The first solid evidence that smoking causes cancer came in the 1950s, followed decades later by revelations that “second-hand” smoke also harms health. On Friday, scientists issued a warning about what they call “third-hand” smoke (THS) — the sticky residue from tobacco puffing that clings to walls and furniture. In mice, at least, exposure to these toxic leftovers causes lower infant weight and alters counts of blood cells associated with the body’s immune system, they reported in the Nature journal Scientific Reports.