Ultrafine particles in the urban air: to the respiratory tract--and beyond?

Abstract

In 1994, when we introduced the ultrafine particle hypothesis stating that ambient ultrafine particles (UFP; < 0.1 µm in aerodynamic diameter) may cause adverse health effects at the first Colloquium for Particulate Air Pollution and Human Mortality and Morbidity in Irvine, California, it was met with friendly skepticism as well as out-right dismissal.Arguments were that UFP are very short-lived and disappear through heterogeneous and homogeneous aggregation within seconds or minutes and therefore are toxicologically irrelevant.These arguments did not recognize that UFP are continuously generated or that ambient UFP contribute very little, if any, mass to ambient PM 10 (particles < 10 µm in aerodynamic diameter) or PM 2.5 (particles < 2.5 µm in aerodynamic diameter).Indeed, the mass distribution of a typical urban aerosol among the different particle sizes may support this point (Figure 1).This attitude of skepticism has changed considerably.Research teams across the world are working now on UFP, forming multidisciplinary alliances between atmospheric scientists, engineers, epidemiologists, clinicians, and toxicologists.They investigate UFP sources, generation, physicochemical characteristics, behavior in ambient air, and potential effects and underlying mechanisms following their inhalation.Still, sound skepticism lingers, as demonstrated by the title of a presentation at the 2002 meeting of the Health Effects Institute: "Nanoparticles: Are They Real?"Obviously, there is no question that UFP are real, but it is also clear that we still do not know enough about them, despite significant progress in our understanding since 1994.Atmospheric UFP derived from gas-to-particle conversions have many sources, natural and anthropogenic, the latter being mostly derived from internal combustion Günter Oberdörster is an inhalation toxicologist who has been investigating effects and toxicokinetics of occupational and environmental particles for more than 25 years, more recently focusing on ultrafine particles.He is Professor of Toxicology and Director of the the U.S. EPA-funded Particulate Matter Center on ultrafine particles