E-cigarette use is rising, particularly among young adults and teens. Recent illnesses and deaths attributed to vaping have caused intense scrutiny of the chemicals in e-liquids and vapor, but little is known about the size of vaping particles and their deposition patterns in human airways. Now, researchers reporting in ACS' Chemical Research in Toxicology have analyzed how e-cigarette particle size and deposition change with factors such as device power, e-liquid composition and vaping practices.