Cigarettes produce two types of smoke: the active and the passive smoke. Active smoke is filtered by the cigarette and inhaled by the smoker. Passive smoke is the smoke that is released by the cigarette in the air. Not many people know that, not being filtered, the passive smoke contains a higher concentration of harmful substances than the active smoke.
Thus, the passive smoke is the type of smoke that has the potential to harm non-smoking people, in particular when they are exposed to it for long periods of time. Children who are exposed to a large amount of passive smoke develop serious diseases such as pneumonia and bronchitis more often than the other kids. It is also more likely that children who have parents who smoke will also start smoking themselves.
When it was still allowed to smoke indoors, most smokers would inhale both the active and the passive smoke, while the rest would often inhale the passive smoke of others. The law that prohibited the smoking indoors was thus protecting both smokers and non-smokers from being continuously subjected to passive smoke and after a first period of adjustments it was widely accepted by all.