It is fairly settled that evident particulate air pollution, such as daily exposure to smoke from wood-fueled cooking fires, has a strongly detrimental effect on long-term health. The mechanisms involved are inflammatory in nature, in that breathing in airborne particles of this nature produces inflammatory signaling that harms tissue function throughout the body, accelerating the onset and progression of all age-related conditions. This is particularly the case for atherosclerosis.
A primary challenge to the study of this correlation between health and particulate air pollution is the confounding effects of wealth and status. Wealthier populations tend to live in areas with lower levels of particulate air pollution, poorer populations in areas with higher levels of particulate air pollution. It is possible to find natural experiments in which the question of wealth is minimized, however. One can look at an interesting paper comparing populations of a similar socioeconomic status in China, for example, showing that higher particulate air pollution correlates with a shorter life expectancy and higher incidence of cardiovascular and respiratory mortality.
Today's research materials are similarly interesting, focused on people who lived in and around Seattle over the past fifty years. This is a small region of the US, but one with enough of a difference in particulate air pollution from site to site to see effects. There is also enough data on income levels and education by location in the Puget Sound to say something about whether or not the burden of particulate exposure falls equally on rich and poor. That said, the way in which this study is constructed leaves open a range of questions about whether the authors have successfully controlled for, say, the effects of medical and public health progress over time since the 1970s. It is worth reading the discussion at the end of the paper in that context.
Using data from two large, long-running study projects in the Puget Sound region - one that began in the late 1970s measuring air pollution and another on risk factors for dementia that began in 1994 - researchers identified a link between air pollution and dementia. In the study, a small increase in the levels of fine particle pollution (PM2.5 or particulate matter 2.5 micrometers or smaller) averaged over a decade at specific addresses in the Seattle area was associated with a greater risk of dementia for people living at those addresses. "We found that an increase of 1 microgram per cubic meter of exposure corresponded to a 16% greater hazard of all-cause dementia. There was a similar association for Alzheimer's-type dementia."
Air pollution may be associated with elevated dementia risk. Prior research has limitations that may affect reliability, and no studies have evaluated this question in a population-based cohort of men and women in the United States. Using the Adult Changes in Thought (ACT) population-based prospective cohort study in Seattle, we linked spatiotemporal model-based PM2.5 exposures to participant addresses from 1978 to 2018. Dementia diagnoses were made using high-quality, standardized, consensus-based protocols at biennial follow-ups. We conducted multivariable Cox proportional hazards regression to evaluate the association between time-varying, 10-year average PM2.5 exposure and time to event in a model with age as the time axis, stratified by apolipoprotein E (APOE) genotype, and adjusted for sex, education, race, neighborhood median household income, and calendar time.
We report 1,136 cases of incident dementia among 4,166 individuals with nonmissing APOE status. Mean 10-year average PM2.5 was 10.1 μg/m3. Each 1-μg/m3 increase in the moving average of 10-year PM2.5 was associated with a 16% greater hazard of all-cause dementia. Our results strengthen evidence on the neurodegenerative effects of PM2.5.