Is second hand smoke very harmful, and if so how does it compare to car exhaust?

Second hand smoke: Yes. The U.S. Environmental protection agency, the U.S. National toxicology program, the U.S. Surgeon general, and the international agency for research on cancer have all classified secondhand smoke as a known human carcinogen (a cancer-causing agent).People who live in a big city for 10 or more years have a slightly higher risk of lung cancer. This is most likely to pollutants like car exhaust.