The intuition (that turns out to be wrong)
If a pack a day is bad, then one cigarette should be about a twentieth as bad, right? That is the natural assumption. It is also wrong, at least for your heart.
What the biggest study found
A 2018 meta-analysis in the BMJ pooled 141 cohort studies. It found that smoking just one cigarette a day carries roughly 40 to 50% of the excess coronary heart disease and stroke risk of smoking twenty a day, not the 5% a straight-line relationship would predict.
In plain terms: one a day roughly doubles the heart risk you started with. Twenty a day roughly triples it. The jump from zero to one is huge; the jump from one to twenty is smaller than you would guess.
Why the curve is so steep at the bottom
The cardiovascular damage from smoking (clotting, inflammation, harm to the lining of your blood vessels) kicks in fast and then plateaus. A little smoke does a lot of that damage; more smoke keeps adding, but at a slower rate.
Cancer works differently: it builds with total lifetime exposure, so cutting down genuinely lowers cancer risk more proportionally. But there is no level of smoking that has been shown to be safe.
So what does that mean for you
Cutting from twenty to one is a real win for cancer risk and worth doing. For heart disease and stroke, it helps far less than the numbers suggest. The only move that takes the cardiovascular risk back toward normal is stopping completely.