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Smoke can be bad because if it goes into the lungs of a person, it can hurt their lungs or even kill them (for example, in a house fire). Most people think that they are always able to see smoke, but people cannot see some types of smoke: they are invisible.
Smoke from tobacco
When people smoke tobacco in cigars or cigarettes, they are burning the tobacco and inhaling the smoke. Tobacco smoke can hurt the mouth, throat, and lungs. It can cause mouth, throat, or lung cancer. It can also cause emphysema and bronchitis (trouble breathing). They continue doing it because the smoke also has a nicotine.
Smoke from factories
Factories and power plants produce large amounts of smoke into the atmosphere which pollutes the air. Eventually the toxic chemicals in smoke fall to the ground, and then they pollute the soil and the water. When the air, soil and water are polluted with toxic chemicals, the chemicals also get absorbed by plants and animals (including humans).