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Abstract
Large epidemiological studies in the United States have shown a statistical association between air concentration of the fine dust fraction PM2.5 in the general environment and increased risk of lung cancer. A quantitative risk assessment for lung cancer based on these studies corresponds to risk estimates based on studies at workplaces with exposure to diesel engine emissions; its magnitude cannot be explained by the known carcinogenicity of organic substances or metals adsorbed to the insoluble particle core. Carcinogenic effects of diesel particles were observed after inhalation in rats independently in several studies. The surprisingly strong effect of diesel particles was partially attributed to their small size. This hypothesis was corroborated by inhalation studies with synthetic nanoparticles virtually free of organic compounds. IARC found sufficient evidence for the carcinogenicity of carbon black and of titanium dioxide in experimental animals. Long-term studies by the method of intratracheal instillation confirmed the carcinogenic effects in rats for an even broader spectrum of synthetic nanoparticles. Non-positive studies with hamsters are not valid because hamsters did not develop lung tumors after inhalation of some known human carcinogens. In recent years, the number of publications reporting in vitro genotoxicity of TiO2 and of carbon black nanomaterials has increased. Overall, there is clear positive evidence for carcinogenicity in rats, together with supporting evidence from human data of structurally related substances. Therefore, the European Union (EU) criteria for category 2 of carcinogenic substances appear to be fulfilled for bio-durable nanoparticles consisting of matter without known significant specific toxicity.
Declaration of interest: The author reports no conflicts of interest. The authors alone is responsible for the content and writing of the paper.
Notes
*The proportion of lung cancer cases among the total number of deaths in a country within a certain time interval is a reasonably good approximation of the lifetime lung cancer (background) risk of this population. For example, within the 5-year period from 1999 through 2003, a total of 5,937,687 male Americans (all races) died, among them 450,483 lung cancer cases, i.e., 7.6%. Another example, same percentage: Of 1,028,152 deaths among white males in the United States in 2005, 78,379 were lung cancer cases, i.e., 7.6% (CDC, 2008). A similarly high frequency of lung cancer is observed among males in other industrialized countries. Doubling of a background risk of 7% means a total risk of 14%.
*The following unit risk values were used (per μg/m3): BaP as reference substance for organic products of pyrolysis, 7 × 10−2; arsenic, 4 × 10−3; cadmium, 1.2 × 10−2; chromium, 1.2 × 10−2; nickel, 4 × 10−4; furthermore, asbestos, 2 × 10−5 per 100 fibers/m3. The following annual average concentrations of these substances were associated with 40 μg PM10/m3 and 20 μg PM2.5/m3, respectively (μg/m3): BaP as reference substance for organic products of pyrolysis, 0.0032; arsenic, 0.0031; cadmium, 0.0013; chromium, 0.0157; nickel, 0.0107; furthermore, asbestos, 100 fibers/m3; these values may be linearly transformed to a concentration of 3 μg PM2.5/m3.
*Category 3 of carcinogenic substances is defined as follows: “Substances which cause concern for man owing to possible carcinogenic effects but in respect of which the available information is not adequate for making a satisfactory assessment. There is some evidence from appropriate animal studies, but this is insufficient to place the substance in category 2. ... Category 3 actually comprises 2 subcategories: (a) substances which are well investigated but for which the evidence of a tumour-inducing effect is insufficient for classification in category 2. Additional experiments would not be expected to yield further relevant information with respect to classification; (b) substances which are insufficiently investigated. The available data are inadequate, but they raise concern for man. This classification is provisional; further experiments are necessary before a final decision can be made.” Only substances which meet the criteria for classification as a category 1 or 2 carcinogen are defined as “carcinogens” according to Directive 2004/37/EC, which has as its aim the protection of workers from the risks related to exposure to carcinogens or mutagens at work and which explicitly contains the precautionary principle.