Secondhand Smoke

[ysabetwordsmith]

 ... may cause behavior problems in children.  Some people have allergies to tobacco; if nothing else, children may act up because the smoke makes them miserable.  

Also it does not help to know adults around you are unwilling or unable to stop putting your life at risk.  That starting bugging me when I was, hmm, four or five?  It'd take longer for most kids, but at least some of them will notice that.