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Different people can respond very differently to the same dose of a chemical, and this variation is due to many factors. Age, sex, and genetics all influence how susceptible people are to chemicals. People can also be more at risk from toxic chemicals if taking certain drugs, if they have certain diseases, and if they are exposed to mixtures of chemicals.

The very young and the old are usually more susceptible to toxic chemicals than the rest of the population. In the young, this is because they can absorb relatively large doses of chemicals, the systems that detoxify chemicals have not yet matured, and parts of their body, such as their brains, are more easily damaged. Older people are more susceptible because they may have larger body stores of certain chemicals, and more of them have diseases which could put them at greater risk from toxic chemicals.

In addition to reproductive and developmental factors, men and women may differ in their response to chemicals in other ways. As women in general have lower body weight than men, they will be more strongly affected by the same dose of chemical. Different genetic make-up and different proportions of body fat may also have an influence. For example, as women have a higher proportion of body fat than men, they are more prone than men to retain organochlorine pesticides such as DDT once ingested. There are reports that women are more susceptible to skin diseases induced by chemicals and to the effects of indoor air pollution including multiple chemical sensitivity. The possibility of differential responses should be taken into account when performing risk assessments (Encyclopedia of occupational health and safety, ILO, 1998).

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