A survey by the Environmental Working Group released on Tuesday found 141 unregulated chemicals and an additional 119 for which the Environmental Protection Agency has set health-based limits. Most common among the chemicals found were disinfection byproducts, nitrates, chloroform, barium, arsenic and copper.

The research-and-advocacy organization compiled findings from the states that agreed to provide data they collected from 1998 to 2003. That data comes from nearly 40,000 water utilities, serving 231 million people. The utilities were required by federal law to report that data to consumers.

For the unregulated chemicals, EPA is still identifying and considering the potential risks for possible future regulations. Nineteen of those chemicals exceeded EPA's unenforced safety guidelines for tap water systems serving at least 10,000 people, according to the advocacy group.

The EPA gathers its own water monitoring data, reviews the latest research and looks at treatment methods and technology, an agency spokeswoman said. States also are free to set their own safety standards for contaminants that may not be detected in other states.

Benjamin Grumbles, who heads EPA's Office of Water, said that "for the chemicals the agency regulates, nearly 100 percent of the community water systems that provide drinking water to the majority of Americans are meeting clean drinking water standards. We also have a process to continuously identify new contaminants for which regulation could reduce risks."

Jane Houlihan, EWG's vice president for research, said the group's findings show that the United States allows millions of people to be exposed to some chemicals for which EPA either has never considered the risks or if it has, has no enforceable limits.

"So in many communities the water that comes out of the tap could be contaminated with scores of chemicals. People shouldn't be alarmed, but they should be concerned. Our system of public health protections isn't working in this case," Houlihan said.

The top 10 states, listed in order of the most contaminants in their drinking water, were: California, Wisconsin, Arizona, Florida, North Carolina, Texas, New York, Nevada, Pennsylvania and Illinois, according to EWG, which listed the biggest sources as agriculture, industry and urban and sprawl developments.

Tom Curtis, a deputy director of the Denver-based American Water Works Association, echoed Grumbles' comments. "That's good news, and it's a reflection of water professionals' ongoing commitment to protecting public health," he said.

Curtis said the EPA has "a systematic approach to determining which substances should be regulated. Those regulations take into account occurrence data and health effects research, and should reflect the best available science."

He also said that "water suppliers support strong regulations that protect public health, and they also support proactive research that identifies and examines new substances found in source waters."

Last week, in setting two new rules to protect public drinking water, EPA Administrator Stephen Johnson called clean drinking water "a key ingredient to keeping people healthy and our economy strong." One rule aims to prevent disease-causing microorganisms from entering public water supplies, while the other is intended to limit the amount of potentially harmful disinfection byproducts.