Are there any studies on health effects of smoking once a month or less?
There's very little out there on very rare smokers, and as such I've been unable to find any studies along these lines for myself.
(Related question here. This question is different to that one, as this one is concerned with studies of even lower frequency of smoking, and that one is specific to pipe smokers.)
Today, most studies about smoking talk about passive smoking, pollution effect and how to compare pollution effect to smoking intensity etc.
Now, what is considered as a light smoker by almost all studies starts about 3cigarettes/day. This is understandable since the pollution effect in crowded places or near highly exposed places is equivalent to 3-10 passively smoked cigarettes/day (see here for an article about air pollution effect near an Amsterdam freeway).
Your concern being about one cigarette/month, you can see it is truly negligible compared to air pollution concerns for example. That's why there are no studies about this.
However, one should acknowledge a last thing. Daily life, involving simple things such as walking under shining sun, exposes us to hazard. Our body is made to be adapted to it, and handles it fairly well (otherwise humanity would have already been annihilated by viruses, skin cancer etc). When one smokes, he increases the risk for his body to develop bad things. Even though the risk is much lower in that case (1cigarette/month), it is still present. It's a bit like playing lottery; if you play very often, well, you might win.
