Air pollution was recognized as a proven carcinogen in 2013. In addition to its effect on the lungs, a large study confirms its role on the brain and the onset of dementia. Published in the journal The Lancet Planetary Health, this meta-analysis reviews more than 51 studies conducted worldwide, involving 30 million patients. Most of them were conducted in high-income countries: 15 come from North America, 10 from Europe, 7 from Asia and 2 from Australia.
The researchers identified a positive and statistically significant association between dementia and three types of pollutants: fine particulate matter smaller than 2.5 micrometers (PM 2.5), nitrogen dioxide (NO2), and black carbon. PM2.5 particles are particles small enough to be inhaled into the lungs. They come from several types of emissions, such as cars, power plants, certain industrial processes, fireplaces or wood stoves, and construction-related dust. They can remain in the atmosphere for a long time and travel far, through the air, between where they are produced and where they are inhaled.
Nitrogen dioxide is one of the main pollutants from fossil fuel industries. It is also released by cars, particularly diesel engines, as well as gas stoves and gas heaters. Exposure to these particles can irritate the respiratory system, causing and worsening certain diseases such as asthma. Finally, black carbon, also known as soot carbon, also comes from automobile emissions and wood burning. When inhaled, it can penetrate deep into the respiratory system and aggravate lung diseases and increase heart risks.
At least one year of exposure
According to the researchers, for every 10 micrograms per cubic meter (μg/m3) of PM2.5, the relative risk of developing dementia would increase by 17 μg/m3. In 2023, the average concentration of PM2.5 measured at the roadside in central London was 10 μg/m3. For comparison, the average concentration of PM2.5 in the air in Paris is around 12 μg/m3, but this can vary by area, with higher concentrations near major roads depending on Airparif. Added to this, for every 10 μg/m3 of nitrogen dioxide, the relative risk increased by 3 %. The average NO2 concentration measured at the roadside in central London in 2023 was 33 μg/m3. At the same time, Paris recorded annual averages of between 18 and 26 μg/m3 depending on the municipality.
Finally, for every 1 μg/m3 of black carbon present in PM2.5, the relative risk increased by 13 %. Across the UK, average annual soot concentrations measured at selected roadside locations in 2023 were 0.93 μg/m³ in London. In Paris, black carbon is typically around 1 μg/m3 in residential areas, but can reach 11 μg/m3 at sites with high traffic, according to AirparifEach time, these figures were calculated on an exposure of at least one year.
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Several risks to the brain
By what process do particles cause dementia? There are direct neurological effects: fine and ultrafine particles can travel to the brain via the bloodstream and the olfactory nerve. Once in the brain, these particles can induce neuroinflammation and oxidative stress, both of which are known to be involved in the onset of neurodegenerative diseases." explains Clare Rogowski, researcher at the University of Cambridge (United Kingdom) and first author of this study.
Pollution also induces a systemic effect on the nervous system. Air pollution can trigger a systemic inflammatory response after being inhaled and entering the bloodstream. These immune responses can interfere with the brain's protective mechanisms, contributing to the accumulation of proteins in the brain. (beta-amyloid proteins involved in Alzheimer's disease, editor's note) and leading to dementia. »
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But despite these new findings, it is not currently possible to attribute the onset of dementia cases specifically to pollution. Rather, it is a range of factors that, when combined, increase an individual's overall risk. Epidemiological studies have demonstrated a statistically significant association between long-term exposure to air pollutants—particularly fine particulate matter (PM2.5) and nitrogen dioxide (NO2)—and an increased risk of dementia. However, these associations do not imply that air pollution is a necessary or sufficient cause of dementia in isolation." insists Clare Rogowski.
Dementia remains a complex syndrome, influenced by a combination of non-modifiable risk factors, such as age and genetic susceptibility. Modifiable factors include cardiovascular health, education level, physical activity, and now, environmental exposures such as air pollution.
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