The most important hazard to health is tobacco smoking. Not only does smoking directly damage the lungs of the one smoking, but most of the smoke from a cigarette is sidestream smoke that goes into the atmosphere and harms those in close contact with smokers.
Smoking is the largest single cause of preventable disease in the world today.
It causes not only chronic bronchitis, emphysema, and lung cancer and makes one more susceptible to all respiratory infections, but it also causes up to one-third of all heart disease, the most common cause of adult deaths in the western world. Unfortunately, people in the developing world are increasing their smoking just as the U.S. and some other Western countries are successfully reducing their smoking habits. A major influence is the desire of the tobacco industry to make money off this killer habit. Here are two links for those who wish to stop smoking. Please read both.