Exposure assessment for atmospheric ultrafine particles (UFPs) and implications in epidemiologic research

Abstract

Epidemiologic research has shown increases in adverse cardiovascular and respiratory outcomes in relation to mass concentrations of particulate matter (PM) < or = 2.5 or < or = 10 microm in diameter (PM2.5, PM10, respectively). In a companion article [Delfino RJ, Sioutas C, Malik S. 2005. Environ Health Perspect 113(8):934-946]), we discuss epidemiologic evidence pointing to underlying components linked to fossil fuel combustion. The causal components driving the PM associations remain to be identified, but emerging evidence on particle size and chemistry has led to some clues. There is sufficient reason to believe that ultrafine particles < 0.1 microm (UFPs) are important because when compared with larger particles, they have order of magnitudes higher particle number concentration and surface area, and larger concentrations of adsorbed or condensed toxic air pollutants (oxidant gases, organic compounds, transition metals) per unit mass. This is supported by evidence of significantly higher in vitro redox activity by UFPs than by larger PM. Although epidemiologic research is needed, exposure assessment issues for UFPs are complex and need to be considered before undertaking investigations of UFP health effects. These issues include high spatial variability, indoor sources, variable infiltration of UFPs from a variety of outside sources, and meteorologic factors leading to high seasonal variability in concentration and composition, including volatility. To address these issues, investigators need to develop as well as validate the analytic technologies required to characterize the physical/chemical nature of UFPs in various environments. In the present review, we provide a detailed discussion of key characteristics of UFPs, their sources and formation mechanisms, and methodologic approaches to assessing population exposures.

Figures

Figure 1

Normalized total particle (A) number and (B) volume concentration, in the size range of 6 –220 nm, as a function of distance from the I-405 freeway. CPC, condensation particle counter (TSI Inc., Shoreview, MN). Reprinted with permission from Zhu et al. (2002b). Copyright 2002 Air and Waste Management Association.

Figure 2

Correlation between traffic density and measured total PN concentration, corrected for wind velocity, 30 m downwind from the freeway. Reprinted with permission from Zhu et al. (2002b). Copyright 2002 Air and Waste Management Association.

Figure 3

UFP size distribution at different sampling locations near (A) the I-405 freeway [reprinted with permission from Zhu et al. (2002b); copyright 2002 Air and Waste Management Association] and (B) the I-710 freeway [reprinted from Zhu et al. (2002a); copyright 2002, with permission from Elsevier].

Figure 4

(A) Normalized PN concentration for different size ranges as a function of distance to the I-710 freeway. (B) Concentrations of BC as a function of distance from the I-405 and I-710 freeways. Reprinted from Zhu et al. (2002b). Copyright 2002, with permission from Elsevier.

Figure 5

Results of a principal component (PC) analysis from the work of Charron and Harrison (2003): (A) factor loadings for PNs; (B) factor loadings for NOx and CO—median weekly factor scores for (C) PC1, (D) PC2, (E) PC3, (F) PC4. Reprinted from Charron and Harrison (2003). Copyright 2003, with permission from Elsevier.

Figure 6

Comparison of decay of PN concentrations in summer and winter in the size range of 6–12 nm near the I-405 freeway. Reprinted from Zhu et al. 2004: “Aerosol Science & Technology: Seasonal Trends of Concentration and Size Distribution of Ultrafine Particles Near Major Highways in Los Angeles.” 38(suppl 1):5–13. Copyright 2004. Mount Laurel, NJ. Reprinted with permission.

Figure 7

Average diurnal variation of NOx, CO, PN and particle volume, (A) working days, and (B) Sundays in various locations in Copenhagen, Denmark. Abbreviations: Diff Jgt.-HCØ, difference between street and urban background sites; Teom_Jgt, tapered element oscillating microbalance (R&P Inc., Albany, New York) measurements of PM10 in the street canyon. Jagtvej (Jagtv.) is located in a street canyon, The second station is located at the roof of the 20 m high H.C. Ørsted Institute (HCØ) and is measuring the urban background concentration. Reprinted from Ketzel et al. (2003). Copyright 2003, with permission from Elsevier.

Figure 8

In-vehicle measurements of (A) PN concentrations (B) number-based particle size distributions, in freeways and urban areas in Los Angeles, CA. PIU, particle instrumentation unit, located at the southern California Supersite in downtown Los Angeles. Long Beach and Pasadena are two other urban areas in Los Angeles. Arrows indicate when the vehicle was off road at these three urban locations. From Westerdahl et al. (2005). Copyright 2005, with permission from Elsevier.

Figure 9

PN concentrations vs. CO, NO, and NO2 in Riverside, California, during summer 2002. (A) 24-hr average PN vs. CO, (B) hourly PN vs. CO, (C) 24-hr average PN vs. NO, (D) hourly PN vs. NO, (E) 24-hr aver- age PN vs. NO2, (F) hourly PN vs. NO2. Reprinted with permission from Sardar et al. (2004). Copyright 2004, Air and Waste Management Association.

Figure 10

Hourly PN vs. O3 concentrations in Glendora, California, during summer 2002, with a lag time of 2 hr (PN vs. O3 2 hr earlier). Reprinted with permission from Sardar et al. (2004). Copyright 2004, Air and Waste Management Association.