Really this is one of the more useless statistics. It's pretty much impossible to OD unless you're a toddler and you accidentally drink juice.

What matters is the affects small amounts of nicotine, repeatedly, have over time. Nicotine increases blood pressure and so it slowly destroys your cardiovascular system. Smoking is, of course, bad for your lungs.

The combination of lung damage and heart damage is deadly. Typically heart failure will require more oxygen in your blood to make up for the fact your heart doesn't work. But the lungs are bad at extracting oxygen when they have pulmonary damage.