Alan, ny take on the medline articles are either immediate toxic exposures or
if you can find a pharmaceutical similar in make-up to the toxin you are
interested in you can find studies on those.  I compare long term chemical
exposure to radiation exposure.  There is dose dependent relations for half of
a population etc.  Unfortunately radiation has been much better quantified and
defined than chemical exposures.  Simple substances like Toluene produce a
"drunkeness" which could be described as chemically induced psychosis.  Here
again you have to look at the literature on drug abuse and reactions to
"huffing" (sniffing glue, household chemicals) to see these effects.  Many
"highs" would be described as a mental illness if you aren't fmiliar with the
particular chemical brand of "drunkenness".  My feeling in that either milder
transitory effects or more biological neurotransmitter-brain nerve ending
effects are probably at lower doses.  Heavy duty neurological damage is heavy
toxic exposure.  It may indeed be fruitless to find these lower level exposures
but there may other body markers which point to this (immune system).  I am
particularly interested in the possibility that the farm workers thought
involved in the O.C. bombing could have exposures from the farm and the
chemistry they were doing.  If so, others need to be tested.