A huge new study has found a link between air pollution and dementia. Led by the University of London and King’s College London, the team stress that the causes of dementia, including Alzheimer’s disease, are still ambiguous and “may be multifactorial.”
Nevertheless, their work should encourage further research, especially as the future global burden of dementia will be substantial.
Published in BMJ Open, the study looked at 130,978 adults aged 50-79, who, prior to 2005, had no history of dementia. Their addresses were used to approximate concentrations of nitrogen dioxide and ozone. These are common components of various air pollution sources, from traffic to fossil fuel-fired power plants, and are linked to a range of negative health outcomes.
Of those adults, 1.7 percent were diagnosed with dementia; 39 percent developed Alzheimer’s and 29 percent got the vascular variety. Adjusting for age, sex, ethnicity, smoking, and body mass index, the researchers found that adults living in areas with the highest degrees of nitrogen dioxide had a 40 percent higher risk of dementia than those in low-pollution areas.
That’s increased risk, not absolute, so here’s another way of looking at it: The paper suggests that if all their subjects were living in the low-pollution zones, 7 percent of the total number of current dementia cases in the UK could still be attributable to air pollution.
Dementia is the leading cause of death in England and Wales. The World Health Organization (WHO), noting that 10 million new cases are diagnosed worldwide each year, has made it a public health priority. It currently has no cure.
As a retrospective cohort study – in which correlations are described, but cause-and-effect mechanisms can’t be established – the interpretation of the data must be taken cautiously, something the authors themselves acknowledge. This paper doesn’t stand in isolation, though: the link between air pollution and dementia, along with cognitive problems, has been looked into several times, and worrying trends are emerging.
A 2017 study, for example, found that people who live near busier roads have higher dementia rates. Again, this was an association, not a causal link, but it still added to an increasingly high pile of circumstantial evidence. It’s also clear that air pollution particles can make their way into the human brain, which won’t exactly be beneficial.
Air pollution, which comes from a variety of outdoor and indoor sources, is indubitably damaging to your health. It kills millions of people all over the world every single year, and is certainly linked to cardiovascular, respiratory, and pregnancy problems, per the US National Institutes of Health. Dementia links remain under investigation, but few would say that no association exists.
Outside experts agree that this new study is solid. Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, told the Science Media Centre that this is a “good, well-conducted study that adds to what is known,” pointing out that such studies are particularly difficult to conduct.
Like others, he suggests interpretations aren’t easy. Other factors linked to dementia, like socio-economic background, couldn’t be taken into account; people’s backgrounds affect how close to areas of high air pollution they reside.
Nevertheless, people living in particularly high-pollution areas still rarely got dementia. As McConway notes, there's “considerable uncertainty about exactly how big the increase in risk is.” Individual exposure to air pollution also couldn’t be determined, which is problematic.
Martie Van Tongeren, Professor of Occupational & Environmental Health at the University of Manchester, praised the study and its “solid data.” Despite the unavoidable shortcomings, Tongeren agrees that the increasingly substantial evidence suggests the best course of action is obvious: authorities must act to reduce air pollution.