Ambient fine particulate matter (particles <2.5 μm in diameter [PM_2.5]) is a major public health issue; its importance as the fifth-ranking global mortality risk factor was powerfully highlighted in the 2015 Global Burden of Diseases Study (1). Whereas the larger, respirable coarse particles (those between 2.5 and 10 μm in diameter [coarse PM]) typically deposit in the tracheobronchial and upper airways of the respiratory tract, the smaller PM_2.5 aerosols penetrate deep into the alveolar regions of the lungs where gas exchange occurs, causing systemic inflammation and oxidative stress (2). Because of the importance of particle size, many researchers have focused their efforts on understanding the health effects of PM_2.5, and studies of coarse PM are few and far between compared with the sizable literature on PM_2.5 (2). However, coarse particles also very likely have health effects. Controlled chamber experiments have shown effects of coarse PM on inflammation and blood pressure in human volunteers (3, 4). Authors of a meta-analysis of epidemiological studies found a tendency for coarse particles to be positively associated with mortality and cardiorespiratory hospitalizations, although important between-study heterogeneity in the association estimates was present (2).

This issue of the Journal includes results from Keet and colleagues’ analysis (pp. 737–746) of a large U.S. dataset of children enrolled in Medicaid (5). The authors report positive associations of PM_2.5 (averaged over the study period) with asthma prevalence, hospitalizations, and emergency department visits. These findings are not surprising, given existing literature (6). More novel are the positive associations of pediatric asthma with coarse PM that persist after statistical adjustment for PM_2.5. The results serve as a reminder that when we ignore coarse PM, we may well be underestimating the public health burden of the particles in our air.

One notable study strength is the analysis of a restricted-access dataset of 7.8 million children enrolled in Medicaid from 34 U.S. states, of which nearly 1 million (12.8%) had prevalent asthma. Access to these ZIP code–level data was made possible via the Federal Statistical Research Data Center at the University of Minnesota, 1 of about 30 such centers in the United States (7). These centers enable researchers to securely access and analyze data at a finer granularity than what is available from public use files. The authors linked the georeferenced patient records with national air quality models of PM_2.5 and coarse PM. Use of this large dataset helps to reduce random error, and the inclusion of children from nearly three dozen U.S. states provides rate ratio estimates with a “nationally averaged” interpretability. One outstanding question pertains to the generalizability of the study findings to non–low-income children. There are some indications that lower-income children may be more susceptible to the effects of ambient air pollutants (8), although research in this area needs further development. Even so, in 2015, there were 36.8 million U.S. children covered by Medicaid (41% of children under age 19 yr) (9). Thus, although better understanding how associations might vary by socioeconomic position is a topic for further research, at a minimum the results from Keet and colleagues’ study should apply well to the 41% of Medicaid-enrolled children.

So, where do coarse particles come from? Whereas PM_2.5 is most commonly formed during energy combustion and via photochemical reactions in the atmosphere, coarse particles are typically generated by mechanical grinding and erosion of solids (2, 10). The origin of coarse PM includes both naturogenic sources, such as crustal material, sea salt, and organic material, as well as made-made sources, such as zinc and iron from tire and brake wear (10, 11). Unfortunately, relatively few U.S. Environmental Protection Agency monitoring sites directly measure coarse PM, and chemically speciated coarse PM measurements (which are required to relate the particles back to their sources) are even less available. Given the limitations of the coarse PM monitoring network, to estimate coarse PM the authors created a national model for PM_2.5 concentrations (using the PM_2.5 monitoring data) and subtracted these estimates from a similar model of particulate matter less than or equal to 10 μm in aerodynamic diameter (PM₁₀). Although subtracting PM_2.5 from PM₁₀ is a reliable approach for estimating coarse PM concentrations at a monitoring site (12), in this study few of the PM_2.5 and PM₁₀ monitoring locations were collocated, which is a source of uncertainty in the exposure model. Unfortunately, understanding the impact of exposure measurement error on the reported health association estimates is difficult, barring formal statistical work. Moving forward, if we are to become more serious about coarse PM health effects, then investments in direct monitoring of chemically speciated coarse PM will likely be required.

Because much of the coarse PM comes from natural sources, it can be challenging to regulate; for example, regulating the occurrence of dust storms is well beyond the abilities of our air quality management colleagues. Other opportunities to control coarse PM, however, are available. For example, increasingly many northern states use saltwater brine to deice winter roads. The brine adheres to the road better than solid rock salt and uses less salt, and tire abrasion generates much less coarse PM (13). Transportation presents another challenge. Over the next 20 years, the Federal Highway Administration predicts an annual 1.07% increase in vehicle miles traveled (14). Even if all the vehicles on our roads were electric, coarse PM concentrations from vehicles would likely go up because of increased tire and brake wear. Intelligent urban development and investments in public transport are needed to lessen the impact of this projected demand.

Keet and colleagues’ paper provides new, supporting evidence that coarse particles at levels observed in the United States impact pediatric asthma, and they conclude “that direct monitoring of coarse PM may need to be implemented and that long-term coarse PM standards should be reconsidered” (5). Although there would be costs to expanding the monitoring network and implementing additional air quality standards, these costs need to be weighed against the benefits to public health and quality of life and, as clearly shown here, costs that taxpayers are already shouldering through programs such as Medicaid.

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