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MUNICH WORKSHOP ON EVALUATION OF FIBER AND PARTICLE TOXICITY: AN INTRODUCTION

In particle toxicology a number of battles have been fought in which standpoints and issues were quite clear. Typically most of these battles were fought over fibers including asbestos, man-made or synthetic fibers that were identified as carcinogens in humans and/or animals, and others were not carcinogenic or heavily discussed (Mossman et al., 1990). Another constant issue, but not unique to particle toxicology, is the extrapolation of carcinogenicity outcomes to a lower dose regimen to define a threshold limit value or an acceptable limit. Until recently, particles escaped emotional battles as just described, and adverse effects such as bronchitis and lung fibrosis and were smoothly handled using cumulative exposure levels that could be recalculated easily to define no-observed-adverse-effect levels. Things started to change when ultrafine particles (<100 nm) came to the forefront (Ferrin et al., 1992). Rat studies showed that even nontoxic particles, also called nuisance-particles, were able to induce lung tumors at high enough dose (review: Nikula, 2000). Today, even dose has become a complicated issue. Abundant data have now demonstrated that particle volume and surface are characteristic s that correlate better with tumor rate and intermediate biological processes than particle mass (Tran et al., 2001; Driscoll et al., 1997). In a breakthrough paper, Morrow (1988) suggested that lung clearance is impaired by the volumetric loading of macrophages, which starts at levels over 6% of the normal macrophage volume. At such a point, particles start to accumulate and no steady-state lung burden will be reached. Morrow’s finding generated enormous discussion, since many animal studies that showed increased lung tumor rates were performed at doses that could be shown to retard the pulmonary clearance rate (Morrow, 1992). In addition, apart from a change in kinetics, overload in the rat appeared to be associated with inflammatory response, interstitial and alveolar accumulation of particles, and cell proliferation (ILSI, 2000; Oberdörster et al., 1995). A body of work primarily conducted by the research teams of Günther Oberdörster (Rochester, NY) and Ken Donaldson (Edinburgh, UK) have dem-