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WebMD:

Is Second-Hand Smoke Harmful to a Person with Asthma?

Second-hand smoke is the combination of smoke from a burning cigar or cigarette and smoke exhaled by a smoker.

Inhaling second-hand smoke, also called "passive smoke" or "environmental tobacco smoke," may be even more harmful than actually smoking. That's because the smoke that burns off the end of a cigar or cigarette contains more harmful substances (tar, carbon monoxide, nicotine, and others) than the smoke inhaled by the smoker.

Second-hand smoke is especially harmful to people who already have asthma. When a person with asthma is exposed to second-hand smoke, he or she is more likely to experience the wheezing, coughing and shortness of breath associated with asthma.

http://www.webmd.com/content/article/45/1660_51090.htm