No amount is smoking is harmless. So drop it right away. Even the smokeless tobacco, social smoking, low-nicotine smoking or even present while others are smoking are equally dangerous. While smoking Carbon Monoxide replaces some of the oxygen in your blood which increases the blood pressure. At this circumstance, your heart is forced to work hard to produce sufficient oxygen for the body.
I don't smoke but never knew if someone else smokes in front of me.. That too be harmful.