Multiple chemical sensitivities (MCS) nowadays more often called idiopathic environmental intolerance (IEI) is characterised by the recurrent occurrences of multiple symptoms involving in multiple organ systems. With respect to the aetiology of IEI toxico-, immune-, and psychogenic theories have been proposed but the exact physiological mechanisms underlying this syndrome are fare from being conclusive. The complex health complaints are triggered by environmental chemicals in very low doses and the upper respiratory tract and the nervous system are two of the most prominent organs of these health complains. Due to the fact that these ‘trigger concentrations’ are usually tolerated by most persons, genetic (e.g. slower detoxification) and acquired hypersensitivities (odour sensitivity) are thought to be characteristic features of IEI patients. In questionnaires assessing sub-clinical levels of chemical intolerance items addressing odour sensitivity are often used. Our experimental research showed that such scores predict a more unidimensional hedonic evaluation (e.g. malodors) of chemicals. Moreover, differentiation between odours was reduced. In line with other researchers we also showed that such self-descriptions are not associated with better olfactory acuity assessed with standardised psychophysical methods. When combining our own experimental findings with epidemiological studies on IEI we came to the conclusion that chemical or odour hypersensitivity is not an increased sensitivity of chemosensory pathways, but it appears to be a uniform and affective style of responding to low-level exposures to volatile chemicals. Such an undifferentiated response might also explain the fact the chemical triggers of IEI symptoms span from natural fragrances, to perfumes, to industrial chemicals like solvents.
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