Risk factors are additive and even multiplicative. Important to know as well is your level of exercise. Any degree of exercise becomes protective but 40 minutes of brisk walking 4 times a week is highly protective. Also know if you are depressed, that is a risk factor. Levels of stress, especially if you have anger and hostility, acts as a risk factor as well probably by raising levels of cortisol and catecholamines.
Smoking seems to damage the heart in many different ways.
First, there seems to be a direct effect on the endothelial cells that line the coronary arteries. This then promotes entry of cholesterol into the sub endothelial spaces. The molecular mechanisms of this endothelial damage are rather intricate and not fully understood, but it occurs. A 3rd mechanism is that smoking increases platelet aggregation, which promotes clot formation and can result in the thrombus that is the final path of a myocardial infarction. I was a pack a day smoker for 20 yrs.