Background: While there is broad consensus regarding the health impact of tobacco use and secondhand smoke exposure, considerable ambiguity exists about the nature and consequences of thirdhand smoke (THS). Objectives: We introduce definitions of THS and THS exposure and review recent findings about (a) constituents, indoor sorption-desorption dynamics, and transformations of THS, (b) distribution and persistence of THS in residential settings, (c) implications for pathways of exposure, (d) potential clinical significance and health effects, and (e) behavioral and policy issues that affect and are affected by THS. Discussion: Physical and chemical transformations of tobacco smoke pollutants take place over timescales ranging from seconds to months and include the creation of secondary pollutants (e.g., tobacco-specific nitrosamines). THS persists in real-world residential settings in the air, dust, and surfaces, and is associated with elevated levels of nicotine on hands and nicotine in urine of nonsmokers residing in former smoker homes. Much still needs to be learned about the chemistry, exposure, toxicology, health risks, and policy implications.
Conclusion: The existing evidence provides strong support for pursuing a programmatic research agenda on THS to close gaps in our current understanding of the chemistry, exposure, toxicology, health effects, as well as behavioral, economic, and socio-cultural considerations and consequences of THS. Such a research agenda is necessary to illuminate the role of THS in existing and future tobacco control efforts to decrease smoking initiation and smoking levels, to increase cessation attempts and sustained cessation, and to reduce the cumulative effects of tobacco use on morbidity and mortality.