People are exposed to toxic substances through a variety of routes by ingesting contaminated food and water, by breathing contaminated air, or by contact with the skin. Other paths to exposure are indirect, for example, parental exposures that harm a fetus developing in the womb. Exposure per se does not necessarily result in an adverse impact. The level of impact depends on several factors; key among them is dose. Exposure is defined as the amount of a substance that reaches a person, whereas dose is the amount actually absorbed by the target organ. The timing and duration of the dose, as well as the amount absorbed, are all important in determining impact. For some toxic substances, there appears to be a threshold level below which no effects occur. Others, by contrast, may exert effects at the lowest possible doses.
Further complicating the connection between exposure and effect is the fact that the same dose will not necessarily elicit the same effect in two different people. As is true with infectious diseases, some people are more sensitive to the ill effects of chemical or physical pollutants than others. Children are particularly at risk, for a number of reasons, including their rapid growth rate, the amount they ingest in relation to their body weight, and the manner in which they are exposed. (See Children’s Special Vulnerability.) The developing fetus is also exquisitely sensitive to a number of chemical agents, especially in the first several months of development. Beyond age, genetics plays a key role, as does an individual’s general state of health and nutritional status. Children deficient in iron or zinc, for instance, absorb more lead than well-nourished children with identical exposures (Bowen and Hu, 1993).
In addition, a given chemical can have a variety of ill effects, or endpoints, ranging from minor irritation to death. These effects may show up immediately (acute effects), as is the case with pesticide poisonings, in several years (chronic effects), or in subsequent generations, as is the case with birth defects. For some effects, similar to certain cancers, the routes and mechanisms of action are relatively well understood because they have been studied extensively. For others, such as substances thought to disrupt the endocrine system, scientific understanding is rudimentary at best.
Adapted from D. Briggs, C. Corval