At this time, there is a new case definition for MCS (multiple chemical sensitivity). It was written in a meeting of doctors/researchers that took place in August of last year (2007). It is now working its way through the peer review process on its way to publication. Difficult to tell how long that will take, but it probably won't come out until early next year, at a guess.
When it is published, we will shout it to the world in the pages of Our Toxic Times.