I became chemically sensitive after a significant exposure to toxic mold. They say one can become sensitive after either one large exposure to a chemical or a series of exposures that pushes you beyond your "threshold". MCS is a very real thing, and it is estimated that 15% of the population has it. The symptoms range from mild to debilitating. More people are coming down with it all the time, largely due to the creation of new chemicals and the overuse of them.