Biota may be exposed to ionizing radiation from the environment from both external and internal exposure from radionuclides taken into the organism. For example, terrestrial organisms are exposed externally to radiation from the nearby soil, and aquatic organisms are exposed externally from the radioactivity in the water. Bottom dwellers or benthic organisms are exposed externally by the radioactivity in sediment and both terrestrial and aquatic organisms are exposed internally by radionuclides taken up by the organism. Therefore, in evaluating exposure it is necessary to obtain or estimate the levels of radioactivity in the abiotic and biotic components of the ecosystem. In some cases, there may be measured data available; however, often models are necessary to provide an estimate of radioactivity in various compartments.

As indicated earlier, a great deal of work has been carried out to estimate the concentrations of radionu-clides in biota that are part of the human food chain. In the absence of actual measurements of radionuclide levels in biota, models developed to estimate the levels of radioactivity in biota for use in assessing dose to humans are also useful in assessing exposure to plants and animals. The work of the International Atomic Energy Agency (IAEA) has been particularly helpful in summarizing methods and data from around the world. Although dynamic models are needed to evaluate the effects of transient concentrations in the environment, equilibrium models, where the radionuclides are assumed to reach equilibrium with each of the environmental compartments, are widely used for assessing risks to nonhuman biota.

Another aspect in determining exposure is that radiation levels can vary with distance (spatially) and with time (temporally). These two considerations are important in the determination of exposure.

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