Parents who smoke often open a window or turn on a fan to clear the air of second-hand smoke, but experts now have identified another smoking-related threat to children’s health that isn’t as easy to get rid of: third-hand smoke. That’s the term being used to describe the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing, not to mention cushions and carpeting, that lingers long after smoke has cleared from a room. The residue includes heavy metals, carcinogens and even radioactive materials that young children can get on their hands and ingest, especially if...