J Allergy Clin Immunol. 2006 Dec;118(6):1257-64.
Multiple chemical sensitivities: A systematic review
of provocation studies.
Das-Munshi J, Rubin GJ, Wessely S.
Section of Epidemiology, Institute of Psychiatry,
London.

A systematic review of provocation studies of persons
reporting multiple chemical sensitivities (MCS) was
conducted from databases searched from inception to
May 2006. Thirty-seven studies were identified,
testing 784 persons reporting MCS, 547 control
subjects, and 180 individuals of whom a subset were
chemically sensitive. Blinding was inadequate in
most studies. In 21 studies odors of chemicals were
probably apparent; 19 of these reported positive
responses to provocations among chemically sensitive
individuals, and 1 study demonstrated that negative
expectations were significantly associated with
increased symptom reporting after provocations.
Seven studies used chemicals at or below odor
thresholds, and 6 failed to show consistent
responses among sensitive individuals after active
provocation. Six studies used forced-choice
discrimination and demonstrated that chemically
sensitive individuals were not better at detecting
odor thresholds than nonsensitive participants.
Three studies tested individuals by using nose
clips/face masks and confirmed response, possibly
mediated through eye exposure. Three studies used
olfactory masking agents to conceal stimuli, and
none of these found associations between
provocations and response. We conclude that
persons with MCS do react to chemical challenges;
however, these responses occur when they can
discern differences between active and sham
substances, suggesting that the mechanism of
action is not specific to the chemical itself
and might be related to expectations and prior
beliefs.