In interviews, some E.P.A. officials conceded that they were frustrated by the limitations they face in scrutinizing chemicals like atrazine. An estimated 33 million Americans have been exposed to atrazine through their taps, according to data from water systems nationwide.

“The public believes that the E.P.A. has carefully reviewed all the chemicals that are used and has the authority it needs to deal with risks, but that’s often not the case,” said Erik D. Olson, director of food and consumer product safety at the Pew Charitable Trusts, and a former lawyer at the E.P.A. and for the Senate Committee on the Environment and Public Works.

“The E.P.A. is working with weak laws, basic research at the agency is often seriously underfunded, and in some cases there’s institutional inertia against change,” he added. “That’s contributed to a sense that the agency is often slow to react to new science showing risks.”

Though the hazards posed by atrazine are far from clear, some scientists and health advocates argue that the chemical deserves special scrutiny because it is so widely used. The European Union, for instance, has banned atrazine as part of a precautionary policy that prohibits pesticides that easily contaminate groundwater. (European regulators did not evaluate the chemical’s health risks.)

Atrazine, which is sold under various brand names including AAtrex, is most commonly used on corn in farming states. But it can also be found on lawns, gardens, parks and golf courses. Sometimes, the only way to avoid atrazine during summer months, when concentrations tend to rise as cropland is sprayed, is by forgoing tap water and relying on bottled water or using a home filtration system.

E.P.A. officials note that anyone using atrazine must complete a short training course and is warned to wear long-sleeve shirts and pants, as well as chemical-resistant gloves and shoes, when spraying. The chemical cannot be applied near lakes, reservoirs or other bodies of water. And local water systems must produce an annual report detailing the highest concentrations of atrazine and other chemicals detected over the previous year.

Some high-ranking E.P.A. officials say there are concerns over atrazine, and that it, among other chemicals, is likely to be closely re-examined by the new E.P.A. administrator, Lisa P. Jackson.