Counterfactual modeling across New York City, Los Angeles, and Atlanta from 2005 to 2019 quantifies what air quality would have looked like without key federal and local pollution controls targeting power plants, vehicles, and ports. Combined policies reduced ambient PM2.5 concentrations by 27–62% at monitoring sites. These reductions translated into 7.1% fewer respiratory emergency department visits in NYC, 4.5% in Atlanta, and 2.4% in Los Angeles. Cardiovascular hospitalizations dropped by 2.6% in NYC and 1.2% in LA; Atlanta showed no statistically meaningful cardiovascular benefit.
This preprint — not yet peer-reviewed — offers one of the more granular city-level assessments of real-world regulatory impact on acute health outcomes, using positive matrix factorization to disaggregate pollution sources rather than relying on total-mass metrics alone. That methodological rigor strengthens the causal narrative, though the counterfactual concentration estimates carry their own modeling assumptions and uncertainty intervals widen considerably in Atlanta's cardiovascular results. The absence of Atlanta cardiovascular benefit may reflect genuine source-mix differences or statistical underpowering rather than true policy ineffectiveness. Critically, ED visits and hospitalizations capture only the most severe health endpoints — subclinical cardiovascular strain, lung function decline, and mortality are not included, likely understating the full benefit. For public health advocates and policymakers, the findings reinforce that multi-sector emission controls compound their benefits over time. The city-to-city variation also underscores that local source profiles and baseline pollution levels materially shape who gains most from federal mandates.