The myriad of problems tobacco causes were pretty much unknown until recent years.
If im not mistaken it has only been in the 60's or 70's that all the things they cause were starting to be discovered.
There are different types of cigarettes/ways to smoke so that also depends. I hear the "pure" tobacco leaf isn't bad and that it even makes your teeth white and helps with calcium. Supposedly.
The modern filtered cigarettes are loaded with carcinogens and harmful chemicals. They didn't even exist during St. Pius X's time.
It is an undisputed fact that cigarettes can give you a number of cancers. Smoking is a coin toss; you either get cancer or you don't. Some are heavy smokers and never get anything during their whole life, some do. You don't know. But i don't believe taking that risk would be far from sinful at all.
Not to mention that they are addictive because of the nicotine. You can't live without them after a certain time and that would certainly be sinful imo.
Like others said they are a massive waste of money (if you buy them in countries where they are expensive like the U.S.) and time.
There is also the fact that second-hand smoking also causes cancer. Not only could you take yourself down but you could also take others down with you.
I firmly believe had all the medical and health implications of smoking had always been known, the Church would have decreed it is sinful to smoke. And smoking in moderation does not even compare to drinking in moderation. You can still get cancer smoking "moderately".
Smoking a pack a day would certainly be mortally sinful, imo. Think about how much time that would take to do, and how addicted the person has to be to smoke that much to begin with.
Smoking is vicious. Period.