Many ultrafine particles comprised classically of low-toxicity, low-solubility materials such as carbon black and titanium dioxide have been found to have greater toxicity than larger, respirable particles made of the same material. The basis of the increased toxicity of the ultrafine form is not well understood and a programme of research has been carried out in Edinburgh on the toxicology of ultrafines aimed at understanding the mechanism. We used fine and ultrafine carbon black, TiO2 and latex and showed that there was an approximately 10-fold increase in inflammation with the same mass of ultrafine compared with fine particles. Using latex particles in three sizes - 64, 202 and 535 nm - revealed that the smallest particles (64 nm) were profoundly inflammogenic but that the 202 and 535 nm particles had much less activity, suggesting that the cut-off for ultrafine toxicity lies somewhere between 64 and 202 nm. Increased oxidative activity of the ultrafine particle surface was shown using the fluorescent molecule dichlorofluorescein confirming that oxidative stress is a likely process by which the ultrafines have their effects. However, studies with transition-metal chelators and soluble extracts showed that the oxidative stress of ultrafine carbon black is not necessarily due to transition metals. Changes in intracellular Ca2+ levels in macrophage-like cells after ultrafine particle exposure suggested one way by which ultrafines might have their pro-inflammogenic effects. Discussion: C.V. Howard (Foetal Toxico-Pathology, University of Liverpool, UK). Do you have the basis in your assays for comparing different substances at equal dosage and equal particle size ranges by inhalation, to construct a table of relative toxicities? It may be that this would be of use to policy makers to help them design strategies for trying to control those processes that produce the most toxic particles. K. Donaldson. We agree that some measure of relative potency by inhalation is desirable but this would be costly. M. Williams (DETR, London, UK). Professor Oberdorster plotted PMN response against mass and showed that ultrafine TiO2 had a larger response than fine TiO2, but, when plotted against surface area, all points fell on one curve. The graphs early on in your talk showed similar behaviour, and in many of your later histograms, the responses seemed to scale with surface area. Would one expect this if (cf. Dr. Jefferson's paper) there was something special about ultrafines and it was not just a surface area effect? K. Donaldson. This is an important question that needs to be addressed by well-defined dose-response studies. However, our impression, from limited data, is that ultrafine particles have extra surface reactivity as well as extra surface, compared with non-uniformities. A. D. Maynard (NIOSH, Cincinnati, OH, USA). There has been significant emphasis on the contribution that low-solubility particle surface area may make to the nature, magnitude and rate of biological interactions. However, characterization of 'biologically relevant' surface area will depend on the length-scale over which these interactions occur. Could you speculate on the order of magnitude of length-scale that is likely to be of greatest relevance in determining interaction mechanisms? K. Donaldson. The only information that I know about regarding the length or distance that cells can resolve in a paper by Wojciak-Stothard et al . (1996). This paper shows that macrophage-like cells align their cytoskeleton along grooves 44 nm in depth. This suggests that cells can discriminate, via one assumes surface receptors, well down into the ultrafine size range. The physiological responses that such a cytoskeletal re-organization might cause are of great potential interest.
CV Howard, Univ Liverpool, Liverpool L69 3BX, Merseyside England