(CNN)Millions of people in Bihar, India, are showing symptoms of arsenic poisoning, which can be linked to cancer, due to consumption of contaminated drinking water.
The state of Bihar, in eastern India, is one of the country’s most impoverished states.
World Health Organization guidelines state that the permissible limit of arsenic in drinking water is 10 ppb, while the Indian government states it to be 50 ppb, five times higher.
Since 2004, Ghosh has tested 44,000 tubewells in Bihar and found that about 30% have water with arsenic concentration higher than 10 ppb.
In 2016, the Mahavir Cancer Institute registered as many as 23,000 new cancer patients, and Ghosh found that many, though not all, of these cases were linked to exposure to this toxin.
"When I joined the cancer hospital, I started checking blood, hair and nail samples of cancer patients and found they have very clear-cut symptoms of arsenic poisoning," Ghosh said.
Arsenic has been linked to various forms of cancer in India, the most common being skin cancer, followed by cancers of the bladder, kidney and lung, says Majumdar.
But since then, millions of tubewells were installed in the Ganges delta, the International Agency for Research on Cancer says, to prevent gastrointestinal diseases such as diarrhea from surface water.
People soon became dependent on groundwater.
These changes mean arsenic — which previously occurred in a relatively harmless insoluble conjugate with iron called arsenopyrite — then split due to overuse, contaminating the groundwater with a soluble ionic form of arsenic that has since been consumed by millions of people for decades.