'Significant evidence' ties air pollution with dementia, study finds

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ST. PAUL, Minn., July 24 (UPI) -- British researchers said Thursday they've found "significant evidence" of a long-suspected link between air pollution and dementia after studying data from nearly 30 million people across four continents.
The authors of a paper published in The Lancet Planetary Health reviewed data compiled from 32 studies that involved more than 29 million participants, mostly from high-income countries in Europe, North America and Asia, as well as Australia, to "provide more robust overarching conclusions" about the suspected connection.
After crunching the numbers from those studies, University of Cambridge researchers said they had identified a "positive and statistically significant association" between dementia and long-term exposure to three types of air pollutants: fine particulate matter with diameters of 2.5 microns or less, nitrogen dioxide, and soot from sources such as vehicle exhaust emissions and burning wood.
They also concluded the risks of dementia go up as the levels of exposure increase, especially for the tiny particulate matter, which is so small it can be inhaled deeply into the lungs. For such "PM2.5," every 10 micrograms per cubic meter (μg/m³) of long-term exposure increases an individual's relative risk of dementia by 17%, they found.
To put that into perspective, they noted the average roadside measurement for PM2.5 in central London in 2023 was 10 μg/m³.
Similarly, for every 10 μg/m³ of long-term exposure to nitrogen dioxide, the relative risk of dementia increased by 3%, while the dementia risk jumped by 13% for each 1 μg/m³ of soot exposure.
The study, which was funded by the European Union's scientific research arm, also noted that while traffic-related air pollution is common in cities in high-income countries, "marginalized groups" from low- and middle-income nations and communities not included in the analysis tend to have higher exposures to air pollution and are likely at even greater risk.
Senior author Dr. Haneen Khreis of Cambridge's MRC Epidemiology Unit said the findings provide "further evidence to support the observation that long-term exposure to outdoor air pollution is a risk factor for the onset of dementia in previously healthy adults."
She added that the mounting evidence makes the case for cleaner air even stronger -- not only for the sake of human health but also for the ability of care systems to cope with the exploding numbers of Alzheimer's disease and other types of dementia.
The 57 million people already affected worldwide are expected to almost triple to 153 million cases by 2050, and reducing that avalanche of future illness will require "less burning of fossil fuels, a move to renewable clean energy, a move to clean transportation options, such as walking and cycling, and enacting climate change mitigation strategies," Khreis told UPI in emailed comments.
"A coordinated effort would be a multisector, systems-level response -- not unlike how we've approached other public health crises."
She and her Cambridge colleagues two years ago created an interactive "evidence map" of hundreds of possible urban policy interventions available by geography to reduce traffic-related emissions and air pollution, which they set up as an aid for planners to become aware of options of which they may not have been previously aware.
"There are indeed many options available," Khreis said. "But in short, we need a whole-of-government and whole-of-society approach that views air pollution mitigation as an upstream intervention for brain health, and so many other outcomes that are now convincingly linked to air pollution. We need political will and courage to address this health hazard."
Suspected in brain inflammation
Scientists has been seeking to establish a definitive link between air pollution and dementia for a decade, as several previous studies have suggested such a connection.
For instance, the British government's Committee on the Medical Effects of Air Pollutants reviewed nearly 70 studies in human populations and reported in 2022 that "it is likely that air pollution does contribute" to a decline in mental ability and dementia in older people, although they qualified that more research is needed before recommendations could be issued "with confidence."
The 2024 Lancet Commission on dementia prevention, intervention and care, meanwhile, identified air pollution as one of 14 modifiable risk factors for dementia.
Most theories about the link center around how air pollution can trigger inflammation in the brain and "excessive oxidative stress," in which the body's ability to combat free radicals with antioxidants is overwhelmed.
There is "a wealth of expanding literature that investigates this question," Khreis said, noting that some research indicates the ultrafine particles may be able to bypass the blood-brain barrier and enter the brain directly via the olfactory nerve, thus triggering inflammation.
Indirectly, these inhaled pollutants can trigger systemic inflammation and immune responses, such as when they enter the circulatory system and travel to solid organs. The particles can cross into the brain and disrupt microglial function and promote amyloid-β plaque formation, which is associated with dementia, she said.
Experts comment
Experts and other researchers who have looked at the possible link between air pollution and dementia, but were not connected to the current study, weighed on its findings when contacted by UPI.
Dr. Isolde Radford, senior policy manager at the British charitable group Alzheimer's Research U.K., agreed that air pollution "is not just an environmental issue -- it's a serious and growing threat to our brain health. If no one were exposed to air pollution, there would be three fewer cases of dementia for every 100 people who develop it now."
She called the current study a "rigorous review [that] adds to mounting evidence that exposure to air pollution -- from traffic fumes to wood burners -- increases the risk of developing dementia," and specifically noted marginalized groups are often exposed to higher levels of pollution, yet remain underrepresented in research.
"Future studies must reflect the full diversity of society -- because those most at risk could stand to benefit the most from action," Radford added, calling on the British government to institute a "bold, cross-government approach to health prevention -- one that brings together departments beyond health ... to take coordinated action on the drivers of dementia risk" and move up existing timelines to produce cleaner air.
Dr. Hao Chen, a researcher at Guangdong Provincial People's Hospital in China and author of a recent study linking atmospheric pollutants with the progression of cardiometabolic disorders and subsequent dementia, called the Cambridge paper an important document.
"This is a landmark study that significantly advances our understanding of the link between air pollution and dementia," he told UPI in emailed comments, noting it is "the largest and most comprehensive meta-analysis to date, significantly advancing previous reviews by including new pollutants and rigorous bias/certainty assessments."
It reinforces the findings of the 2024 Lancet Commission, he said, adding, "The public health implications are profound. The findings suggest that efforts to reduce air pollution -- particularly from traffic and industrial sources -- could play a crucial role in lowering the global burden of dementia.
"Investing in cleaner air is not only beneficial for respiratory and cardiovascular health; it is a critical investment in protecting our brain health and cognitive longevity. This research provides a compelling, evidence-based mandate for policymakers worldwide to implement stricter air quality standards as an urgent dementia prevention strategy," Chen said.
Victoria Williams, an assistant professor of geriatrics and gerontology, and noted Alzheimer's disease researcher at the University of Wisconsin, said the findings suggest "a moderate level of certainty that outdoor air pollution exposure increases risk for dementia, with the studies included considered to be at minimal risk for bias."
Since there is currently no cure for dementia, "prevention is key to reducing its overall incidence in our society," she added. "Thus, minimizing exposure to air pollution does reflect a promising strategy in reducing the overall burden of dementia," and unlike other known risk factors linked to lifestyle choices such as smoking and a poor diet, societal action could make a difference.
"Limiting exposure to ambient air pollution is well adept to being addressed at a policy level where reductions can offer broad protective effects to a society as a whole," Williams said.