There is a large body of literature showing that fine particulate matter (PM2.5) is associated with premature death, with the most persuasive evidence coming from long-term epidemiological studies. The epidemiological evidence for a similar relationship with ultrafine particles (UFP) is limited and inconclusive; the proposed study aims to help fill this research gap. The overall objective of the proposed study is to determine whether any feature of UFP's (number, surface area, or mass) is positively associated with premature death in California. The proposed study, which will combine statewide modeling of UFP distribution with monitoring data, will greatly strengthen the exposure analysis for UFPs relative to previous work that relied on central site monitors. The results of this study will provide ARB with a clearer understanding of the health impacts associated with exposure to UFP, including health risk at ambient concentrations.