Only if done, in excess… and in a manner which would endanger the health of the person smoking. Or those around him, such as infants… children… and elderly, who can’t avoid it. The Catechism of the Catholic Church (CCC) underlines respect for ones health and the health of others… in connection to the Fifth Commandment “Thou shalt not kill”. Here is what the CCC has to say:
(2288) “Life and 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.”
(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 drunkeness or a love of speed, endanger endanger their own or others’ safety on the road, at sea, or in the air.”
… Medical Science has proven that smoking and “second hand smoke” increases the risk of many types of cancer and other diseases, such as asthma and emphysema. So it would seem logical, that smoking could be a sin… IF the smoker is doing it to the extent of harming himself or others around him.
Hope this answers your question. God bless.