Inflammatory bowel disease refers to a group of chronic diseases of unknown etiology related to both genetic and environmental factors. In this 1990-1993 study, the authors investigated associations between the content and quality of drinking water and the incidence of inflammatory bowel disease. They used data from a population-based cohort recruited in southeastern Norway and a registry of water quality derived from Norwegian waterworks that contained measurements of iron, aluminum, acidity (pH), color, turbidity, and coliform bacteria. The authors found that risk of developing inflammatory bowel disease, including ulcerative colitis and Crohn's disease, was associated with high iron content. The relative risk of developing inflammatory bowel disease increased by 21% (95% confidence interval: 9, 34) when the iron content in the drinking water increased by 0.1 mg/L. They found no association between the diseases and aluminum in the water, color of the water, and turbidity of the water. The authors suggest that the observations can be explained by 2 mechanisms. First, high iron concentration works as a catalyst for oxidative stress, which will cause inflammation and/or increase the rate of cell mutations. Second, iron content stimulates the growth of bacteria and increases the likelihood of inappropriate immune responses in genetically predisposed individuals.