It s like walking behind a smoky bus when it starts up. That s an allergic response. Humans are allergic to bus exhaust. The eyes water to cleanse the eyes, and the coughing reflex kicks in to forcibly expel the toxic fumes before they get inhaled. It happens in an instant.
Tolerance is a big problem.
That means when we get used to an irritant, the body gives upon trying to expel it. Consider your average bus mechanic. After a few weeks or months of breathing those fumes every day, the body doesn t try so hard. He s becoming less sensitized to the poisonous carbon monoxide. The irritant is no longer triggering the strong cleansing response that it used to.