I used to think so too. I've never smoked, but I remember going to business meetings where just about everyone else was smoking, with clouds of smoke billowing through the room. Plus, I remember coming back from business meetings via airline, and people were still allowed to smoke on planes. Most passengers would light up as soon as we were off the ground. I'd come home from such business trips with my clothes reeking of cigarette smoke. But the smoke never bothered me. Today, however, I can't even be in a room with a smoker. It severely irritates my throat, and obviously my lungs as I go into coughing fits. I don't have a medical explanation as to why my body has developed such a low tolerance for second-hand smoke, but I certainly know it isn't healthy for me.
I hear you! Of course, I smoked back then, myself. I think our noses were just burned out so we couldn't notice.