Bottom-up and top-down integration of human and ecological risk assessment.

The World Health Organization has published a framework for integrating human health and ecological risk assessments, and many others have argued for greater integration of health and environmental concerns in assessments. However, those who argue for integration do not agree on the expected benefits. This article provides a conceptual organization of the rationale for integration. The bottom-up line of argument begins with the premise that the mechanisms of transport, fate, exposure, and effects of chemicals on humans and nonhuman organisms are largely common. Therefore, integrated assessment should be more efficient and should employ the highest quality of science. The top-down line of argument begins with the premise that humans are organisms that reside in ecosystems. Therefore, changes in the environment imply changes in human health and welfare. These include changes in the many services of nature that contribute to human health, such as air and water purification, sentinel functions, and provision of recreation and food supplies. They also include the direct health benefits that have been associated with exposure of humans to plants, animals, and natural ecosystems. Integration should proceed from both the top and bottom.