If baby (formula-fed or breastfed) is around secondhand cigarette smoke on a regular basis, he could be more prone to respiratory illnesses and colic, could have lowered levels of HDL ("good cholesterol") and a heightened risk of dying from SIDS. The nicotine from secondhand smoke has been shown to enter your milk supply in small amounts, but even if you can't avoid being exposed to secondhand smoke, you should continue breastfeeding. Thing is, formula-fed babies are more prone to respiratory illnesses, colic, and lowered levels of HDL than breastfed babies (even without the smoke), and they're more likely to die from SIDS.
If you live with a smoker, ask him to smoke outside — not just in another room. (Or even better, ask him to quit.)
Only about 15 percent of cigarette smoke is inhaled by the smoker — the rest hangs out in the air you're breathing. Risk increases with exposure, so more temporary secondhand smoke (like hanging out in a building with smokers for a short time) is less risky…but it's still not healthy. Steer clear when you can.