Air Pollutants, Especially From Traffic, Independently Increase Stroke Risk

Long-term exposure to air pollutants independently increases risk for stroke among older adults, according to a large, nationwide cohort study published in Global Epidemiology.

Worldwide, stroke is the second leading cause of death and the leading cause of long-term disability. There has been mounting evidence that air pollution may influence stroke risk. As such, the World Health Organization and National Academy of Science have prioritized efforts to establish which air pollutants contribute more to risk.

The data from this study were sourced from the Medicare Chronic Conditions Warehouse, Medicare dominator file, and the Environmental Protection Agency, Air Quality Systems databases. Between 2000 and 2017, all Medicare beneficiaries (N=17,443,900) aged 65 years and older were evaluated for incidence of stroke and risk was related with exposure to particulate matter with aerodynamic diameter <2.5 μm (PM2.5), nitrogen dioxide (NO2), and ground-level ozone (O3). Pollutant exposure was predicted using an ensemble-based machine learning approach.

The study population was aged 65-74 years (88.2%), 58.2% were women, and 88.3% were White. During the study period, 12.8% (n=2,240,586) had an incident stroke.

Mean annual PM2.5 exposure was 9.4 (standard deviation [SD], 3.7) μg/m3, NO2 exposure was 17.5 (SD, 12.4) ppb, and warm-season O3 was 42.8 (SD, 6.5) ppb. According to the EPA’s annual standards for PM2.5 and NO2, 66% and 99% of the cohort was exposed to safe levels, respectively.

PM2.5 exposure was highest in the eastern region and West Coast of the US, NO2 was concentrated in urban centers, and warm-season O3 was increased in the western region. The average annual ZIP code exposure levels were correlated between PM2.5 and NO2 (r, 0.38), PM2.5 and O3 (r, 0.28), and NO2 and O3 (r, 0.21).

After adjusting for potential covariates, long-term exposure to all 3 pollutants increased risk for incident stroke in all models. A 12.4 ppb elevation in mean annual NO2 increased risk for stroke the following year (hazard ratio [HR], 1.073; 95% CI, 1.068-1.079). Similarly, an increased PM2.5 level of 3.7 μg/m3 associated with a HR of 1.056 (95% CI, 1.051-1.061) and O3 of 6.5 ppb a HR of 1.035 (95% CI, 1.031-1.039). Tri-pollutant models did not attenuate the relationship between pollutants and stoke risk.

The pollutant that had the strongest relationship with stroke was NO2. If exposure to NO2 could be reduced by 12.4 ppb, an estimated 6% of strokes would be avoided.

In an effect modification analysis examining gender, race, Medicaid eligibility, age, neighborhood-level income, and education, all modifiers except race were robust in the single- and tri-pollutant models.

The major limitation of this study is the potential misestimation of exposure to pollutants.

The study authors noted that this study provided epidemiological evidence supporting the relationship between long-term exposure to pollutants with increased stroke risk.

“The strongest relationship we found with incident stroke was for NO2 among the 3 pollutants, suggesting that traffic-related air pollution may play a particularly significant role in increased risks of stroke,” researchers concluded.

Reference

Ma T, Yazdi MD, Schwartz J, et al. Long-term air pollution exposure and incident stroke in American older adults: A national cohort study. Global Epidemiol. 2022;4:100073. doi:10.1016/j.gloepi.2022.100073