Carbon nanotubes (CNT) are of great commercial interest. Theoretically, during processing and handling of CNT and in abrasion processes on composites containing CNT, inhalable CNT particles might be set free. For hazard assessment, we performed a 90-day inhalation toxicity study with a multiwall CNT (MWCNT) material (Nanocyl NC 7000) according to Organisation for Economic Co-operation and Development test guideline 413. Wistar rats were head-nose exposed for 6 h/day, 5 days/week, 13 weeks, total 65 exposures, to MWCNT concentrations of 0 (control), 0.1, 0.5, or 2.5 mg/m(3). Highly respirable dust aerosols were produced with a proprietary brush generator which neither damaged the tube structure nor increased reactive oxygen species on the surface. Inhalation exposure to MWCNT produced no systemic toxicity. However, increased lung weights, pronounced multifocal granulomatous inflammation, diffuse histiocytic and neutrophilic inflammation, and intra-alveolar lipoproteinosis were observed in lung and lung-associated lymph nodes at 0.5 and 2.5 mg/m(3). These effects were accompanied by slight blood neutrophilia at 2.5 mg/m(3). Incidence and severity of the effects were concentration related. At 0.1 mg/m(3), there was still minimal granulomatous inflammation in the lung and in lung-associated lymph nodes; a no observed effect concentration was therefore not established in this study. The test substance has low dust-forming potential, as demonstrated by dustiness measurements, but nonetheless strict industrial hygiene measures must be taken during handling and processing. Toxicity and dustiness data such as these can be used to compare different MWCNT materials and to select the material with the lowest risk potential for a given application.

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One of the most important quantitative outputs from toxicity studies is identi cation of the highest exposure level (dose or concentration) that does not cause treatment related effects that could be considered relevant to human health risk assessment. A review of regulatory and other scienti c literature and of current practices has revealed a lack of consistency in de nition and application of frequently used terms such as No Observed Effect Level (NOEL), No Observed Adverse Effect Level (NOAEL), adverse effect, biologically signi cant effect, or toxicologically signi cant effect. Moreover, no coherent criteria were found that could be used to guide consistent interpretation of toxicity studies, including the recognition and differentiation between adverse and nonadvers e effects. This presentation will address these issues identi ed rst by proposing a standard set of de nitions for key terms such as NOEL and NOAEL that are frequently used to describe the overall outcome of a toxicity study. Second, a coherent framework is outlined that can assist the toxicologist in arriving at consistent study interpretation. This structured process involves two main steps. In the rst, the toxicologist must decide whether differences from control values are treatment related or if they are chance deviations. In the second step, only those differences judged to be effects are further evaluated in order to discriminate between those that are adverse and those that are not. For each step, criteria are described that can be used to make consistent judgments. In differentiating an effect from a chance nding, consideration is given inter alia to dose response, spurious measurements in individual parameters, the precision of the measuremen t under evaluation, ranges of natural variation and the overall biological plausibility of the observation. In discriminating between the adverse and the non-advers e effect consideration is given to: whether the effect is an adaptive response, whether it is transient, the magnitude of the effect, its association with effects in other related endpoints, whether it is a precursor to a more signi cant effect, whether it has an effect on the overall function of the organism, whether it is a speci c effect on an organ or organ system or secondary to general toxicity or whether the effect is a predictable consequenc e of the experimental model. In interpreting complex studies it is recognised that a weight of the evidence approach, combining the criteria outlined here to reach an overall judgment , is the optimal way of applying the process. It is believed that the use of such a scheme will help to improve the consistency of study interpretation that is the foundation of hazard and risk assessment .

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A formal validation study was performed, in order to investigate whether the commercially-available reconstructed human epidermis (RHE) models, EPISKIN®, EpiDerm™ and SkinEthic®, are suitable for in vitro skin absorption testing. The skin types currently recommended in the OECD Test Guideline 428, namely, ex vivo human epidermis and pig skin, were used as references. Based on the promising outcome of the prevalidation study, the panel of test substances was enlarged to nine substances, covering a wider spectrum of physicochemical properties. The substances were tested under both infinite-dose and finite-dose conditions, in ten laboratories, under strictly controlled conditions. The data were subjected to independent statistical analyses. Intra-laboratory and inter-laboratory variability contributed almost equally to the total variability, which was in the same range as that in preceding studies. In general, permeation of the RHE models exceeded that of human epidermis and pig skin (the SkinEthic RHE was found to be the most permeable), yet the ranking of substance permeation through the three tested RHE models and the pig skin reflected the permeation through human epidermis. In addition, both infinite-dose and finite-dose experiments are feasible with RHE models. The RHE models did not show the expected significantly better reproducibility, as compared to excised skin, despite a tendency toward lower variability of the data. Importantly, however, the permeation data showed a sufficient correlation between all the preparations examined. Thus, the RHE models, EPISKIN, EpiDerm and SkinEthic, are appropriate alternatives to human and pig skin, for the in vitro assessment of the permeation and penetration of substances when applied as aqueous solutions.