Breathing polluted air may affect more than the lungs and heart. A new study suggests that common outdoor air pollutants are also linked to changes in parts of the brain that are especially vulnerable to Alzheimer’s disease.
The research was led by scientists at the USC Mark and Mary Stevens Neuroimaging and Informatics Institute at the Keck School of Medicine of USC. The findings were published in the journal NeuroToxicology.
Alzheimer’s disease is the most common cause of dementia and gradually damages memory, thinking and independence. Scientists know that age and genes play major roles, but they are increasingly studying environmental factors that may influence how quickly the brain ages.
Air pollution is one possible factor because nearly everyone is exposed to it. Previous studies have connected long-term pollution exposure with poorer thinking abilities and a higher risk of dementia, although researchers are still working out what happens inside the brain.
The new study included 1,484 adults who did not have dementia and had no history of stroke. Researchers compared MRI brain scans with estimates of pollution around the places where participants had lived during the three years before their scans.
The team focused on two widespread pollutants. One was PM2.5, tiny particles that are less than 2.5 micrometers across and can come from traffic, power generation, wildfires and other forms of burning.
Because these particles are so small, they can travel deep into the lungs and some may influence the body beyond the respiratory system. The second pollutant was nitrogen dioxide, or NO2, a gas strongly associated with traffic and the burning of fossil fuels.
The researchers measured the thickness of the cerebral cortex, the folded outer layer of the brain. This tissue is involved in memory, language, decision-making and many other abilities, and it normally becomes thinner as people grow older.
They paid particular attention to four areas that are often affected by Alzheimer’s disease. These regions help with memory and the recognition and processing of information and can show changes before severe dementia develops.
The study involved two very different groups. One included 1,097 women from the Women’s Health Initiative Memory Study who were about 78 years old on average, while the other included 387 men from the Vietnam Era Twin Study of Aging whose average age was about 62.
Among the older women, greater exposure to both PM2.5 and nitrogen dioxide was associated with a thinner cortex in the Alzheimer’s-sensitive brain regions. Higher PM2.5 exposure was also linked with thinning across many other parts of the brain.
The researchers estimated that each additional microgram per cubic meter of PM2.5 was associated with a difference in cortical thickness similar to about 13 months of normal aging. Each additional part per billion of nitrogen dioxide corresponded to roughly three months of aging.
These comparisons do not mean pollution literally aged each person’s brain by a fixed number of months. They are a way of showing the size of the statistical differences seen in the study.
The results in the younger men were unexpectedly different. Higher pollution exposure was associated with a thicker, rather than thinner, cortex in the same Alzheimer’s-vulnerable regions.
At first glance, thicker brain tissue might sound healthier. But researchers say that is not necessarily true, because some disease processes may temporarily make tissue appear thicker before later damage and tissue loss occur.
Possible explanations include inflammation, swelling or changes inside nerve cells. Early buildup of proteins connected with Alzheimer’s might also play a role, although the study did not measure amyloid, tau or brain inflammation and therefore cannot establish any of these explanations.
Age provided another interesting clue among the men. The link between PM2.5 and thicker cortex became weaker from the mid-50s into the 60s and appeared to reverse after about age 65, although the later association with thinning was not statistically strong enough to rule out chance.
Lead author Lauren Salminen said the contrasting results suggest that the brain’s response to pollution may change during aging. A possible explanation is that early biological stress produces temporary thickening, followed years later by tissue loss, but this remains a hypothesis.
There is another major complication: the younger group consisted of men and the older group consisted of women. Because age and sex differed at the same time, researchers cannot tell whether the contrasting findings were caused by age, biological sex, other differences between the groups or some combination of these factors.
The study also looked at participants at one point in time. It therefore cannot prove that pollution caused their brain differences, and it cannot show whether people with thinner or thicker cortex later developed Alzheimer’s disease.
Still, the findings add to a growing body of research suggesting that the environment may influence brain aging. Air pollution is especially important because exposure can affect entire cities and communities rather than only individuals with a particular lifestyle or genetic risk.
The strongest part of the study is its use of two independent groups, detailed residential pollution estimates and MRI measurements across many brain regions. The broad thinning seen among older women also suggests that pollution-related changes may not be limited to areas traditionally associated with Alzheimer’s.
Its biggest weakness is the difficulty of interpreting the opposite findings in the two groups. Without following the same men and women over many years, researchers cannot know whether thicker and thinner cortex truly represent different stages of the same process.
Future studies will need repeated brain scans along with measurements of memory, inflammation, amyloid and tau. If they show that pollution exposure predicts a clear sequence of brain changes and later dementia, the case for a direct role in Alzheimer’s disease would become much stronger.
For now, the study should not be read as proof that living in a polluted area will cause Alzheimer’s. Instead, it offers another reason to investigate cleaner air as a possible way to protect not only the lungs and heart, but also the aging brain.
If you care about brain health, please read studies about vitamin D deficiency linked to Alzheimer’s and vascular dementia, and extra-virgin olive oil could boost brain function.
For more information about brain health, please see recent studies about antioxidants that could help reduce dementia risk, and strawberries could help prevent Alzheimer’s disease
Source: Keck School of Medicine of USC.
