Association between long-term ultrafine particulate matter exposure and premature death

Ultrafine particles (UFPs) have diameters ≤ 100 nm allowing them to penetrate into regions of the human body that are not accessible to larger particles. Numerous laboratory studies over the past two decades have found that UFPs are toxic but the epidemiological evidence over this same time period is inconsistent. Past epidemiological studies have typically focused on particle number concentration measured at central site monitors, but this approach cannot adequately represent the sharp spatial gradients in UFP exposure. New techniques are needed to better represent the spatial gradients for UFP number and mass concentrations to fully explore UFP public health impacts. The objective of the current project is to estimate exposure to UFP number and UFP mass using new approaches that capture variations in space and time over the past two decades. A regional chemical transport model was created to predict UFP concentrations across California by combining criteria pollutant emissions inventories with measured particle size and composition profiles that extend into the UFP size range. Model calculations include all of the atmospheric processes that influence UFP concentrations including emissions, transport, deposition, coagulation, condensational growth, and nucleation. UFP concentration fields generated for California between the years 2000 – 2016 are in reasonable agreement with the available measured UFP number concentrations and measured UFP mass concentrations. UFP source apportionment information contained in the predicted UFP exposure fields is also in good agreement with measured UFP source apportionment calculated using the Chemical Mass Balance (CMB) model at Los Angeles, East Oakland, and San Pablo over a 12 month period in 2015 2016. Looking across all of California, UFP concentrations were highest in large cities with dense populations. Major UFP sources included on-road vehicles, wood burning, food cooking, airports, and natural gas combustion. UFP concentrations associated with on-road vehicles and biomass combustion declined between the years 2000 – 2016 due to the effects of targeted emissions control programs. Final UFP exposure fields were combined with the health data from the California Teachers Study (CTS) cohort (+90,000 participants) to analyze health effects using a Cox proportional hazard model. The risk estimate on all-cause mortality per 10 μg/m3 of PM2.5 calculated using the CTS cohort agrees with corresponding published estimates from the ACS and Medicare cohorts. Specifically, the hazard ratio (HR) associated with a 10 μg/m3 of PM2.5 were 1.07 (95% CI = 0.99, 1.06), 1.09 (0.99, 1.22) and 1.20 (1.02, 1.45) for all-cause, cardiovascular and ischemic heart disease mortality, respectively. Extending the analysis to the UFP size range, multiple UFP metrics were associated with all-cause mortality at the 95% confidence level including mass, EC, OC, metals, and on-road diesel. UFP metrics associated with CV mortality at the 95% confidence level included Cu, EC, and SOA. UFP metrics associated with IHD mortality at the 95% confidence level included Cu, Fe, EC, metals, SOA, gasoline, off-road diesel, food cooking, and natural gas. The sum of primary PM0.1 from all fossil fuel combustion activities is associated with all-cause, CV, and IHD mortality at the 95% confidence level. There is a suggestion from our results that UFP may correlate with the mortality outcomes as well or better than PM2.5. However, given the high correlations between the PM2.5 and UFP mass and species and the likely differential in measurement error, it is difficult to reach a firm conclusion on this issue. Two pollutant models of UFP individual species plus mass provide additional support for the effects of individual species. Page | 21 Future studies are needed to analyze the UFP exposure fields using additional cohorts to create further weight of evidence for the health effects of UFPs in epidemiological studies.