Health Risk Assessment of Ultrafine Particles and Nonexhaust Emission Metals by Near-Road Monitoring

Ultrafine particles (UFPs; <100 nm) can carry redox-active metals that deposit efficiently in the respiratory tract, yet size-resolved density and source-specific risks remain uncertain. We conducted a winter near-road campaign in Taipei, combining scanning mobility and aerodynamic sizing, positive matrix factorization (PMF), and an automated DMA-APM method to continuously measure effective density (50–150 nm). Size-segregated metals in the 56–180 nm aerodynamic diameter range (measured via Nano-MOUDI/ICP-MS) were coupled with multiple-path particle dosimetry (MPPD) to estimate regional lung deposition and deposition-adjusted health risks. A four-factor PMF solution separated road dust resuspension and secondary aerosols, oil combustion and regional combustion influence, aged traffic exhaust/soot mode, and fresh nucleation traffic. MPPD indicated peak pulmonary deposition fractions of ∼0.13–0.26 for 50–150 nm particles, with total respiratory deposition of ∼0.26–0.50 depending on size. Deposition-adjusted hazard quotients for individual metals were on the order of 10–4-10–3 (Mn) and ∼10–3 (Ni), while Cr(VI) dominated carcinogenic risk (∼10–7); Ni and Mn dominated noncancer hazard, and Cr(VI), As, and Ni dominated cancer risk. Among traffic-related factors, road dust/secondary aerosol carried a disproportionate share of the lung-deposited metal dose despite its low particle counts. Results highlight the emerging importance of nonexhaust controls under tightening tailpipe standards and demonstrate the value of density-aware dosimetry for interpreting UFP metal risks in near-road microenvironments.