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Thirdhand smoke (THS) is the residue from tobacco smoke that clings to virtually all surfaces long after a cigarette has been extinguished. The burning of tobacco releases many semivolatile organic compounds (SVOCs) such as nicotine that adsorbs strongly onto indoor surfaces (walls, floors, carpeting, drapes, and furniture). Nicotine can persist on those materials for days, weeks, and even months. Our recent studies reveal that nicotine deposited on surfaces can react with other indoor pollutants, including ozone, which seeps in from outdoors, and nitrous acid, which is common in houses with gasburning appliances. These reactions lead to the formation of carcinogenic tobacco-specific nitrosamines (TSNAs) and ultrafine particles (UFP < 100 nm) that could exacerbate or cause asthma. These findings shed light on the longterm exposure to residual tobacco smoke in indoor environments.