"No one told me not to," he said.

The two children are victims of what the World Health Organization has declared the "largest mass poisoning of a population in history." Their hand-operated water pump in this town in northwestern Bangladesh produces water laced with arsenic. The poison gradually debilitates its victims before spawning cancers that kill them.

As many as 20 million Bangladeshis drink water contaminated with arsenic, according to American and Bangladeshi researchers; estimates vary widely, but some experts believe arsenic poisoning, if left unchecked, could ultimately cause cancers that could kill millions. . Dr. Allan H. Smith, director of the arsenic research program at the University of California, Berkeley, called the situation "the highest environmental cancer risk ever found," a threat worse than Chernobyl or Bhopal.

A decade after Bangladeshi officials confirmed the widespread arsenic contamination, some signs of progress are emerging, according to Bangladeshi officials and American experts. But even as initial steps are taken to counter arsenic poisoning, new complications arise.

In a step forward, a $30 million project financed by the World Bank and the Swedish government has tested 5 million of the country's 11 million tube wells and found 1.4 million, or roughly 29 percent, to be contaminated, according to Bangladeshi officials. Alternative water supplies are being constructed in 10,000 of the worst affected areas, they said. And after initially refusing to admit the problem, the Bangladeshi government has adopted a national arsenic mitigation strategy.

But the international and national response remains far too slow, according to Bangladeshi arsenic victims and American experts. They contend that comparatively meager efforts are being made. "There are simple things to do," said Richard Wilson, a Harvard University physicist and arsenic poisoning expert. "But they're not being done."

Bangladeshi government and World Bank officials say they are doing their best to counter a staggering problem. Testing and possibly replacing millions of tube wells in Bangladesh, one of the world's poorest countries, is a gargantuan task.

They also point out that more immediate killers, like bacterial contamination of drinking water, compete for the same resources and attention. Khwaja M. Minnatullah, a senior water and sanitation specialist for the World Bank, said bacteria in drinking water continued to kill about 110,000 children a year in Bangladesh.

"This is still the major killer related to drinking water contamination in Bangladesh and across the developing world," Mr. Minnatullah said. "There should be a balanced response to address the whole issue of contamination of drinking water."

Chapai Nawabganj illustrates the new complications that have emerged.

A year after inspectors declared the water at the pump in the impoverished neighborhood of Rajarampur Master Para contaminated, some families use it only to wash clothes and floors, according to residents. But other families, and many neighborhood children, continue to drink from the pump.

Muhammad Yad Ali, 42, a neighborhood yarn trader, said inspectors painted the water pump's spout red as a warning. But over time, the paint has washed away, and fears of poisoning have faded. Hot and thirsty after playing games, children drink the water in great gulps.

"When they first put the red color, everyone became afraid," Mr. Ali said. "Now, they became more relaxed and don't think about the drinking."

A new system of piped water is being built, he added. But the slow-moving project has yet to reach Rajarampur Master Para.

A second neighborhood, Swarupnagar, illustrated another problem. Munir ul Haq, 26, a motor-parts shop owner, said one team tested his neighborhood tube well and declared the water safe to drink. A year later, a second team tested it, declared the water unsafe and painted the spout red. He said residents had stopped drinking the water but were unsure what to believe.

A half mile away, Sumon Ali, a 20-year-old mechanic, said inspectors declared water from the tube well behind his auto repair shop contaminated two years ago. Ten months ago, he dug a new well a few feet away to create more space for his garage. No one knows if the new tube well is also contaminated.

The arsenic problem in Bangladesh was born of good intentions. For 20 years, government and United Nations officials, as well as Bangladeshi aid groups, urged Bangladeshis to stop drinking unclean pond water, to prevent the lethal diseases it bred. Instead, they were to invest in tube wells to tap into underground aquifers. Unfortunately, no one tested the aquifers for arsenic.

Salma Begum, 30 and the mother of three, lives in the village of Abirpara in central Bangladesh. She is one of 38,118 arsenic patients identified by the World Bank study. (Some experts contend that the number is far higher.)

Ms. Begum said her poisoning stemmed from years of drinking from an arsenic-laced well in her home village. She began drinking clean water five years ago, but her health continues to worsen.

Arsenic kills quickly in high doses, but is a slow and unpredictable attacker in small amounts. It takes 2 to 20 years to unleash its damage, affecting different people in different ways. Some people, including all of Ms. Begum's relatives, drank arsenic-laced water for years and are unaffected.

All over Ms. Begum's tiny, slowly withering body, signs of arsenic poisoning have emerged. Ugly and painful boils cover her hands and feet. Her veins protrude from her skin. Dark spots cover her arms, legs and parts of her body she declines to show. Her skin itches endlessly when exposed to sunlight.

She said her largest problem was a creeping weakness and constant ache in her arms and wrists. Her arms have grown so feeble that she struggles to hold her 2-year-old son and care for her husband and two daughters. As she speaks, she constantly kneads the muscles in her forearms and wrists, as if trying to wring the pain from her flesh.

Ms. Begum said she could not afford to buy medicine to counter some of the symptoms. She is not alone. Bangladesh's government has adopted a protocol to treat people identified with arsenic poisoning, but the outreach group that aids her, Dhaka Community Hospital, says it does not have adequate financing to provide treatment.

Ms. Begum, meanwhile, grows weaker and more frustrated by the day. Teams conducting surveys for arsenic victims have interviewed her, she said, but the poison continues to slowly devour her body.

"Visitors come and write down our problems in the notebook," she said, as her children clutched at her legs. "But no action is taken."