Cigarette smoke contains myriad compounds that are known mutagens and carcinogens, and the health risks associated with active smoking and secondhand smoke are well established. Nearly 10 years ago, researchers at Berkeley Lab identified another potentially hazardous source of tobacco exposure: “thirdhand smoke,” the toxic residues that linger on indoor surfaces and in dust long after a cigarette has been extinguished. A team led by Antoine Snijders, Jian-Hua Mao, and Bo Hang in Biosciences’ Biological Systems and Engineering (BSE) Division have determined that early thirdhand smoke exposure is also associated with increased incidence and severity of lung cancer in mice.