At what concentration in the body does nicotine become harmful/addictive?

Question:

Answers:
Any! It's bad for you and you shouldn't do it!

any whatsoever.cos then you get hooked.I smoke so I cant say too much, but just dont do it period.one of the biggest mistakes I ever made was starting smoking.its stays with you, hell I got a friend of mine that I havent seen smoke in four years and all of a sudden he is like, can I get one of those?