Dear Andrew:

If we purposely smoked with the intention of killing ourselves then that would be a mortal sin, but the sin is not the smoking, it is the suicide. I cannot imagine anyone actually trying to commit suicide on purpose through smoking. Such a thing would be rather prolonged and not guaranteed.

On a basic moral front, we have an obligation to respect our bodies and its health, but not to the point that we become a "health nut". To put it more academically, here is the Catechism:

Respect for health

2288 Life and physical health are precious gifts entrusted to us by God. We must take reasonable care of them, taking into account the needs of others and the common good.

Concern for the health of its citizens requires that society help in the attainment of living-conditions that allow them to grow and reach maturity: food and clothing, housing, health care, basic education, employment, and social assistance.

2289 If morality requires respect for the life of the body, it does not make it an absolute value. It rejects a neo-pagan notion that tends to promote the cult of the body, to sacrifice everything for its sake, to idolize physical perfection and success at sports. By its selective preference of the strong over the weak, such a conception can lead to the perversion of human relationships.

2290 The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others' safety on the road, at sea, or in the air.

2291 The use of drugs inflicts very grave damage on human health and life. Their use, except on strictly therapeutic grounds, is a grave offense. Clandestine production of and trafficking in drugs are scandalous practices. They constitute direct co-operation in evil, since they encourage people to practices gravely contrary to the moral law.

From this we see that although we are to avoid excess and abuse of tobacco, it is not listed as a grave sin. Drunkenness and love of speed ARE listed as grave sins as well as the illicit use of drugs and the production and trafficking in drugs.

As for how much is too much to qualify as an excess or abuse, I have no idea. That would probably have to be determined on a case-by-case basis. In any event, even excess or abuse of tobacco is not automatically mortal sin. I am not sure it is even grave matter which would be a requirement for smoking to be even potentially mortal.

Smoking could easily be seen as a violation of the virtues of temperence and prudence, but I am not sure that it can be seen as a grave sin.  In any event, if smoking can be seen as grave, the culpability of an abuser of tobacco would likely be mitigated due to addiction.

By the way, Pope St. Pius X was an avid cigar smoker.

God Bless,
Bro. Ignatius Mary