Ronald Crystal of Weill Cornell Medical College in New York says he saw the difference between airway cells from non-smokers and from smokers who had no sign of lung disease.
Crystal says some hypothetical man smoking outside a building may think he's OK, but he's not.
"When you look at the cells lining his airways, the biology of those cells are markedly different, and they are clearly abnormal," Crystal says in a statement.
Crystal says quitting tobacco can let lungs heal from smoke damage, but some cells never fully recover -- so it's best never to start.
The findings were published in the journal Stem Cell.