Second-hand vapor may also be harmful. In one study, people sitting in a chamber filled with secondhand smoke and others in a separate one with vaping aerosols (designed to mimic real-world conditions) had similar blood levels of nicotine after sitting in their respective confines. ("Nicotine is a highly addictive substance with negative effects on animal and human brain development, which is still ongoing in adolescence," the authors write.) Another study "measured indoor pollution from 3 people using e-cigarettes over a 2-hour period in a realistic environment modeled on a café," the authors wrote. "They found elevated nicotine, 1,2-propanediol, glycerin, aluminum, and 7 polycyclic aromatic hydrocarbons classified as probable carcinogens by the International Agency for Research on Cancer in the room air."