[Editor’s Note: This is a HUGE step for the EPA.]

The Environmental Protection Agency plans to conduct a new study about the potential health risks of atrazine, a widely used weedkiller that recent research suggests may be more dangerous to humans than previously thought.

Atrazine – a herbicide often used on corn fields, golf courses and even lawns – has become one of the most common contaminants in American drinking water.

For years, the E.P.A. has decided against acting on calls to ban the chemical from environmental activists and some scientists who argued that runoff was polluting ecosystems and harming animals.

More recently, new studies have suggested that atrazine in drinking water is associated with birth defects, low birth weights and reproductive problems among humans, even at concentrations that meet current federal standards.

The E.P.A. is expected to announce on Wednesday that it will conduct a new evaluation of the pesticide to assess any possible links between atrazine and cancer, as well as other health problems, such as premature births. The E.P.A. may determine that new restrictions are necessary.

The decision by E.P.A.’s administrator, Lisa P. Jackson, who took over the agency in January, is a significant departure from the policies of the E.P.A. under President George W. Bush.

For years, agency officials said that atrazine in drinking water posed almost no risk to humans or the environment. As recently as this summer, E.P.A. staff members argued that current regulations were adequate.