How harmful is smoking one or two cigarettes a week?

Not very harmful: But very few people can accomplish not smoking more if they smoke that many. However, for those who can, it is not likely to cause many problems, but there is still a statistically detectable effect on health, just as there is for second-hand smoke exposure, especially in terms of more likely colds, sinus infections, bronchitis etc., but probably no effect on heart disease or cancer/emphysema etc.