WASHINGTON — The process used to sterilize Congressional mail after the anthrax attacks last fall could itself be making Capitol Hill workers sick, a report to be issued on Tuesday says.

The Office of Compliance, which is responsible for Congressional workplace safety, said its inquiry into more than 200 complaints "concluded that handling irradiated mail for substantial periods" might cause or contribute to health problems reported by Congressional employees.

The findings are the first to suggest that irradiation could be harmful. The report called for monitoring workers, further studies on long-term exposure to irradiated mail and some protective steps, including airing the mail before delivering it.

An earlier review for Congressional administrative officials found no evidence that handling irradiated mail affected health. The Postal Service has also said that testing showed the mail is not harmful.

Kristin Krathwohl, a spokeswoman for the Postal Service, said the agency would "certainly read the report of the general counsel of the Office of Compliance with great care." The Postal Service, Ms. Krathwohl added, was guided by assurances from the Centers for Disease Control and Prevention and the National Institutes of Occupational Safety and Health that irradiation was safe. "We rely on them," she said.

Senator Charles E. Grassley, Republican of Iowa, who sought the investigation in February, said the report suggested that the earlier Congressional inquiry "may have been too quick to conclude irradiated mail was harmless, and they may not have taken employees' health concerns seriously enough."

"Irradiating the mail," Mr. Grassley added, "was and is a big experiment."

Hundreds of thousands of pieces of mail to Congress and federal agencies in Washington are sent each day to a radiation center in New Jersey for treatment before being distributed. Wider use of irradiation has been considered. The process, more often used on food and medical equipment, exposes mail to an electron beam that renders any anthrax harmless.

The irradiation of Washington mail began last November after letters containing anthrax spores were sent to Capitol Hill, causing the Hart Senate Office Building to be closed for months. Anthrax-laced mail in Washington and elsewhere killed five people. The Postal Service resumed delivering mail to Congressional offices in January.

Dozens of House and Senate employees soon complained of ailments that included headaches, nausea, bloody noses, rashes and other irritations. Postal workers also reported problems. The compliance office said that in May it resurveyed Congressional employees who had filed health complaints and found that 55 percent of those contacted continued to report problems, though they were often milder.

"The continued symptoms in employees is the single most important reason in suggesting the need for more study," said Gary Green, general counsel of the office.

The report, Mr. Green said, was compiled with the help of two specialists in occupational medicine and two industrial hygienists. Independent environmental testing by the investigators was also critical to the findings, he said.

The report said tests of the air from irradiated mailbags and the House and Senate mail rooms detected small amounts of chemical irritants that could have been produced from the paper during irradiation.

"While we do not believe these chemical irritants are life-threatening," the report said, "we believe further study is essential to determine the effects of extended exposure to irradiated mail, particularly in restricted work areas."

Earlier this year, the Postal Service said the irradiation could produce carbon monoxide, and it began holding the mail in a ventilating area for 24 hours to let any gas dissipate. But officials said tests showed no threat to health.

The study for a Congressional task force by the National Institutes of Occupational Safety and Health attributed the staff complaints to "multiple factors" including odors that could trigger reactions and "heightened awareness and resultant employee stress from recent terrorist attacks."

Copyright 2002 The New York Times Company

###