A study investigating the relationship between air pollution, specifically PM2.5 particles, and dementia rates in the United States found that individuals residing in areas with higher levels of this type of air pollution were marginally more likely to develop dementia. PM2.5 particle concentrations originating from agriculture and wildfires showed the most significant correlation with increased dementia rates. The findings were published in JAMA Internal Medicine.
PM2.5 particles, or fine particulate matter, are microscopic airborne particles measuring 2.5 micrometers or smaller in diameter. These particles are so small that they can penetrate deep into the lungs and even enter the bloodstream. Sources of PM2.5 include coal-fired power plants, agricultural activities, vehicle emissions, industrial processes, residential heating, wildfires, and natural occurrences like dust storms and volcanic eruptions.
Research has linked PM2.5 exposure to various health issues. Short-term exposure can cause irritation to the eyes, nose, throat, and lungs, leading to symptoms such as coughing, sneezing, and shortness of breath. Long-term exposure is associated with more severe health problems, including an increased risk of respiratory and cardiovascular diseases like asthma, heart attacks, and strokes, as well as a heightened risk of premature death, especially among older adults and those with pre-existing conditions. Due to their significant impact on human health, monitoring and regulating PM2.5 levels is essential for public health efforts worldwide.
Study author Boya Zhang and her team noted that PM2.5 exposure has also been linked to cognitive health problems. However, PM2.5 can originate from various sources, and particles from different sources possess distinct chemical and physical characteristics. For instance, PM2.5 from traffic tends to contain nitrates and black carbon, while particles from agricultural activities are more likely to contain ammonium. The question of whether PM2.5 from all sources affects the brain in the same way, or if different types of particles have varied effects on brain health, remains unanswered.
To address this knowledge gap, the study authors analyzed data from the Environmental Predictors of Cognitive Health and Aging (EPOCH) study, which combined biennial survey data from 1998 to 2016 of participants in the Health and Retirement Study with estimates of PM2.5 particle concentrations at the participants’ residences.
The study included data from 27,857 individuals over 50 years old who did not have dementia at the beginning of the study. The average age of the participants was 61 years, and 56.5% were female. Over the study period, 4,105 participants developed or began to show signs of developing dementia.
PM2.5 levels were estimated based on over 300 characteristics, including nearby transportation, land use and cover, population density, emission sources, and vegetation, allowing the researchers to derive annual mean PM2.5 particle concentration estimates for each survey respondent between 1990 and 1999. This methodology produced estimates of both total particle concentrations and concentrations of PM2.5 particles from specific sources.
The results indicated that the median concentration of PM2.5 particles during the study period was 11.2 μg/m^3. Particle concentrations from agriculture, traffic, energy production, and other industrial sectors were generally higher in the Midwest and lower in the West of the U.S. Conversely, concentrations of particles from wildfires and windblown dust were higher in the West and Southwest. The concentrations tended to be higher in areas where older participants, those with less formal education, lower wealth, and non-Hispanic Blacks resided, except for particles from wildfires and windblown dust.
The study found that individuals living in areas with higher concentrations of PM2.5 particles were slightly more likely to develop dementia, with an 8% increase in risk per interquartile range increase in PM2.5 concentrations, compared to those in areas with lower concentrations. The greatest increase in risk was associated with PM2.5 particles from agricultural activities (17% increase per interquartile range), while PM2.5 particles from windblown dust were not linked to an increased risk of developing dementia. Following agriculture, PM2.5 particles from traffic also showed a significant association with increased dementia risk, whereas the links between PM2.5 particles from wildfires and coal combustion and dementia risk were less pronounced.
“Our cohort study suggests that reducing PM2.5 and perhaps selectively targeting certain sources for policy interventions might be effective strategies to reduce the burden of dementia at the population level, although more research is needed to confirm our findings,” the study authors concluded.
The study makes an important contribution to the scientific understanding of links between air pollution and cognitive health. However, it should be noted that all the observed associations are weak and the associated increases in risk are only slight. Additionally, the study design does not allow for any cause-and-effect conclusions to be drawn from the results. While it is possible that air pollution leads to dementia it might also be that individuals prone to dementia are for some reason slightly more likely to live in areas with higher air pollution. There are other possibilities as well.
The paper, “Comparison of Particulate Air Pollution From Different Emission Sources and Incident Dementia in the US,” was authored by Boya Zhang, Jennifer Weuve, Kenneth M. Langa, Jennifer D’Souza, Adam Szpiro, Jessica Faul, Carlos Mendes de Leon, Jiaqi Gao, Joel D. Kaufman, Lianne Sheppard, Jinkook Lee, Lindsay C. Kobayashi, Richard Hirth, and Sara D. Adar.