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New research has strengthened the link between pollution and the development of dementia. Felix Cesare/Getty Images
  • A new study in rodents has found a link between traffic-related air pollution and a higher risk of age-related dementia.
  • In the study, exposure to traffic-related air pollution accelerated Alzheimer’s disease characteristics in animals who express the risk gene and in wild-type rats.
  • The researchers concluded that traffic-related air pollution might decrease the time to Alzheimer’s onset and accelerate disease progression.

A growing number of studies suggests that air pollution may be associated with a higher risk of developing dementia or Alzheimer’s disease.

The World Health Organization (WHO) reports that around 50 million people globally have dementia.

Alzheimer’s disease, which is an irreversible, progressive neurological condition that causes memory loss and cognitive decline, is the most common form of dementia. It accounts for an estimated 60–80% of dementia cases.

A new study by researchers at the University of California, Davis adds to the body of research suggesting that there may be a link between traffic-related air pollution and an increased risk of developing age-related dementia, including Alzheimer’s disease.

For their study, which now appears in the journal Environmental Health Perspectives, the researchers set up a rodent vivarium near a well-used traffic tunnel in Northern California. They did this to replicate the pollution that humans might have exposure to while in traffic.

Senior study author Dr. Pamela Lein, a professor of neurotoxicology at the University of California, Davis, told Medical News Today about a key difference between this research and other efforts.

She said, “Because epidemiologic studies can provide evidence regarding the strength of association between exposure and outcome but cannot establish a cause-effect relationship, there has been a need for experimental animal studies to confirm causality.”

“However,” added Dr. Lein, “the criticism of much of the published animal data to date is that the exposures used have not faithfully mimicked human exposures. [This is because the] animals have been exposed to a subset of components that make up traffic-related air pollution and/or because animals have been exposed to very high concentrations of traffic-related air pollutants, often for relatively short periods of time.”

“Our study is relatively unique in that animals were exposed to ambient traffic-related air pollution in real time over the course of their lifetime, providing strong data to corroborate the epidemiologic data.”

– Dr. Pamela Lein

For their study, the researchers exposed male and female rats to either filtered air or polluted air for up to 14 months. They drew the polluted air from the busy tunnel in real time and delivered it straight to the animals unchanged.

The team divided the rats into two groups: One group comprised those who expressed Alzheimer’s disease risk genes that are relevant to humans, and one group comprised wild-type rats.

The researchers conducted this testing in 3-, 6-, 10-, and 15-month-old rats. They quantified the expression of Alzheimer’s disease characteristics and carried out hyperspectral imaging and behavioral testing.

The researchers found that having exposure to chronic traffic-related air pollution accelerated and exacerbated traits relevant to Alzheimer’s disease in the rats who were genetically susceptible to the condition. They also saw the same effect in wild-type rats.

“Our data demonstrated that traffic-related air pollution decreases the time to onset and increases the severity of disease in rats who expressed genetic risk factors for Alzheimer’s disease,” Dr. Lein told MNT.

This, she explained, suggests “that differences in exposure histories may contribute to the differing clinical profiles observed in individuals with similar genetic backgrounds.”

Interestingly, she also told MNT that their “data suggest that even individuals who do not express [Alzheimer’s disease] risk genes are at increased risk [of] Alzheimer’s disease if they are chronically exposed to [traffic-related air pollution].”

Dr. Heather Snyder, the vice president of medical and scientific relations at the Alzheimer’s Association, told MNT that she appreciated that the researchers behind this study are “looking at what might be a mechanism, what might be the biological underpinnings” that contribute to the onset of Alzheimer’s disease.

She cautioned that it is too early to say how the findings will translate to humans. “Alzheimer’s, and actually all causes of dementia, are […] complex diseases, and there are likely a number of things that are contributing to a person’s risk,” Dr. Snyder said.

For her part, Dr. Lein is excited to tackle additional research. “Our immediate goal is to determine which component(s) of the traffic-related air pollution mediate the effects on the aging brain,” she told MNT.

“Is it the particulate matter, or the gases, or both? Are components from light duty (most cars) or heavy duty (trucks, including diesel trucks) more important in promoting Alzheimer’s disease phenotypes? The answer to this will be critically important for regulatory policymaking.”

Additionally, Dr. Lein explained that she and the other researchers behind this study are eager to understand how traffic-related air pollution promotes Alzheimer’s disease characteristics. They also want to learn more about windows of sensitivity.

“Do early life exposures to traffic-related air pollution cause changes in the brain that manifest as [Alzheimer’s disease] phenotypes in old age?” she asked. “Or is it only exposures [in] mid-late life that are critical for increasing risk? Or do you really need chronic exposure across life to see the effects of traffic-related air pollution on the brain?”

The goal of her research, Dr. Lein told MNT, is to identify specific environmental factors that are associated with an increased risk of developing Alzheimer’s disease. These, after all, are easier to address than genetic risk factors.

This study and future research, Dr. Lein explained, will help lawmakers who “need data identifying specific environmental risk factors in order to enact meaningful policy changes.”