By Amarnath Tewary
BBC News, Bihar
Arsenic levels have risen to alarming levels in many districts of the north-eastern Indian state of Bihar, officials say.
Arsenic was recognised as a problem in South Asia in the 1980s
A state government survey has revealed that arsenic levels in up to 12 districts of the state of Bihar are now a threat to life.
The state capital, Patna, is among the affected areas.
Reports say villagers in some districts are suffering from bone deformations and a variety of skin complaints.
The study by Bihar's Public Health and Engineering Department (PHED) reveals that the average arsenic content in drinking water in the 12 districts is 500 parts per billion (ppb).
The World Health Organisation says that levels above 10 ppb present health hazards.
The health authorities in India have fixed the permissible limit at 50ppb.
"If arsenic content goes beyond the specified limits it can cause slow death," Madan Kumar, the PHED's chief engineer told the BBC.
As well as Patna, the other worst affected districts are Darbhanga, Bhojpur, Vaishali, Bhagalpur, Samastipur, Buxar, Khagaria, Begusarai, Katihar, Chapra and Munger.
Arsenic, an odourless and tasteless semi-metal element, occurs naturally in the environment and sometimes as a by-product of agricultural and industrial activities.
"Drinking water with high arsenic content may cause deadly diseases like gangrene and cancer of the intestines, liver, kidney and bladder," a leading Patna doctor, Gauri Shanker Singh, said.
"Its effect is slow as the impact remains invisible for a long time."
In many villages falling in the 12 districts people have complained of weakening and bending of the bones and dreadful rashes and lumps on the skin.
The reasons why these areas are suffering from rising arsenic levels is not clear.
"It might be because of an alarming drop in water levels. The water level in these areas has gone down to a great extent in last five years," Mr Kumar said.
With the help of the United Nations children's agency, Unicef, Bihar's Public Health & Education Department has been marking hand pumps and tube wells with different colours to indicate whether the water is safe to drink.
The first signs that arsenic-contaminated water might be a major health issue emerged in the 1980s, with the documentation of poisoned communities in Bangladesh and the Indian state of West Bengal, which neighbours Bihar.