After surveying wells along the Mekong, which flows through Lao PDR, Cambodia and Vietnam, the UN Children's Fund (UNICEF) and governments concluded that as many as 1.7 million people were at risk of arsenic poisoning, whose long-term symptoms include skin lesions and cancer.
Twenty-one percent of the Vietnamese population is exposed to arsenic above the World Health Organization's acceptable level of 10ppb (parts per billion). It is found not just in groundwater but in bottled water, tap water, even fish, according to the Vietnam Ministry of Health.
In Cambodia and Laos, the precise numbers of people exposed to arsenic contamination is not yet known, though UNICEF and government agencies are compiling a report to be released later this year.
In some provinces along the Mekong River in Vietnam and Cambodia, residents are exposed to 30 times the acceptable level of arsenic, according to data from the Vietnam Ministry of Health.
Water containing arsenic above 300ppb could cause cancer within three to four years, the Health Ministry said.