- First-hand smoke: You smoke cigarettes, cigars, pipe, etc.
- Second-hand smoke: You are exposed to somebody's tobacco smoke.
- Third-hand smoke: You are exposed to the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing.
- Fourth-hand smoke: You see through the window somebody smoking.
The fourth and last exposure to smoke was added as a joke by an experienced allergist. All forms of smoking are harmful however.
See this comment by a reader of the clinical case about bullous emphysema:
"My dad is fighting for his life at the hospital because of severe breathing difficulty. He has bullous emphysema. Why is this happening? Smoking. He's been a smoker since age 16, he is now 63, and he looks like a 93 year old man because of smoking. His last words before taking him to he hospital were: "I wish I could have a couple smokes."
A New Cigarette Hazard: ‘Third-Hand Smoke.’ NYTimes.