How big are the cigarettes, how deep do you inhale, how strong are the cigarettes you smoke, type of filter... So many factors.

But no simple X is safe and X+n is bad formular to appease any concerns. Everybody is different and all these Recommended Daily limits are all based on an average that none of us fully fit into.

I will say though that anything in moderation is the only rule of thumb that still carries, after all - stress does kill and if by not having a few cig's you increase your stress then you may find that your reducing your health even more due to the stress.

Moral being, be happy and if you have to question it, then you already know the answer that's best for you.

The study has a per-factor breakdown here: https://www.bmj.com/content/368/bmj.l6669

"Ever smoking" looks like "smoked any amount but doesn't presently" rather than "occasional smoker through present", and the lowest bucket for current smokers is 1-14 cigarettes per day. <10 is, I suppose, 1/3 of a cigarette per day.

"Ever" and "1-14" show clear and substantial health effects, which isn't promising, but <10/month looks to be below any of the actual measured brackets.

The problem with cigarettes is that it is incredibly difficult to stick to a very low count overtime. I smoked for 20 years, roughly 20 cigarettes every 2 days in the end. I managed to quit thanks to vaping.

Just don't start smoking, even small in quantities. It might not have significant drawbacks when you are young, but it absolutely will take a toll on your health eventually, just like drinking.

No, smoking is usually defined as smoking daily. But there's so few people that smoke less-than-daily that they aren't usually included in these sort of studies so it's harder to say what sort of effect it has.