A growing body of evidence over the last few years has demonstrated that air pollution is a significant risk factor for developing dementia in old age. It shouldn’t be much of a shock. “When sensitive people breathe in polluted air from outdoors, very small particles can penetrate the lungs and get into the circulation system,” Jiu-Chiuan Chen, a neuroscientist at the University of Southern California, told The Daily Beast. “These ‘toxic’ responses can make the blood-brain barrier leaky and cause damage to the brain.” It’s not all that different from how other risk factors like smoking can negatively impact human health, leaving it more vulnerable to degenerative conditions like dementia.
The big question scientists have had in the last few years was whether the impact of air pollution on dementia risk was permanent—or whether this was something that could be reversed. New findings published in Proceedings of the National Academy of Sciences bring good news: Improved air quality over several years is associated with a reduced risk of dementia in elderly women.
The findings bolster suspicions that external pollutants can contribute to accelerated aging in the brain (which is effectively what dementia is). But more critically, they also show that this aging can be slowed down if exposure to those pollutants decreases.
Led by Chen and others from USC, a group of researchers assessed the yearly physical and cognitive health of 2,239 women in the U.S. aged 74 to 92, from 2008 to 2018. (Chen and his colleagues focused on women since they are disproportionately affected by neurodegenerative diseases like Alzheimer's, which can lead to dementia). Those women were geographically spread out over the country, and all were dementia-free from at least 2008 to 2012. The researchers compared those assessments with yearly average concentrations of outdoor pollution from 1998 and 2012 to determine in what locations air quality was trending to healthier levels. The study focused on measurements for fine particulate matter and nitrogen dioxide—two very common pollutants resulting from traffic exhaust.
Over those 10 years, 398 women were diagnosed with dementia. But the researchers found that locations with larger improvements in air quality showed a smaller incidence in dementia diagnoses among the women who took part in the study. While the reduction in risk varies with other factors, Chen explained that lowering air pollution exposure to roughly 15 percent below the EPA’s current standard threshold led to a 20 percent reduction in dementia risk.
Moreover, the link between lower dementia risk and better air quality did not differ significantly by age, education, geography, or cardiovascular risk factors, suggesting that air pollution actually plays a bigger role in dementia than previously thought.
The study is far from perfect. The findings would probably translate to older men, but we can’t be certain yet without actual data to confirm it. Moreover, some women may have been exposed to much different levels of air pollution in their everyday lives than what the general air quality of their surroundings suggests. Other factors like green spaces may have affected air quality at the local level.
But there’s little reason to doubt the overall takeaway: There’s no disentangling human health from environmental health.