Ernani F. Choma, John S. Evans, José A. Gómez-Ibáñez, Qian Di, Joel D. Schwartz, James K. Hammitt, and John D. Spengler

PNAS December 21, 2021 118 (51) e2107402118

ENRIRONMENTAL SCIENCE

Significance

Despite decades of reductions in vehicle emissions in the United States, their impacts remain large, and the offsetting effects of different factors on benefits achieved in recent years are not well understood. We assess benefits from 2008 to 2017 on a fine spatial resolution using the latest epidemiological evidence and emissions inventories. We find that regulation continues to yield large benefits: $270 (190 to 480) billion in 2017 from reduced PM_2.5-attributable mortality and greenhouse gas emissions. Traffic-related PM_2.5-attributable mortality would have been 2.4 times as large in 2017 if vehicles had still been emitting at 2008 levels, accounting for most benefits. Urban passenger light-duty vehicles have become increasingly important, and major health gains require more stringent policies to curb their emissions.

Abstract

Decades of air pollution regulation have yielded enormous benefits in the United States, but vehicle emissions remain a climate and public health issue. Studies have quantified the vehicle-related fine particulate matter (PM_2.5)-attributable mortality but lack the combination of proper counterfactual scenarios, latest epidemiological evidence, and detailed spatial resolution; all needed to assess the benefits of recent emission reductions. We use this combination to assess PM_2.5-attributable health benefits and also assess the climate benefits of on-road emission reductions between 2008 and 2017. We estimate total benefits of $270 (190 to 480) billion in 2017. Vehicle-related PM_2.5-attributable deaths decreased from 27,700 in 2008 to 19,800 in 2017; however, had per-mile emission factors remained at 2008 levels, 48,200 deaths would have occurred in 2017. The 74% increase from 27,700 to 48,200 PM_2.5-attributable deaths with the same emission factors is due to lower baseline PM_2.5 concentrations (+26%), more vehicle miles and fleet composition changes (+22%), higher baseline mortality (+13%), and interactions among these (+12%). Climate benefits were small (3 to 19% of the total). The percent reductions in emissions and PM_2.5-attributable deaths were similar despite an opportunity to achieve disproportionately large health benefits by reducing high-impact emissions of passenger light-duty vehicles in urban areas. Increasingly large vehicles and an aging population, increasing mortality, suggest large health benefits in urban areas require more stringent policies. Local policies can be effective because high-impact primary PM_2.5 and NH₃ emissions disperse little outside metropolitan areas. Complementary national-level policies for NO_x are merited because of its substantial impacts—with little spatial variability—and dispersion across states and metropolitan areas.

See https://www.pnas.org/content/118/51/e2107402118

Fig. 1.

Emissions in the 2008 to 2017 period by pollutant and vehicle category. Unlike the three most recent NEIs, the 2008 NEI (24) does not present refueling emissions separately. The color bars represent actual emissions in each year, whereas the light gray represents the amount added when VMT is adjusted to 2017 levels. 1 tonne = 1 metric ton.