prioritized for future research.