Air pollution and the risk of dementia

The connection between air pollution and dementia needs to be better understood. We don’t currently know how the level, time and life stage of being exposed to air pollution affects dementia risk. There is also evidence that tiny air pollution particles can enter the brain, but more research is needed to fully understand the effect of air pollution on brain health.

There are a growing number of studies looking at exposure to pollution around the world to answer these questions.

In 2022 the Committee on the Medical Effects of Air Pollutants undertook a review of 70 studies in human populations. The evidence suggests a link between air pollutants and the development of memory and thinking problems. It is thought that the effects of air pollution on the heart and circulatory system may impact the blood supply to the brain and lead to vascular dementia.

The report also considered whether there is a direct link between tiny air pollutant particles entering the brain and risk of dementia. The evidence does not currently suggest this plays an important role in the development of dementia.

A form of iron called magnetite is found within fine particulate matter and can be studied in the body due to its magnetic properties. 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 confirmed that magnetite from air pollution can pass into the brain – possibly via the bloodstream or the thin lining of the nose.

The particles were seen inside protein deposits called amyloid plaques, which are abundant in the brains of people with Alzheimer’s disease. This led 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 the death of brain cells. It could be that the magnetite particles that enter the brain end up in amyloid plaques as a consequence of the brain’s waste disposal processes.

Studies on rodents found that exposure to air pollution resulted in poorer learning, memory and motor skills. They have also shown changes in the brain such as cell loss and inflammation. In mice bred to develop certain aspects of Alzheimer’s disease, exposure to air pollution worsened the problems in the brain associated with dementia.

Studies on people have involved looking at people’s natural exposure to air pollution and their thinking abilities. Some studies show that those who are exposed to high levels of pollutants perform poorer on thinking tests over time. But this does not mean they have or will develop dementia.

The most convincing evidence so far comes from a study of 6.6 million people from Ontario, Canada published in 2016. The study found that those living within 50 metres of a major road were 7% more likely to develop dementia than people living more than 300 meters away.

Another huge study in Quebec, Canada found an increase in dementia rates occurred with increased nitrous oxide or fine particulate matter exposure. They also saw an increase in relation to living closer to a major road.

Pollution can also arise from burning wood in the home and this type of pollution has also been suggested to increase dementia risk.

Another study took brain scans of around 18,000 people with memory and thinking problems. They found that people living in areas with worse air quality were more likely to have a build-up of amyloid in their brains.

These studies don’t offer proof that air pollution causes dementia. They can only suggest a relationship between thinking problems or dementia and air pollution. There are other factors associated with living on a busy road that may affect the risk of dementia, including high noise pollution, stress and lack of sleep.