MCS patients did not have unusually heightened sense of smell but did have a stronger emotional response to unpleasant and pleasant smells. Combined with the results of other work it would seem that MCS cases have an unusually strong interpretation of perceived exposures.
Evidence from:
D Papo et al. J Psychosom Research. (2006) Vol. 60 p 199 – 209
“Chemosensory function and psychological profile in patients with multiple chemical sensitivity: Comparison with odor-sensitive and asymptomatic controls”
further detail: