toxicology and health physics and allows scientists to predict toxic or adverse health effects. The dose-response relationship is expected because interactions between organisms and chemicals and energy deposition occur according to the basic laws of physics and chemistry and therefore are predictable.

Dose-response relationships are of two types: one describes the response of an individual to different doses of an agent, and one describes the distribution of responses of a population of individuals to different doses. When toxic or adverse effects are considered, individual dose-response relationships are characterized by a dose-related increase in the magnitude of the response. Interpretation of individual dose-response relationships can be confused by the multiple sites of action of most toxic agents. Each site has its own dose-response relationship. Population dose-response relationships consist of a specific end point and the dose required to produce that end point for each individual in the population.

Three assumptions are made when considering dose-response relationships:

  1. The response is due to the agent administered. Although this assumption seems trivial in laboratory studies, it is not so apparent in epidemiologic studies. For example, epidemiologic studies might find an association between a response (disease) and one or more variables. Use of the term "dose-response" relationship in this context is always suspect until the variable is shown to be a representative factor of the putative causative agent.

  2. The response is related to the measurement of the dose. The most accurate way to determine dose-response curves is to measure the dose actually reaching the site at which an effect is detected within a cell. However, measuring the dose at the site of action generally is prohibitively expensive and has been done in only a few cases. Some measurement of exposure is nearly always substituted for a true measurement of dose.

  3. A quantifiable method of measuring and a precise means of expressing toxicity are available. Early in an investigation of the toxicity of an agent, the best end point for effects might not be apparent, but as more is known about the manifestations of toxicity, the dose-response relationship should become more quantifiable.

These assumptions hold true for all types of toxic agents, presumably including extremely-low-frequency electric and magnetic fields, if such fields are found to exhibit toxicity.

Types of Animal Studies Used in Descriptive Toxicology

Two main principles underlie all descriptive animal studies of toxicity (as reviewed by Klaassen and Eaton 1991). The first is that the effects produced by an agent in laboratory animals are applicable to humans. The second is that exposure of laboratory animals to toxic agents in high doses is a valid method of discovering possible hazards in humans. Toxicity tests are not designed to



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