A number found links between health effects in babies and the amount of fish their mothers had eaten. But the mothers in various studies had eaten different amounts of fish and the health effects found in the children were not identical. In the meantime, techniques for identifying and measuring the contaminants in the mothers' and children's blood were improving significantly. So by our analogy, the researchers were not only looking at different slides through their microscopes, they were looking through different lenses as well. In 1990, the U.S. Congress passed amendments to the Federal Water Pollution Control Act that called for a more comprehensive assessment of health effects of water pollutants in the Great Lakes basin.
Epidemiologic studies compare the health of populations exposed to different levels of contaminants.
In combination with other analytical tools, they usually provide the most convincing evidence of human health effects. They can also provide the understanding needed to target control actions such as health advisories, regulations or medical intervention. But they are costly and time consuming.