Passive smoking can be equally dangerous because it exposes the person to the same harmful effects of smoke.
Passive smoking means being in the same room or place, where some one is smoking and getting exposed to smoke in the surrounding area.
Once inhaled, the smoke can trigger mucus release in the bronchioles that blocks the airways. This induces coughing. But prolonged exposure can lead to bronchitis, emphysema, respiratory tract infections and eventually lung cancer.