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Constantinos Sioutas,¹ Ralph J. Delfino,² and Manisha Singh¹

¹Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, California, USA; ²Epidemiology Division, Department of Medicine, University of California, Irvine, California, USA

Abstract
Epidemiologic research has shown increases in adverse cardiovascular and respiratory outcomes in relation to mass concentrations of particulate matter (PM) ≤ 2.5 or ≤ 10 µm in diameter (PM_2.5, 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 µm (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. Key words: cardiovascular diseases, cytokines, diesel, epidemiology, oxidative stress, particle size, photochemistry, respiratory diseases, toxic air pollutants, ultrafine particles. Environ Health Perspect 113: 947-955 (2005) . doi:10.1289/ehp.7939 available via http://dx.doi.org/ [Online 16 March 2005]

Address correspondence to C. Sioutas, Department of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Ave., Los Angeles, CA 90089 USA. Telephone: (213) 740-6134. Fax: (213) 744-1426. E-mail: sioutas@usc.edu

This work was supported by grant ES-12243 from the National Institute of Environmental Health Sciences (NIEHS) , National Institutes of Health (NIH) ; the contents of this article are solely the responsibility of the author and do not necessarily represent the official views of the NIEHS, NIH. This work was also supported by the Southern California Particle Center and Supersite (SCPCS) funded by the U.S. Environmental Protecton Agency (U.S. EPA ; STAR award R82735201) .

This manuscript has not been subjected to the U.S. EPA peer and policy review and therefore does not necessarily reflect the views of the agencies. No official endorsement should be inferred.

The authors declare they have no competing financial interests.

Received 14 January 2005 ; accepted 16 March 2005.