Natural arsenic salts are present in all waters, with natural concentrations of less than 10 parts per billion (ppb). Unfortunately, there is an increasing number of countries where toxic arsenic compounds in groundwater, which is used for drinking and irrigation, have been detected at concentrations above the World Health Organization's determined safe drinking limit of 10ppb. Of particular concern is the situation in Bangladesh, where it is estimated that there are more than 1 million people drinking arsenic-rich water (above 50ppb). It is imperative that people stop using wells where arsenic levels are high.