Currently, the main exposures of the general population to nanoparticles are likely to arise from breathing the air. Nanoparticles are very abundant in theatmosphere and have been present since time immemorial in the vicinity of anycombustion process. In most areas with heavy human occupancy, concentrationsare highly elevated above unpolluted background concentrations andmajor exposures are unavoidable. There have been frequent suggestions thatbecause of their enhanced toxicity per unit mass, nanoparticles are responsiblefor a large component of the adverse effects due to exposure to airborneparticles. However, nanoparticles, although dominating the number concentrationof particles in the atmosphere, represent only a relatively small proportionof the mass of PK_(10) or PM_(a2,nd the .ava5ilable)pidemiologicalevidence does not present a compelling case for indicting them as the majorcontributor to airborne particle toxicity.
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