Human factor which is smoking a clear explanation for the choice

Your sentence makes no sense as written. It lacks a verb. ("is" is part of a separate clause).

You call smoking a "human factor". It is certainly a factor affecting human health. What "choice" are you talking about?

A similar question is asked more clearly at
http://www.jiskha.com/display.cgi?id=1308655446

Check the answer there.