A new study published in March 2020 in the journal JAMA Neurology shows an association between air pollution and the development of dementia. However, the role played by cardiovascular disease (CVD) is yet to become apparent.
Association Between Cardiovascular Disease and Long-term Exposure to Air Pollution With the Risk of Dementia. Image Credit: VanderWolf Images / Shutterstock
Dementia is a condition in which the memory fails, accompanied by confusion, disorientation, failure of judgment and decision making, and other cognitive skills. The person typically becomes dependent over time unless death supervenes.
Scientists forecast that there will be three times as many patients with dementia in the next 30 years, and there is no cure for the condition at present.
The current study is focused on the identification of the role of air pollution in brain degeneration, oxidative stress, and inflammation of the nervous tissue. Some research has shown that intellectual skills suffer more rapidly when the levels of pollutants in the air are higher. CVD is also a known outcome of higher levels of air pollution.
CVD and dementia
Air pollution is a global reality today, which lends urgency to the task of reducing this modifiable risk factor to reduce the burden of dementia in the future.
How does air pollution affect the brain? Some think that fine particles may enter the bloodstream and cause inflammation throughout the body. This, in turn, activates inflammatory cells inside the brain by the damage caused to the blood-brain barrier.
Only one recent study from Canada has looked into the role played by CVD in the association of dementia with air pollution. It found that high levels of PM2.5 and NOx exerted an indirect effect on dementia risk by increasing the risk of CVD. The current study supports this finding as well as those of numerous studies that have reported links between air pollution and dementia. One US study pegged the increase in the risk of dementia with a significantly high level of air pollution, above the regulatory standards for that area, at 92% above that reported for people living in unpolluted areas.
Indirect damage to the brain can also occur because of the harmful effects of air pollution on the heart and blood vessels by increasing the heart rate, thickening the blood, promoting blood clotting, and making the arterial plaques more fragile. These changes could result in heart attacks, heart failure, and strokes. All of these are known to increase the rate of dementia changes or to cause the early onset of dementia.
The data for the current cohort study was taken from the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K). The study included over 2,900 elderly people without dementia at baseline. They were followed up to 2013 for a mean period of 6 years.
The investigators examined the levels of two major air pollutants: particulate matter ≤2.5 μm (PM2.5) and nitrogen oxide (NOX). Yearly measurements were taken from 1990 onwards. The scientists used statistical methods to derive the at-home levels of these two pollutants for those who were part of the study.
They used these measurements and the number of new cases of dementia to estimate the risk of developing this condition. They also tried to find out how CVD was associated with this risk – was it the mechanism through which air pollution caused dementia, or did it modify the risk?
The study was carried out on people from a city with a relatively low air pollution level.
The participants were aged 74 years, on average, and about 63% were female. There were 364 new cases of dementia in the group.
The per-quartile change in the mean levels of each pollutant over the preceding 5 years was linked with a rise in the risk of dementia by 14% and 50% for PM 2.5 and NOX, respectively. In other words, with each increase of 0.88 μg/m3 rise in PM2.5 levels, the risk of dementia went up by 54%. With a rise of 8.35 μg/m3 NOx, the risk went up by 14%.
When the individual also had heart failure, the risk of dementia rose by 93% in the presence of increasing PM2.5, and by 43% with rising NOx levels. If the person had ischemic heart disease, dementia risk increased by 67% and 36% in the presence of PM2.5 and NOx, respectively.
Additionally, almost half of dementia cases related to air pollution were caused by strokes found in the study.
The study found that dementia risk increases when a person is exposed to air pollution over a long period. This link is stronger when there are heart failure and ischemic heart disease. On the other hand, much of the dementia risk caused by air pollution is because of the occurrence of stroke in highly polluted areas.
The study is even more important when we remember it comes from a comparatively, less polluted town. The exposure over the five years preceding the study period seems to contribute more to the poor outcome compared to the 6-11 years before that.
Almost 7 out of 10 people in the world may live in cities by 2050, exposing them to high levels of air pollution. In addition, the mean age of the world population is continuously increasing. These two may act in tandem to increase the risk of dementia. The researchers say, “It might imply that reducing air pollutant levels today could yield better outcomes already in the shorter term, reinforcing the need for appropriately set air quality standards.” Better control and treatment of CVD could help reduce dementia risk in polluted areas still further.
Grande, G., Ljungman, P. L. S., Eneroth, K., Bellander, T., and Rizzuto, D. Association between cardiovascular disease and long-term exposure to air pollution with the risk of dementia. JAMA Neurol. doi:10.1001/jamaneurol.2019.4914. Published online March 30, 2020. https://jamanetwork.com/journals/jamaneurology/fullarticle/2763459