You can go round making studies of what the concentration of these chemicals are in the cabin till the cows come, the reality is that knowing the concentrations required to damage health of each chemical on it's own is one thing, but in combination is another. From my reading (but I maybe mistaken) the answers to those questions are not well known.
What's really required is a long term study of crew health with controls etc. Funding is an issue, and the science of detecting the effects and indeed the presence in the body is somewhat in it's infancy. However some are making progress - this paper makes interesting reading:
AUTOANTIBODIES TO NERVOUS SYSTEM-SPECIFIC PROTEINS ARE ELEVATED IN
SERA OF FLIGHT CREW MEMBERS: BIOMARKERS FOR NERVOUS SYSTEM INJURY