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The relationship between air pollution and cardiovascular harm was supported by another study, this time down to the level of specific types of pollution that appeared to have varying effects according to geographical region.

Analysis of individual components of fine particular matter (PM_2.5) suggested that greater estimated exposure to certain pollution sources was consistently accompanied by more atherosclerotic cardiovascular disease (ASCVD) deaths on the population level:

• Oil combustion: RR 1.051 per interquartile range increase in PM_2.5 mass (95% CI 1.049-1.052)
• Industrial pollution: RR 1.054 (95% CI 1.052-1.056)
• Coal and biomass burning: RR 1.065 (95% CI 1.062-1.067)
• Motor vehicle pollution: RR 1.044 (95% CI 1.042-1.045)
• Soil and dust: No association

The exposure patterns and associations varied regionally, however -- there being no significant link between ASCVD and motor vehicle source pollution in the Southeast U.S., for example, and soil and dust pollution only showing harm in the eastern regions and the Midwest, reported Maayan Yitshak-Sade, MPH, PhD, epidemiologist of Icahn School of Medicine at Mount Sinai in New York City, and colleagues in .

"These regional differences require further investigation, as these differences can be attributed to a combination of diverse factors. For example, each U.S. region has a distinct climate. Atmospheric temperature, as well as other climate variables, were found to modify PM_2.5-associated health effects," the study authors wrote.

"Furthermore, components of the built environment such as greenness levels and the socioeconomic, racial, and ethnic composition of the population can also contribute to regional differences," they added.

It is hard to see how the regional differences would be biologically plausible, according to an by economist C. Arden Pope III, PhD, of Brigham Young University in Provo, Utah, and Joshua Apte, PhD, air quality engineer and researcher of University of California Berkeley.

"Such discordant differences in responses to PM_2.5 from different sources are not likely the result of regional population differences in human physiology. One must wonder how these results are affected by regional differences in exposures or if they result from artifacts of the source apportionment procedures," Pope and Apte wrote.

Otherwise, the "overall results are consistent with other studies that suggest that fine particles from combustion and industrial sources are associated with an increased rate of cardiovascular mortality -- even at levels below current air quality standards," the duo wrote.

Indeed, Yitshak-Sade's group reported that even in areas meeting the current National Ambient Air Quality Standard (NAAQS; PM_2.5 <9 µg/m³), ASCVD mortality tracked with increasing PM_2.5 mass (RR 1.028, 95% CI 1.026-1.029). "These results highlight the need to close the gap between the NAAQS and the World Health Organization recommendation -- currently set at <5 µg/m³," they suggested.

"These results emphasize the role of geographical context and the need to consider local population characteristics and exposure patterns when assessing health risks associated with air pollution," Yitshak-Sade and colleagues concluded.

For their study, the researchers had on hand ASCVD mortality rates compiled from Medicare records spanning the years 2000 to 2016. They also relied on machine learning models providing PM_2.5 exposures -- down to a 50-m or 1-km spatial resolution -- and adjusting for individual-level demographic data, zip code-level socioeconomic characteristics, meteorological variables, and other covariates.

The study population ultimately included over 65 million Medicare beneficiaries who entered the dataset at 71 years of age on average (56% women, 85% white).

The 14 PM_2.5 components of interest included sulfate, nitrate, organic and elemental carbon, silicon, lead, silicon, and copper. Different combinations of these components helped the investigators characterize the type of particulate matter likely being exposed to residents in each area.

During the study period, an estimated 7.2% of people died of ASCVD. The incidence was highest in the Northeast (9.80 per 1,000 person-years) and lowest in the West (7.71 per 1,000 person-years), the researchers found.

Important study limitations, they cautioned, include potential exposure misclassification and lack of accounting for each person's residential history prior to Medicare enrollment. Moreover, the study did not cover ASCVD other than ischemic heart disease and stroke.

And then there are the possible impacts of those "artifacts of the source apportionment procedures" on different regional results, as stressed by Pope and Apte.

"As finely resolved source-specific exposure estimates become more widely available across the United States, there may be increasing opportunities to leverage the full statistical power of large cohorts (such as Medicare data) to investigate these questions," the editorialists commented.

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