The most dangerous ubiquitous (which means present or appearing) chemical found in drinking water is arsenic which interferes with receptors for estrogen (steroid hormones that develop and maintain female characteristics of the body), testosterone (steroid hormones that develop and maintain the male sexual characteristics) and progesterone (steroid hormones that stimulates the uterus to prepare for possible pregnancy).
It is a known poison that is normally found in soils and water around the world and can cause vomiting, diarrhea, pain, skin eruptions and in some cases, death. This is the reason why America and other first world countries made it possible to have an endless supply of safe and clean water for their daily needs so they can avoid being contaminated with arsenic.