Scientists Find Shocking Link between Dirty Air, Dementia

In one of the largest studies of its kind, researchers looked at data from almost 30 million people and found that long-term exposure to common air pollutants, like car exhaust and industrial fumes, is linked to a higher risk of dementia. These tiny particles in the air can travel deep into the body, and the study suggests they may contribute to memory loss and cognitive decline, the Lancet Planetary Health reported.

A comprehensive review of nearly 30 million people’s health data has brought new attention to the connection between air pollution and a higher risk of developing dementia. This includes pollution from everyday sources such as vehicle exhaust.

Dementia, including Alzheimer’s disease, currently affects over 57 million people around the world. That number is projected to rise sharply, reaching an estimated 152.8 million cases by 2050. The consequences are far-reaching, placing a heavy burden on individuals, families, caregivers, and society as a whole.

Some encouraging trends show a decline in dementia rates in regions like Europe and North America. This suggests that it may be possible to reduce risk at a population level. However, the situation in other parts of the world remains far more concerning.

In recent years, researchers have identified air pollution as a possible contributor to dementia risk. While several studies have pointed to specific pollutants, the overall strength and consistency of the evidence have varied.

To better understand the connection, scientists from the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge conducted a large-scale review and meta-analysis, recently published in The Lancet Planetary Health. By combining and analyzing results from a wide range of studies, including those with conflicting outcomes, the researchers were able to draw more reliable conclusions about the link between pollution and dementia.

The final analysis included 51 studies and drew from data on over 29 million people, primarily from wealthier nations. Among these, 34 studies were part of the meta-analysis: 15 came 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 air pollutants and dementia. These were:

Particulate matter with a diameter of 2.5 microns or less (PM2.5), a pollutant made up of tiny particles small enough that they can be inhaled deep into the lungs. These particles come from several sources, including vehicle emissions, power plants, industrial processes, wood burning stoves and fireplaces, and construction dust. They also form in the atmosphere because of complex chemical reactions involving other pollutants such as sulphur dioxide and nitrogen oxides. The particles can stay in the air for a long time and travel a long way from where they were produced.

Nitrogen dioxide (NO2), one of the key pollutants that arise 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 high concentrations of nitrogen dioxide can irritate the respiratory system, worsening and inducing conditions like asthma and reducing lung function.

Soot, from sources such as vehicle exhaust emissions and burning wood. It can trap heat and affect the climate. When inhaled, it can penetrate deep into the lungs, aggravating respiratory diseases and increasing the risk of heart problems.

According to the researchers, for every 10 micrograms per cubic meter (μg/m³) of PM2.5, an individual’s relative risk of dementia would increase by 17%. The average roadside measurement for PM2.5 in Central London in 2023 was 10 μg/m³.

For every 10 μg/m3 of NO2, the relative risk increased by 3%. The average roadside measurement for NO2 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%. Across the UK, annual mean soot concentrations measured at select roadside locations in 2023 were 0.93 μg/m³ in London, 1.51 μg/m³ in Birmingham and 0.65 μg/m³ Glasgow.

Senior author Dr. Haneen Khreis from the MRC Epidemiology Unit said: “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.”

Several mechanisms have been proposed to explain how air pollution may cause dementia, primarily involving inflammation in the brain and oxidative stress (a chemical process in the body that can cause damage to cells, proteins, and DNA). Both oxidative stress and inflammation play a well-established role in 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. Air pollution can also enter circulation from the lungs and travel to solid organs, initiating local and widespread inflammation.

The researchers point out that the majority of people included in the published studies were white and living in high-income countries, even though marginalised groups tend to have a higher exposure to air pollution. Given that studies have suggested that reducing air pollution exposure appears to be more beneficial at reducing the risk of early death for marginalised groups, they call for future work to urgently ensure better and more adequate representation across ethnicities and low- and middle-income countries and communities.

Joint first author Clare 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 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.”

Further analysis revealed that while exposure to these pollutants increased the risk of Alzheimer’s disease, the effect seemed stronger for vascular dementia, a type of dementia caused by reduced blood flow to the brain. Around 180,000 people in the UK are thought to be affected by this type of dementia. However, as there were only a small number of studies that examined this difference, the researchers did not class it as statistically significant.

Joint first author Dr. Christiaan Bredell from the University of Cambridge and North West Anglia NHS Foundation Trust said: “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.”

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