Long-term exposure to air pollution increases risk of dementia

An analysis of studies involving nearly 30 million people has found a role for air pollution - including car exhaust emissions - in the increased risk of dementia.

Forms of dementia such as Alzheimer's disease are estimated to affect more than 57.4 million people worldwide, and this number is expected to almost triple to 152.8 million cases by 2050. The consequences for the individuals themselves, their families, carers and society as a whole are enormous.

While there are some signs that dementia prevalence is declining in Europe and North America – suggesting that the risk of the disease may be declining at the population level – the situation in other regions looks less encouraging.

Air pollution has recently been identified as a risk factor for dementia, and a number of studies point to specific pollutants. However, the strength of the evidence and the ability to establish a causal relationship varies.

In a paper published in The Lancet Planetary Health, a team led by researchers from the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge carried out a systematic review and meta-analysis of the existing scientific literature to examine this link in more detail. This approach allowed them to combine data from individual studies that might have been weak or conflicting on their own, and draw stronger overall conclusions.

Overall, the researchers included 51 studies in their analysis, covering more than 29 million participants, mostly from high-income countries. Of these, 34 articles were included in the meta-analysis: 15 from North America, 10 from Europe, seven from Asia, and two from Australia.

The researchers found a positive and statistically significant association between three types of pollutants and dementia:

1. Particulate matter with a diameter of 2.5 micrometers or less (PM2.5) is a pollutant made up of tiny particles small enough to penetrate deep into the lungs when inhaled. These particles are formed from a variety of sources, including vehicle exhaust, power plants, industrial processes, wood stoves and fireplaces, and construction dust. They can also form in the atmosphere as a result of complex chemical reactions involving other pollutants, such as sulfur dioxide and nitrogen oxides. The particles can remain airborne for long periods of time and travel long distances from where they form.
2. Nitrogen dioxide (NO₂) is one of the main pollutants produced by burning fossil fuels. It is present in vehicle exhaust (especially diesel), industrial emissions, and gas stoves and heaters. Exposure to high concentrations of NO₂ can irritate the respiratory system, aggravate and cause diseases such as asthma, and reduce lung function.
3. Soot – from sources such as car exhaust and wood burning. It can trap heat and affect the climate. When inhaled, it penetrates deep into the lungs, worsening respiratory diseases and increasing the risk of cardiovascular problems.

According to the researchers:

• Every 10 micrograms per cubic metre (μg/m³) of PM2.5 increases the relative risk of dementia by 17%. In 2023, the average PM2.5 reading by a road in central London was 10 μg/m³.
• Every 10 µg/m³ of NO₂ increases the risk by 3%. Average roadside levels in central London in 2023 were 33 µg/m³.
• Every 1 μg/m³ of soot (as part of PM2.5) increases the risk by 13%. In 2023, average annual concentrations of soot measured near roads were: in London - 0.93 μg/m³, in Birmingham - 1.51 μg/m³, in Glasgow - 0.65 μg/m³.

Lead author Dr Haneen Khreis, from the MRC Epidemiology Unit, said:

"Epidemiological data are key to determining whether and by how much air pollution increases the risk of dementia. Our work provides further evidence that long-term exposure to outdoor air pollution is a risk factor for dementia in previously healthy adults."

"Tackling air pollution can deliver long-term benefits for health, society, the climate and the economy. It can reduce the enormous burden on patients, families and carers, and ease the pressure on overstretched health systems."

Several mechanisms have been proposed to explain how air pollution may cause dementia, primarily through inflammation in the brain and oxidative stress (a chemical process that can damage cells, proteins, and DNA). Both of these processes are well-known factors in the onset and progression of dementia. Air pollution may trigger them either directly, by entering the brain, or through the same mechanisms that underlie lung and cardiovascular disease. Pollutants can also pass from the lungs into the bloodstream and reach internal organs, causing both local and systemic inflammation.

The researchers note that most of the study participants were white and lived in high-income countries, although marginalized populations tend to be exposed to higher levels of pollutants. Given that previous research has shown that reducing air pollution is particularly effective in reducing the risk of premature death in these groups, the authors call for more complete and adequate representation of ethnic groups, low- and middle-income countries, and communities in future studies.

Joint first author, Claire Rogowski, also from the MRC Epidemiology Unit, said:

"Efforts to reduce exposure to these key pollutants are likely to help reduce the burden of dementia in society. Tighter controls on a range of pollutants will be needed, targeting the main sources – transport and industry. Given the scale of the problem, urgent regional, national and international policies to tackle air pollution equitably are needed."

Further analysis showed that while exposure to these pollutants increased the risk of Alzheimer’s, the effect was greater in vascular dementia, a type of dementia caused by a disruption of the blood supply to the brain. In the UK, about 180,000 people have this type of dementia. However, because there were limited studies comparing differences between the types, the authors did not consider the result statistically significant.

Joint first author Dr Christian Bredell, from the University of Cambridge and North West England NHS Trust, added:

"These findings highlight the need for a multidisciplinary approach to dementia prevention. Preventing dementia is not just a health issue: this study reinforces the position that urban planning, transport policy and environmental regulation play an equally important role."