oa Editorial [Hot Topic: Recent Advances in Understanding of Cigarette Smoke Free Radicals and Their Relationship to Smoking Related Diseases (Guest Editors: Chuan Liu and Kevin McAdam)]

The links between cigarette smoking and diseases such as lung cancer, chronic obstructive pulmonary disease and cardiovascular disease are now well established. Significant efforts have been made over the last 50 years to establish links between specific smoke constituents and the onset of these smoking related diseases. In recent years risk assessment approaches have also been developed to prioritise key toxicants amongst the thousands of identified species in cigarette smoke. The main focus of these models has been on stable, easily measured smoke constituents which have documented toxicity data. However, on a quantitative population basis, these models fail to predict the observed incidence of disease. A number of researchers have suggested alternative aspects of smoke which may also contribute to the risk of cigarette smoking. Amongst these, cigarette smoke free radicals have a long history of research findings which point to their potential to induce oxidative stress in smokers. However, due to the analytical challenges in identifying and quantifying these species, and the lack of quantitative risk data, they are not amenable to analysis using current risk assessment models. This special issue therefore reviews recent advances in our understanding of cigarette smoke free radicals with an emphasis on examining evidence for their involvement in smoking related diseases. Investigation of cigarette smoke chemistry is a specialised field, and this issue begins with an overview of recent advances in understanding of cigarette combustion and smoke formation, with focus on the sensitivity of smoke composition and chemistry to the methods used for smoke generation, trapping and analysis. Regulatory interest in smoke composition and the current generation of cigarette smoke risk assessment models are also discussed (Liu et al.). Interest in the chemistry of cigarette smoke free radicals is driven primarily by evidence for the participation of in vivo free radical species in disease processes, particularly oxidative stress. Current understanding of the contribution of oxidative stress to smoking related diseases is summarised by Fearon et al., together with evidence for the involvement of reactive oxygen species, reactive nitrogen species and biological free radicals. A review of the role of these reactive species in vivo leads to the conclusion that reactive oxygen species are major species involved in damage at molecular, cell and organism levels. An important biological mechanism for the generation of oxygen free radicals is decomposition of hydrogen peroxide to hydroxyl radicals via the Fenton reaction (Sharan et al.). Biomarkers of oxidative stress have been used as a surrogate measure of radical damage in smokers; these biomarkers are critically reviewed in light of the consistency of their relationship with smoking status (Lowe and Cemeli)......