Air pollution associated with higher rates of dementia, poor health

Key takeaways:

• Researchers attributed PM_2.5 exposure to nearly 188,000 new dementia cases in the U.S. per year.
• Air pollution and potentially toxic and hazardous sites may contribute to racial health disparities.

Greater residential levels of fine particulate matter air pollution, especially from agriculture and wildfires, were linked to higher rates of dementia, according to study results published in JAMA Internal Medicine.

A separate study in the same journal found an association between hazardous environmental conditions and poor or fair health, particularly in neighborhoods with a large proportion of Black and Hispanic residents.

Together, the findings demonstrate the health effects of environmental conditions.

Pollution and dementia

Fine particulate matter (PM_2.5) can originate from a variety of environmental sources, like wildfires, agricultural emissions, coal-fired power plants and traffic, according to Boya Zhang, PhD, a graduate student research assistant at University of Michigan School of Public Health, and colleagues.

“Air pollution has recently been recognized as a risk factor for dementia,” they wrote. “Fine particulate matter may affect cognitive function via neuroinflammation as a result of systemic inflammation or oxidative stress following lung irritation.”

It is thought that variations in emission sources contribute to the differences that have been observed in associations across studies of cognitive health and PM_2.5, Zhang and colleagues wrote. However, because PM_2.5 is usually quantified by total mass, it is not known whether PM_2.5 from all emission sources similarly affects the brain “despite clear relevance for designing strategies to reduce PM_2.5 pollution,” they added.

“Unlike many other common risk factors for dementia (eg, hypertension, stroke, and diabetes), exposures to air pollution can be modified at the population level, making it a prime target for large-scale prevention efforts,” they wrote.

So, the researchers conducted a cohort study to evaluate connections between dementia incidence among older adults and exposure to PM_2.5 — both long term and from nine unique emission sources in the United States. To do so, they used survey data from the Health and Retirement Study from 1998 to 2016. The analysis included 27,857 participants aged at least 50 years who did not have dementia at baseline. About 15% developed dementia during a mean follow-up of 10.2 years.

Zhang and colleagues found that, overall, higher concentrations of total PM_2.5 were linked to higher incident dementia rates (HR = 1.08; 95% CI, 1.01-1.17). Also, PM_2.5 from every source except dust was linked to higher dementia rates in single pollutant models. The strongest associations were for:

• agriculture;
• coal combustion;
• traffic; and
• wildfires.

The researchers noted that only wildfires (HR = 1.05 per IQR; 95% CI, 1.02-1.08) and agriculture (HR = 1.13; 95% CI, 1.01-1.27) “were robustly associated with greater rates of dementia.”

“In our secondary analyses, we estimated that nearly 188,000 new cases per year of dementia were attributable to total PM_2.5 exposure in the U.S. if these associations were causal,” they wrote.

Zhang and colleagues concluded that their findings offer further evidence to support reducing PM_2.5 as a “population-based approach to promote healthy cognitive aging” and suggest that interventions on key emission sources may be valuable.

“These findings support the hypothesis that airborne particulate matter pollution is associated with the likelihood of developing dementia and suggest that selective interventions to reduce pollution exposure may decrease the life-long risk of dementia; however, more research is needed to confirm these relationships,” Zhang and colleagues wrote.

Racial disparities in environmental justice

Exposure to air pollution may also be contributing to health disparities, according to Vishal R. Patel, BS, a student at Dell Medical School at the University of Texas at Austin, and colleagues.

“Decades of underinvestment have disproportionately exposed Black and Hispanic communities in the U.S. to hazardous environmental conditions,” they wrote. But how much the exposures have contributed to differences in health is not yet known.”

The researchers conducted a cross-sectional study using the CDC’s Environmental Justice Index — a recently designed “national small area-based measure of cumulative environmental and social burden” — to analyze the association between self-reported health status at the neighborhood level and environmental justice. They evaluated 71,669 neighborhoods in 2020, which represented 98.1% of all conterminous neighborhoods in the U.S.

Patel and colleagues reported finding a direct connection between the prevalence of poor or fair health and neighborhood environmental injustice. However, the association significantly varied based on neighborhood race concentration; neighborhoods with high proportions of people from underrepresented racial populations had a higher prevalence of poor or fair health in areas with the highest environmental injustice scores (P < .001).

For neighborhoods with high proportions of people from underrepresented racial populations, health status variations were connected to potentially hazardous and toxic sites and exposure to air pollution.

Patel and colleagues concluded that the findings are “consistent with historical patterns of systemic racism, de jure segregation, and housing discrimination.”

“Persistence of these disparities emphasizes the need for intentional efforts at the federal policy level to address them,” they wrote. “The Justice Initiative is one such policy that directs 40% of federal investments to disadvantaged communities. These new, small area-based measures of environmental injustice and self-reported health may facilitate investment implementation and measurement of impact.”

References:

• Patel VR, et al. JAMA Intern Med. 2023;doi:10.1001/jamainternmed.2023.2835.
• Zhang B, et al. JAMA Intern Med. 2023;doi:10.1001/jamainternmed.2023.3300.