This chapter provides definitions and descriptions of MCS and reviews the prevalence studies regarding who gets MCS.

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Many study samples have been predominantly women. Caress and Steinemann (2003) reported that two-thirds of their chemically sensitive respondents (when adjusted for sample bias) were women. My samples have been about 80% women. Although no one knows for certain why women appear to be more susceptible to MCS, there are several hypotheses that may explain these percentages. Some simple explanations may be that women are physically smaller on average than men and, therefore, may be less able to metabolize chemicals. Women also have a greater total percentage of body fat, which stores chemicals. There is an enzyme called alcohol dehydrogenase that detoxifies not only carbohydrates, sugar, and alcohol, but also chemicals. Men have much more of this enzyme than do women (Freeza, Padova, Pozzato, et al. 1990; Rogers 1990). Also levels of butylcholinesterases, which scavenge chemicals, are lower in females over the age of ten than in males, and decline further in women over the age of thirty (Wilson 1997).