People living in greener, more walkable neighborhoods may face a lower risk of developing dementia, while those exposed to more air pollution might face a higher risk, according to a large review of existing studies. The findings, published in Ageing Research Reviews, highlight connections between environmental factors and brain health as people age.

Dementia is a growing global concern, affecting nearly 50 million people worldwide. It encompasses a range of disorders that impair memory, thinking, and daily functioning, including Alzheimer’s disease. With an aging population and no available cure, finding ways to prevent or delay dementia has become a pressing goal in public health. While many studies have examined genetic and lifestyle factors, researchers are now turning their attention to environmental influences. Previous studies have hinted that elements such as air quality, traffic noise, and neighborhood design may affect brain health, but results have been inconsistent and often based on subjective reports.

To address this gap, a team of researchers conducted a systematic review and meta-analysis. This means the researchers carefully examined and combined data from many previously published studies. A systematic review aims to gather all relevant evidence on a topic, while a meta-analysis uses statistical techniques to combine the results of those studies and provide a clearer picture of how strong the observed effects are. Only studies that used objective measures—such as satellite data, geographic information systems, and pollution monitoring—were included to ensure the most reliable results.

“My interest in this research topic is because of the urgent global challenge posed by dementia. As dementia affects millions of people and effective treatments remain limited, I was driven to explore modifiable risk factors that could help delay or even prevent cognitive decline,” said study author Linlin Da, a PhD student at the University of Georgia.

“I became particularly interested in investigating the role of the built environment, and how factors such as air pollution, urban design, and the availability of green spaces might influence cognitive health. By utilizing objective measures to assess these environmental exposures, I aimed to address inconsistencies in previous studies and contribute to a clearer understanding of how urban planning and environmental policies could support dementia prevention.”

The researchers searched five major scientific databases and initially identified over 1,800 studies. After a thorough screening process, they selected 54 high-quality studies for their systematic review, 21 of which were also suitable for meta-analysis. The included studies came from around the world, with sample sizes ranging from under 200 to over 63 million individuals, most of whom were aged 60 and older. These studies measured environmental exposures like fine particulate matter in the air (known as PM2.5), nitrogen dioxide, proximity to green spaces, and access to local services. Outcomes included diagnosed dementia or measures of cognitive decline over time.

The researchers found that people exposed to higher levels of air pollution were more likely to develop dementia. Specifically, exposure to fine particulate matter—tiny particles that can be inhaled into the lungs and bloodstream—was associated with a 9% higher risk. Nitrogen dioxide exposure was linked to a 10% higher risk. Living near major roads, often a source of both air pollution and noise, was also tied to a 10% increase in dementia risk.

In contrast, people living in neighborhoods with more green and blue spaces—such as parks, forests, rivers, or lakes—had a 6% lower risk of developing dementia. These areas were also associated with slower cognitive decline. Neighborhoods with more connected street layouts and better walkability also supported cognitive health, possibly by encouraging physical activity and social engagement. Access to food stores, community centers, and healthcare services was similarly beneficial.

While these associations don’t prove direct cause and effect, they are consistent with what scientists already know about how the environment can affect the brain. Air pollution is known to cause inflammation and oxidative stress in the body, which can harm brain tissue and potentially contribute to dementia. On the other hand, green spaces may help protect the brain by reducing stress, encouraging exercise, and improving sleep—all of which are known to support cognitive health.

The study did have some limitations. The researchers found considerable variation across the studies they analyzed. This so-called heterogeneity reflects differences in study designs, populations, and how environmental factors were measured. Some factors, like indoor air quality or socioeconomic status, were not always taken into account, and the methods used to estimate environmental exposure may not perfectly reflect individuals’ real-life experiences.

Still, the authors emphasize that their findings provide strong evidence that environmental features play a meaningful role in brain health. By focusing on studies that used objective measurements and following participants over time, the researchers were able to offer a more reliable picture than many past efforts.

Song hopes the findings will help shape future research and public health efforts. “These numbers show that living in a dementia-friendly environment is important to delay or prevent cognitive decline and the onset of dementia,” said Suhang Song, an assistant professor at the University of Georgia.

Urban planning decisions—like adding parks, improving air quality, and making neighborhoods more walkable—may offer a promising path toward protecting cognitive health at the population level. Future research may focus on how to better measure these environmental exposures over time, and how they interact with genetics, lifestyle, and other personal risk factors.

The study, “Objectively measured environmental features and their association with cognition and dementia: A systematic review and meta-analysis,” was authored by Linlin Da, Xia Song, Zimu Jia, Nicholas Gary Lamont Myers, Jin Sun, Jingkai Wei, Daniel Jung, Feiyang Li, and Suhang Song.