Question: Most of Rand's heroes (at least in Atlas Shrugged and The Fountainhead) smoked. This was a common, unquestioned practice during the time in which the novels are set. We in the 21st century, however, have more scientific knowledge of the habit and its dangers. Isn't smoking an irrational decision and inherently anti-life?
Answer: Ayn Rand herself smoked, and many Objectivists still do. Objectivism has no particular position on smoking as such. However, I will comment on how I think the facts generally known about smoking fit into the Objectivist ethics.
Let me first remark that we see here the contextual nature of ethical knowledge. Smoking tobacco has common stimulative effects that any smoker can perceive to the degree that he experiences them. The alertness that smoking can induce is a very useful state to be in for most productive tasks, and so if one had no reason to think smoking was otherwise harmful, one would have good reason to conclude that smoking was likely to be beneficial, and that smoking was essentially virtuous. This is the basis for Ayn Rand's treatment of smoking in her novels, besides the fact that she liked the symbolism of a person being able to hold fire at his fingertips, representing man's control over nature.
Since Ayn Rand began writing, ample medical evidence and solid causal explanations have been developed that give us good reason to conclude that smoking does tend to have deleterious and currently irremediable health consequences. It is a cause of emphysema and cancers, is associated with other ailments, and can be expected to reduce one's life expectancy, other things equal. This would indicate that smoking is essentially a vice, because in Objectivism one's own life is one's ultimate value.
Some Objectivist smokers argue as follows: Smoking enhances their alertness and sense of well-being, hence enhancing their productiveness. Over the course of a lifetime, this benefit in terms of values achieved outweighs the health effects of smoking. And in any case, the health effects of smoking are not uniform: The averages depend on various traits individuals have, on other ways in which they behave (exercise, diet, and so on), and on certain random events. So if one knew one didn't have the physical traits likely to result in cancers, heart disease, or emphysema, and one took care to practice healthful exercise and dietary habits, one might well be able to judge that smoking was far less harmful in one's own case than in the average case, to say nothing of the aforementioned benefits of smoking.
I am no health expert, and I personally have never considered smoking, since for one thing I come from a family in which several people have suffered from smoking-induced emphysema and died far too young and far too unhappily. However, based on what I know, it is my own view that the health risks of smoking far outweigh any putative benefits. There are many stimulants that don't have such bad health effects. I also have the impression that smoking's effects are mostly random. Smoking over a lifetime makes these random events more likely occur, in the same way flipping a coin fifty times raises the chances that "heads" will turn up at some point. So in my view, smoking is a vice.
This point, however, allows me to remark that Objectivism uses moral terms in a distinctive way. To say smoking is a vice (or a virtue) is not to say that it violates Supernatural Law or Social Obligation. No Categorical Imperative of duty is involved. An Objectivist vice is simply a course of action that can be expected (based on one's own knowledge) to make one less likely to survive as a rational being. Vices are bad for whoever practices them. And so, when I say smoking is a vice, I am simply saying that it is bad for the smoker.