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Measurements on the retention of cigarette smoke constituents in the human respiratory tract have been undertake for more than 100 years. The first studies on nicotine retention were begun by Lehmann in Germany in 1903 and published in 1908. The first studies on the retention of smoke particulate matter were published by Baumbereger in the United States in 1923. Since those early publications, many studies have been undertaken, more or less continuously. This article is a review of the work that has been done over the last 100 years, including a large number of unpublished studies undertaken by British American Tobacco in Southampton, UK. The techniques used have evolved over the years and there is a certain amount of variation in the data. However, the general trends in the results are reassuringly consistent. The bulk of the studies indicate that, on average, 60 to 80% of the mainstream smoke particulate matter is retained in the lungs after inhalation. For nicotine, carbon monoxide, nitric oxide, and aldehydes the total retentions are of the order of 90-100, 55-65, 100, and approximately 90%, respectively, during cigarette smoke inhalation. For most smoke constituents the retentions in the mouth only are considerably smaller than in the whole respiratory tract. The lung retention values for smoke particulate matter are dependent on the depth of inhalation, hold time in the lungs, exhalation volume, and other factors. However, the degree of nicotine retention following inhalation is not markedly influenced by changes in respiratory parameters. Furthermore, the percentage retentions for smoke particulate matter and nicotine are smaller for nonsmoking subjects exposed to environmental tobacco smoke than with active smoking. The smoke retentions are related to properties of the smoke aerosol particles and gases and their behavior as they travel through the respiratory tract. This includes particle growth in the respiratory tract and evaporation of gases out of the particles, and relevant aspects of these processes are also reviewed.