BOSTON--An internal investigation of a January radioactive spill in Building D of the Harvard Medical School has concluded the accident produced no health risk, according to a report summary prepared by the school and the University's Environmental Health and Safety Office.

The summary said radioactive Phosphorus-32 dripped onto the floor in the Medical School's Building D on January 13 from a cart used to transport radioactive waste away from the laboratories. While the spill was not detected for two hours, "no radioactive material was found on individuals, except for trace amounts on the bottom of the shoes of a few employees," the summary said.

The report listed four causes of the spill: a mistake by a researcher in the preparation of a jar for waste disposal, a possible leaky cover for the jar containing Phosphorus-32, the failure of the waste collector to secure the jar in the cart and deficiencies in the cart.

Several members of the Environmental Health and Safety Office did not return repeated phone calls this week.

The summary sought to downplay the radiological significance of the spill. It said that if the total amount of radioactivity that dripped from the cart were ingested by one worker, it would only be half the annual limit on intake established by federal regulations.

But, three months after it happened, signs of the spill remain. Walking areas on the first four floors of Building D are covered with half-inch thick plywood, with 18 planks of wood alone stretching over large parts of the first floor. The floor of the building's main elevator also has a plywood covering.

Graduate students and other lab users, who spoke on condition of anonymity, were told by environmental health and safety officials not to discuss the details of the spill. Many of the users interviewed this week, who spend as much as 14 hours a day working in Building D, said they had not yet seen the report. In fact, 10 graduate students and lab staff members who agreed to interviews criticized the Medical School for keeping them in the dark about the possible risks.

"People didn't really know then what happened," said one graduate student on the first floor. "They still don't know."

Lab officials said there had been a meeting with Harvard Environmental Health and Safety officials, but many said they were not satisfied with the response.

Students were more critical of the evacuation of the building on the day of the spill. Students said the Building D fire alarm was mistakenly set off, allowing several people to leave the building before exits were sealed. That mistake, lab users said, may have allowed people to walk outside with radioactive shoes.

"My personal biggest concern is that the folks in charge keep telling us that a little radioactivity wasn't that big a deal," said another graduate student. "If it's not such a big deal, then why do they go after us in such a strong way if there's a little [radioactive] spot on your desk?