What is passive smoking?

I don’t smoke but live in the same room with two friends who smoke. Another friend warned me that I am at the mercy of passive smoking through my smoker friends. Kindly educate me about passive smoking especially if it is hazardous.
Brimo (by SMS)

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Passive smoking is the inhaling of smoke in smoke-filled atmospheres by people who don’t smoke. These people inhale a good deal of sidestream smoke- that is, smoke that is not drawn through the cigarette. Sidestream smoke contains much higher percentages of tar, nicotine and noxious gases than exhaled smoke since exhaled smoke has been filtered by the smoker’s lungs. It is therefore a major indoor pollutant and is dangerous to people with certain kinds of heart disease. It can also cause breathing difficulties and can set off strong allergic reactions in others. Tobacco smoke inside a room tends to hang in mid-air rather than disperse. Hot smoke rises, but tobacco smoke cools rapidly, which stops its upward climb. Since the smoke is heavier than the air, the smoke starts to descend. A person who smokes heavily indoors creates a low-lying smoke cloud that other householders have no choice but to breathe. Second-hand smoke has been confirmed as a cause of lung cancer in humans by several leading health authorities.