Of the thousands of molecules released when someone lights up a cigarette, 2,5-dimethylfuran has been deemed the best “smoking gun.” This marker molecule could be used to evaluate workplace safety and quantify cancer risks. Insurance companies could use it to certify that someone is smoke-free. Juan M. Sanchez of the University of Girona, in Spain, led a team that sampled the air in 56 local cafés and restaurants frequented by smokers in order to identify a molecule that uniquely signals the presence of cigarette smoke without interference from cars, cooking, or other burning and combustion activities (Environ. Sci. Technol., DOI: 10.1021/es1016075). Molecules such as nicotine, benzene, toluene, xylenes, phenol, limonene, and naphthalene have been suggested as markers. Those compounds can be present in the air from other pursuits, however, whereas 2,5-dimethylfuran is unique to lighting up a cigarette, the researchers note. When the team sampled the breath of two nonsmokers who worked in smoky environments, they detected the presence of 2,5-dimethylfuran as a consequence of secondhand smoke inhalation.