Metallic elements in lung tissues: results of a meta-analysis

Several studies have investigated the levels of metallic elements in the pulmonary tissues of healthy subjects, patients with lung diseases and occupationally exposed subjects. The present meta-analysis was aimed at both assessing the possible contribution of metal exposure to the development of lung diseases, including lung cancer, and evaluating systematically the role and the weight of variability factors affecting the results of such studies. A literature research covering the period 1980-2007 was conducted using the public database PubMed. A standard scoring method was elaborated with a minimum score of 5 for inclusion and evaluation. Selected papers underwent a meta-analytical assessment. Fifty-eight papers were retrieved, but 21 of them could not be admitted to further analysis, due to failure to achieve the minimum score. The main limitations of individual studies included: limited sample sizes, poor control of smoking habits and differences in subjects' ages, lung tissue topography, sampling methods, storage procedures and data analysis. Copper and zinc were the most represented elements (121.96 +/- 0.74 and 12.98 +/- 0.07 microg/g dry weight, respectively). Among toxic metals, the highest concentrations were observed for chromium and lead (2.42 +/- 0.12 and 2.14 +/- 0.04 microg/g, respectively). Tissue concentrations were similar in unaffected tissues from both controls and lung cancer patients, whereas they were lower in lung tumor samples. A considerable intra- and inter-individual variability was noted. Such a variability of measures, combined with the very low metal concentrations calls for the definition and use of standardized procedures of sample collection, storage, and analysis.