This got me curious: are most Americans exposed to these pollutants at a meaningful level?

In short: no.

In long:

This study was done in China, and asked "what would happen if we lowered 10 micron particulate pollution levels to the US EPA standard?" Air pollution in China is substantially worse than in the US, and 10 micron particulates (PM10) are one of the largest points of divergence: China relies heavily on unfiltered coal burning for electricity, as well as conducting mining and road construction with minimal particulate control.

Unfortunately I can't get the API data appendix for the study, and the paper's figures are functions of mean API, but the data shouldn't be paper specific. All sources I can find agree: fine particulate pollution levels in China are horrifying. Almost all of China is consistently exposed to more than 40 micrograms per cubic meter of PM10 particulates, and urban centers are consistently much higher. Recent progress has lowered average rates in many cities into the 40-100 microgram window, but in 2010 annual average rates commonly exceeded 150 micrograms, which is the acute (24 hour) threshold set by the EPA.

The benchmark value in the study was 50 micrograms per cubic meter, which was (until 2006, when the standard was scrapped), the EPA's safe limit for annual average exposure. In America, across several hundred sites monitored in 2017, zero exceeded 100 micrograms, only four exceeded the 50 microgram threshold, and only twelve exceeded 40 micrograms. For 2016, those numbers are zero, four, and ten. The hardest-hit sites are all in the Southwestern US, which suffers from both dust and wildfire particulate matter frequently.

Further, not all PM10 particulates are created equal. They're grouped in a single number because the usual focus of concern is inhalation leading to lung problems, but when discussing cognitive effects they're almost certainly not equivalent. In particular, PM2.5 (2.5 micron particles) are a subset of PM10, and are both more likely to enter the bloodstream and more likely to be bioactive substances like metal, organics, or soot. Particulates between 2.5 and 10 microns are more likely to be dust, pollen, or ash - damaging to lungs, but much less likely to cause systemic harm. This study did admirable work controlling for or disproving confounders - I'm genuinely very impressed by how well they covered that base. But the PM10/PM2.5 conflation was inescapable since PM2.5 is a subset of the variable and wasn't measured separately.

(The study relied on an Air Pollution Index observing sulfur dioxide, nitrogen dioxide, and PM10, with the value controlled by the highest subfactor. From the references exclusively to PM10 and from China's other data, I assume that it was consistently the dominant factor, as it is in the US, but if it correlates with the other factors then one of them could potentially drive the effect. Nevertheless, SO2 and NO2 are essentially acute-only problems in the US, with average rates well below EPA thresholds.)

The highest-PM10 sites in the US look prone to dust and ash pollutants, and indeed checking the data says that their PM2.5 levels, while high, trend linearly up from other sites while their PM10 levels rise exponentially. The worst PM10 sites have PM2.5:PM10 ratios of around 20%. The highest PM2.5:PM10 ratios are between 50% and 70%. These are predictably found in industrial Rust Belt cities like Dearborn and Steubenville, but the total levels are still relatively low. 13 cities exceeded the annual EPA standard of 12 micrograms, and none exceeded 17 micrograms. 21 cities had 98th percentile rates in excess of the EPA daily standard of 35 micrograms; excluding outlier Phoenix (146 micrograms) the highest was 67 micrograms.

Meanwhile, China's PM2.5 rates have only recently been measured, but are terrifyingly high. China sets 0-35 micrograms as a Grade I success and 35-75 micrograms as Grade II success. Wuhan, selected as an average Chinese city for a study in 2013, exceeded the Grade II standard 75% of the time, and exceeded 150 micrograms (which, again, is the acute PM10 threshold!) 25% of the time. National samples show all cities profiled have PM2.5:PM10 ratios above 50%, and some exceed 80%. Most days in most cities in China, the PM2.5 level is higher than the highest observed rates for any site in the US except Phoenix.

Where does this leave us? China's PM10 rates are vastly higher than the worst American rates. China's PM2.5 rates, which are likely more relevant, are even more divergent. Annual NO2 and SO2 rates in the US are insignificant. The available evidence suggests that under any plausible shape for this effect - linear or exponential, threshold or LNT - its effects would be immeasurable anywhere in the US except perhaps 2-3 weakly affected cities. In all cases, other known impairing pollutants like carbon monoxide would be far more impactful.

This has been quite a tangent, but overall it's increased my appreciation for this study, while leading me to believe this result is basically irrelevant to US analyses.