More evidence points to the idea that air pollution may increase the risk of Alzheimer disease. A new study suggests that tiny polluting particles carried by dirty air can enter the brain, possibly contributing to cognitive decline.

It is no secret that ambient air pollution is an important risk factor for various health concerns.

According to the World Health Organization (WHO), 9 in every 10 people worldwide breathe highly polluted air. Importantly, poor air quality contributes to millions of deaths globally per year.

Although it is easy to understand how the air we breathe might affect our respiratory system—with research showing that ambient pollution contributes to lung cancer—it can be harder to understand how, or even if it impacts the health of other major organs.

In fact, an increasing amount of evidence suggests that air pollution is associated with cognitive decline, and that it may contribute to the ever growing number of Alzheimer disease cases.

For example, one study that Medical News Today covered in 2018 suggests that people with the most exposure to top air pollutants have a higher risk of dementia, while another went as far as to argue that poor air quality causes around a fifth of dementia cases.

New research in the journal Brain adds to the evidence linking air pollution and Alzheimer's disease. It also points out a potential mechanism through which pollutants may affect brain function.

"This is the first study to really show, in a statistical model, that air pollution was associated with changes in people's brains and that those changes were then connected with declines in memory performance," says study co-author Andrew Petkus.

Petkus is an assistant professor of clinical neurology at the Keck School of Medicine at the University of Southern California in Los Angeles.

'Another piece of the Alzheimer puzzle?'

The researchers analyzed the data of 998 female participants, aged 73–87, all of whom were enrolled in the Women's Health Initiative. The participants had undergone up to two brain scans, 5 years apart, as part of the larger study.

In the course of their research, the investigators gave each brain scan a cognitive decline score. They did this by using a machine learning model that used data from brain scans of people with Alzheimer disease.

The team then combined this information with data on where the study participants lived, and with information on the levels of pollution in those areas. The latter measure allowed the investigators to roughly determine the participants' degree of exposure to fine particle pollution.

Fine particles, or PM2.5 particles, are tiny pollutants measuring about one-thirtieth of the width of a human hair. They enter ambient air through traffic exhaust fumes and smoke.

Because PM2.5 particles can remain airborne for a long time, they are easily inhalable, which means that they can accumulate in unhealthful amounts in people's bodies—including inside the brain.

By analyzing all these data, the researchers found that the higher a person's exposure to fine particle pollution, the likelier they were to experience cognitive function impairments, such as problems with memory.

This association remained even after the investigators adjusted their analysis to account for confounding factors, including income, education level, geographic location, and smoking status.

For this reason, the study authors believe that a buildup of PM2.5 particles in the brain may contribute to the process of cognitive decline.

"Our hope is that by better understanding the underlying brain changes caused by air pollution, researchers will be able to develop interventions to help people with or at risk [of] cognitive decline," says Petkus.

"This study provides another piece of the Alzheimer's disease puzzle by identifying some of the brain changes linking air pollution and memory decline. Each research study gets us one step closer to solving the Alzheimer's disease epidemic."

—Andrew Petkus