Diesel particulate matter health impacts in California: Trends, source apportionment, and policy implications

Diesel particulate matter (DPM) has been recognized as a carcinogen and identified as a toxic air contaminant (TAC) in California and other jurisdictions. In response to this identification, the California Air Resources Board (CARB) has adopted numerous regulations aimed at reducing DPM emissions from various sources. This study utilized an integrated modeling approach to simulate ambient DPM concentrations for individual emission sectors separately for the two years 2012 and 2017. The associated health impacts, including cancer risk and non-cancer effects, were then assessed. This assessment provided a basis for apportioning emission sources, analyzing reduction trends, and informing further regulatory efforts when combined with future emissions projections. Our results showed a significant reduction in DPM-related cancer risk in California between 2012 and 2017. Specifically, population weighted DPM cancer risk decreased by 42 %, and mortality attributable to DPM exposure decreased by about 50 % statewide. Additionally, census tracts with higher population densities consistently experienced more significant reductions in DPM cancer risk from 2012 to 2017. Source apportionment analysis indicated that, as of 2017, on-road mobile sources were the largest contributor to overall DPM risk, followed by off-road mobile, area, and stationary sources. Our findings further suggest that while the overall health risk from DPM will continue to decline with emissions, the relative contributions of each emission sector to DPM risk may shift over time depending on the major regulations in place, and how the emission reductions impact nearby population. When accounting for how emissions have changed since 2017 and are projected to change in the future, new emission reduction efforts will likely need to prioritize off-road mobile sources (e.g., seaports, airports, locomotives) and area sources (e.g., construction and agricultural sectors) to achieve further risk reductions, especially beyond 2025.