Health Effects of Ambient Ultrafine Particles - Aerosols: Measurement, Dosimetry, and Health Effects - page 214

with trace metals, ammonium, and sulfates, but there are few data on the temporal or spatial
variations in the composition.
High concentrations of ultraines are emitted from vehicles and other combustion sources and
people are exposed to them in vehicles and alongside roadways. These small particles deposit very
eficiently in the respiratory system, and when inhaled, the evidence derived from both experimental
and occupational exposures indicates that they are more toxic than larger particles of the same
composition. Some epidemiological studies associate ambient UFPs with morbidity and mortality
[12]. No mechanism has as yet been demonstrated by which these very small particles exert their
toxicity on people or in animals at exposure levels that could occur in ambient air.
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