Health Effects of Ambient Ultrafine Particles - Aerosols: Measurement, Dosimetry, and Health Effects - page 205

8 Health Effects of Ambient

Ultraine Particles

Beverly S. Cohen

CONTENTS
8.1 Introduction .......................................................................................................................... 205
8.2 Formation.............................................................................................................................. 207
8.3 Composition..........................................................................................................................209
8.4 Lung Deposition ...................................................................................................................209
8.5 Toxicology ............................................................................................................................ 210
8.6 Epidemiological Studies ....................................................................................................... 213
8.7 Summary .............................................................................................................................. 213
References...................................................................................................................................... 214

8.1 INTRODUCTION
More than 90% of all airborne particles are generally found in nuclei less than 150 nm in diameter
[1-3]. Number concentrations in an urban environment vary, depending on local sources of particles
and gaseous precursors, season, and weather. In a boreal forest in Finland, the measured concentration
of ultraine particles (UFPs) was about 1000 cm -3 [4]. When there is substantial vehicular trafic,
the number concentration is frequently on the order of 10 4 -10 5 particles cm -3 . In a quiet rural
environment, or indoors in an undisturbed clean room, the concentration more typically ranges from
a few hundred to a few thousand particles cm -3 . These particles, on average, represent only about
one half of 1% of the total airborne particulate volume and mass. Thus, they have not generally
been regarded as important contributors to the toxic effects of inhaled ambient air. However, recent
evidence suggests that they may have an important role in health decrements associated with ambient
particulate matter (PM). Other evidence suggests that particle number may also be a better metric
than mass on which to base risk estimates for certain occupational diseases [2,5].
Various boundaries are in current use for both the upper and lower limits for the diameter of
UFPs. UFPs include those in the atmospheric nuclei mode, plus some of the smaller particles
classiied as “accumulation” mode particles. The lower limit is generally regarded as about 1 nm,
but some reports refer to those up to 10 nm as “nanometer” particles, reserving ultraine for
particles with diameters larger than 10 nm. A convenient upper limit of 100 nm is frequently used.
This is because a 100 nm diameter particle is an acceptable lower size limit to the accumulation
mode, and an upper limit to the nucleation mode (modes are deined in Chapter 2). Also, the term
“manufactured nanoparticles” generally refers to particles smaller than 100 nm. In this size range,
particle aerodynamic behavior is dominated by Brownian diffusion, and particle size is adequately
described by a thermodynamic diameter. The thermodynamic diameter is the diameter of a sphere
of unit density that would have the same diffusion coeficient in air as the particle of interest. An
upper limit value of 150 nm is chosen in this chapter, because it represents the particle size at which
gravitational and inertial effects are of little importance when particles are inhaled. Thus, UFP
behavior in human airways is dominated by diffusion.

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