4. Conclusions

This very partial review of the literature shows that the question of the specific toxicity of ultrafilters is an extremely complex one. A few points can be highlighted, without making them absolutes:

• the surface area or number of PUFs plays a fundamental role in the expression of their pulmonary toxicity; the surface area may be affected by very different factors;

• The nature of the substance, crystallinity (linked to certain surface properties), chemical reactivity, bioavailability, the presence of sufficient quantities of metals and particle size also play important roles. Not forgetting the role of endotoxins in the effects of environmental particulate pollution;

• synergistic or independent biological effects are likely to be present simultaneously;

• ...