The Lancet study shows that in the last decade up to 77 million people in Bangladesh could have been exposed to toxic levels of arsenic in their water supply.
Over a period of ten years researchers studied almost 12,000 people living in one area in the capital Dhaka.
The research found that the arsenic, which is naturally occurring, was the cause of 20% of deaths in the area.
It's
thought that the poisoning began in the 1970's when hand pumps were installed to tap the groundwater.
Researchers chose Bangladesh as the subject because for almost 90% of the population groundwater is the main source of their water supply.
According to scientists cancers in the lung, kidney, bladder and of the skin can be caused by long term exposure to even a small amount of arsenic.
