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Marginalized groups appeared to benefit more from decreasing levels of PM_2.5 (fine particulate matter less than 2.5 microns in diameter), according to a study using over 600 million person-years of Medicare data.

While the hazard ratio associated with decreasing PM_2.5 from 12 μg/m³ to 8 μg/m³ was 0.963 (95% CI 0.955-0.970) for higher-income white populations, the equivalent hazard ratios for marginalized populations were lower:

• Higher-income Black population: 0.931 (95% CI 0.909-0.953)
• Lower-income white population: 0.940 (95% CI 0.931-0.948)
• Lower-income Black population: 0.939 (95% CI 0.921-0.957)

"Results from sensitivity analyses showed consistent patterns," wrote Francesca Dominici, PhD, of the Harvard T.H. Chan School of Public Health in Boston, and co-authors in the .

"Point estimates from all three alternative modeling strategies suggested that exposure-response curves were steeper for Black persons than for white persons (regardless of income) and for Black higher-income persons than for white higher-income persons," they continued. "Sensitivity estimates for low-income exposure-response curves were less stable, but all models suggested similar overall patterns."

While marginalized groups stand to benefit the most from air quality improvements, decreases in PM_2.5 would likely have widespread benefits.

"Among Black persons but not white persons, higher-income persons were exposed to lower PM_2.5 levels than low-income persons. Overall, racial identity, not income, was the primary sociodemographic characteristic distinguishing PM_2.5 exposure levels among the subpopulations we analyzed," Dominici and team noted. "Our findings provide strong evidence that lower PM_2.5 levels -- and thus lower PM_2.5 NAAQS [National Ambient Air Quality Standards] -- would benefit all aging Americans, regardless of racial identity or socioeconomic position."

They emphasized that structural racism and social exclusion of low-income Americans have led to a clear disparity in pollution effects.

"For example, exposure inequities caused by 'disparate siting,' in which roadways and pollution-emitting facilities are disproportionately built in Black or low-income communities (among other marginalized groups), are well documented and observed in both urban and rural areas," they wrote. "Consequently, although Black Americans produce proportionally less air pollution than white Americans, they breathe more of it."

While the Environmental Protection Agency (EPA) said that there has been a in the national average of PM_2.5 concentration from 2000 to 2021, at 12 μg/m³. Earlier this year, the EPA announced plans to improve this primary standard, changing it to 9-10 μg/m³.

"Studies in the past several years have shown substantial adverse health effects at PM_2.5 levels well below 12 μg per cubic meter," Dominici and team noted.

"Consequently, no PM_2.5 level may be low enough to fulfill the EPA mandates to 'protect the public's health' and 'achieve environmental justice' by mitigating environmental health inequities," they added. "Nevertheless, lowering the PM_2.5 NAAQS from its current level would generate substantial benefits for the health of our nation."

For this study, Dominici and colleagues analyzed 623 million person-years of Medicare data from 73 million people ages 65 and older from 2000 through 2016. About 47% of this full population had ever smoked.

The authors noted some limitations to their study, including some potential errors in their PM_2.5 model. In addition, data on race and ethnic identity may have misclassified some people, and this misclassification may have resulted in bias.

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