ABSTRACT
What is the source of this variability? One very obvious reason that people react differently is that they are born differently; i.e., they have different genetic makeups. The chemicals produced in each individual are unique, not only in the exact forms of each chemical produced but also in the amount of each in the body. Thus each individual contains a different response system, which affects that person's ability to function under normal conditions and when challenged by toxic substances. This is obvious when we think about disease resistance. At one extreme we have the few cases of children who are hom without disease resistance (the boy in the bubble) and at the other we have people who "have never been
A third reason that one person's sensitivity differs from another's is their respective ages. Young children and very old people often are more sensitive than individuals in other age groups. This can be largely explained by the differences in body chemistry that occur with age. The very young often do not have fully developed body mechanisms that can effectively detoxify foreign chemicals. The very old may have lost this same ability, at least to some degree. In addition, certain types of toxic effects can only occur while development is still going on, so that adults would not be susceptible. For example, the full development of the brain takes about six or seven years after birth, and children who are younger than this are susceptible to toxic chemicals that can affect brain functions, such as learning ability or behavior.
