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Is there a link between air pollution and Alzheimer's disease? – Dr Tom Russ comments

Tuesday 27 September 2016

Dr Tom Russ is an Intermediate Clinical Fellow and honorary Consultant Psychiatrist at the Centre for Dementia Prevention and Alzheimer Scotland Dementia Research Centre, both at the University of Edinburgh. His research focuses on geographical variation in dementia risk and environmental risk factors for dementia.

Find out more about Dr Russ here:

Dr Russ commented based on the following headlines, versus the findings of the study to which they refer:

  • “Air Pollution Particles Linked To Alzheimer's Found In Human Brain” (Sky News)
  • “Alzheimer’s disease: Condition linked to exposure to air pollution” (Daily Express)
  • “Toxic air pollution particles found in human brains” (Guardian)

Were the headlines right or were they misleading in some way?

This paper attracted a great deal of publicity. Their findings from examining brain tissue from 38 people are exciting and suggest that more attention be paid to the influence of the environment – particularly air pollution – on brain health.

In general, the reporting was appropriately circumspect, though some newspapers did lead with the potential link with dementia. This link is, in fact, an indirect one: magnetite is toxic to the brain and one of the ways it is toxic – by producing reactive oxygen species – has been linked to neurodegenerative diseases.

How important is this story/study for furthering dementia research? Should we be excited?

The researchers examined samples from Mexico City and Manchester using techniques to identify the magnetic content in the brain – being scrupulous to avoid ‘contaminating’ the samples magnetically when they were preparing them. They identified two types of magnetite (an iron ore) in their samples: (1) angular particles which are likely to have been formed in the brain (and which have been previously observed); and (2) rounded particles which resembled those seen in urban air pollution (e.g. from vehicle exhausts and other sources). The formation of such particles requires extremely high temperatures and so they must have been formed outside the body and made their way in, presumably by being inhaled.

We already knew from animal studies that inhaled particulate matter could get into the brain, but here is evidence from humans that air pollution can directly affect the brain. This is an extremely interesting finding but, as is often the case, requires further careful investigation at this early stage. However, this may be the first step towards clarifying exactly how air pollution could be detrimental to brain health.

What might be the impact of this story/study in the scientific community?

There are likely to be numerous environmental factors which increase the risk of developing dementia, including air pollution, exposure to excessive amounts of some trace elements, pesticide use, and vitamin D deficiency (which relates to sunlight exposure). However, several of these – including air pollution – are potentially modifiable and, if this were possible at a population level, it would really be worth pursuing.

Furthermore, the growth in the number of people with dementia in the coming decades is projected to take place predominantly in low-to-middle income countries. It might well be these places where legislation to reduce air pollution could have the biggest effect on health. For example, Mexico City has extremely high levels of air pollution which has been studied in detail by the senior author of this paper (according to the World Bank, Mexico is an upper-middle-income country).

Approximately a third of dementia risk remains unexplained; genetic factors and known modifiable risk factors (smoking, diabetes, hypertension and obesity in midlife, low educational attainment, a lack of physical activity, and depression) also explain roughly a third each. Environmental factors – including air pollution – might well explain a proportion of this unexplained risk. Therefore, we urgently need more research in this area.

What are the next steps?

As with all novel findings, they should be replicated. However, the evidence we already have from observational studies is consistent in its support for the association between air pollution and dementia so this move to elucidate the mechanism is exciting. However, one unanswered question remaining for epidemiology is when exposure to air pollution is most detrimental to brain health – is it a gradual accumulation of damage or are there sensitive or critical periods of exposure? This life course epidemiology perspective, combined with further mechanistic work, should help shed light on the origins of Alzheimer’s disease and other neurodegenerative conditions and lead us closer to our ultimate goal – preventing dementia.