It is well established that it takes years of smoking to cause cancer and heart disease. Those who get these diseases have all smoked for many years. Almost all yeshiva guys who smoke as bochurim quit within a few years after getting married. This is a fact, and there are almost no guys over 30 in yeshiva that smoke.
So when we talk about yeshiva bochurim smoking we are talking about smoking for a few years, which is not much of a health risk. And if anyone disagrees with this show a single study what was done on those that smoked only a few years and shows how dangerous that is.