This silent threat is around us. It’s not invisible, but yet overlooked. And the destruction it causes? Brain damage. We are talking about a common health threat that is secretly lurking around us. A new study has revealed that long exposure to this hazard can increase the risk of developing dementia, including Alzheimer’s disease and vascular dementia, conditions which were earlier believed to be mainly linked with lifestyle and genetics. A study led by the researchers at the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge found that long-term exposure to outdoor air pollution can increase the risk of dementia. The findings are published in The Lancet Planetary Health.Dementia is on the rise worldwide
Dementia, such as Alzheimer’s disease, affects more than 57.4 million people worldwide. This number is expected to triple to 152.8 million cases by 2050. Though some indications suggest that the prevalence of dementia is dropping in certain regions, such as Europe and North America, it may be possible to reduce the risk of the disease at a population level; elsewhere, the picture is less promising.Air pollution and dementia
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The researchers of the new study have found a surprising link between air pollution and dementia risk. Though several studies have pointed to the possibility of an association between dementia and air pollutants, there were less evidence to prove it. The researchers at MRC carried out a systematic review and meta-analysis of existing scientific literature to examine this link further. They looked at 51 studies, which included data from 29 million participants, mostly from high-income countries. 34 papers were included in the meta-analysis: 15 originated in North America, 10 in Europe, seven in Asia, and two in Australia. They found a positive and statistically significant link between three types of air pollutants and dementia. These include:
- PM2.5: Particulate matter with a diameter of 2.5 microns or less. This pollutant is small enough to be inhaled deep into the lungs. The source of these particles includes vehicle emissions, power plants, industrial processes, wood-burning stoves and fireplaces, and construction dust. They form in the atmosphere as well, due to complex chemical reactions involving other pollutants such as sulphur dioxide and nitrogen oxides. These particles stay in the air for a long time and travel a long way.
- Nitrogen dioxide (NO2): This pollutant emerges from burning fossil fuels. It is found in vehicle exhaust, especially diesel exhaust, and industrial emissions, as well as those from gas stoves and heaters. Exposure to NO2 in high concentrations can irritate the respiratory system, worsening and inducing conditions like asthma and reducing lung function.
- Soot: This pollutant is formed from vehicle exhaust emissions and burning wood. It can trap heat and affect the climate. When inhaled, it can go deep into the lungs, aggravating respiratory diseases and increasing the risk of heart problems.
The researchers found that every 10 micrograms per cubic meter (μg/m³) of PM2.5, increases the relative risk of dementia by 17%. In cities like Central London, PM2.5 levels on average roads have been recorded at 10 μg/m³ in 2023.When it comes to NO2, the researchers found that for every 10 μg/m3, the relative risk increased by 3%. It is worth nothing that an average roadside measurement for this pollutant in Central London in 2023 was 33 µg/m³. For each 1 μg/m³ of soot as found in PM2.5, the relative risk increased by 13%. “Epidemiological evidence plays a crucial role in allowing us to determine whether or not air pollution increases the risk of dementia and by how much. Our work provides 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. Tackling air pollution can deliver long-term health, social, climate, and economic benefits. It can reduce the immense burden on patients, families, and caregivers, while easing pressure on overstretched healthcare systems,” senior author Dr Haneen Khreis from the MRC Epidemiology Unit said, in a statement. How air pollution leads to dementia
There are several mechanisms by which air pollution causes dementia. One of the proposed explanations is that it causes inflammation in the brain and oxidative stress, both contributing to the onset and progression of dementia. Air pollution is thought to trigger these processes through direct entry to the brain or via the same mechanisms underlying lung and cardiovascular diseases.“Efforts to reduce exposure to these key pollutants are likely to help reduce the burden of dementia on society. Stricter limits for several pollutants are likely to be necessary, targeting major contributors such as the transport and industry sectors. Given the extent of air pollution, there is an urgent need for regional, national, and international policy interventions to combat air pollution equitably,” joint first author Clare Rogowski, also from the MRC Epidemiology Unit, said.Exposure to air pollutants was linked to a higher risk of Alzheimer’s, but the effect appeared stronger for vascular dementia, which affects about 180,000 people in the UK. However, more research is required on this. “These findings underscore the need for an interdisciplinary approach to dementia prevention. Preventing dementia is not just the responsibility of healthcare: this study strengthens the case that urban planning, transport policy, and environmental regulation all have a significant role to play,” joint first author Dr Christiaan Bredell from the University of Cambridge and North West Anglia NHS Foundation Trust added.
