Over one million U.S. women have had silicone breast implant operations, and in light of recent findings by the Food and Drug Administration (FDA), they may have cause to worry as well.

The most popular type of implant, a silicone envelope filled with silicone gel, has been linked to silicone associated connective tissue disease, scleroderma, systemic lupus erythematosus and rheumatoid arthritis.

The passage of microscopic silicone particles through the envelope is considered the most probable cause of these and other implant-related health problems. These particles lodge in the surrounding breast tissue and in the lymph nodes, from which they are passed throughout the rest of the body.

And though there is no evidence that implants increase the risk of breast cancer, the silicone implants are opaque to X-rays, making it difficult for doctors to diagnose cancer at an early stage.

The FDA's ruling was prompted by information in reports presented by manufacturing companies to the agency last November.

"The information we've acquired since the last panel meeting, rather than dispelling doubts, has increased our concerns about the safety of these products," said FDA Commissioner David Kessler in a prepared statement.

The reports indicated that the officials of at least one major manufacturer of silicone implants, Dow Chemical, have known about the problems associated with the implants for well over a decade.

The FDA's and manufacturing companies' admission of the possible risks of breast implants comes only after a long process of research and reclassification.

Invented in the early part of the century, implants soon became used on an increasingly widespread basis. Their use was unmonitored until a new law regulating the use of medical devices gave the FDA jurisdiction over them in 1976.

At that point, they were placed in an "assumed to be safe" category. In 1982, however, the FDA reclassified them as "risky." In 1988 they received a new classification yet again, this time as "requiring proof for safety and efficacy".

Pressure on the FDA increased over the last few years as doctors, patients and women's health activists continued to express concern about the safety of the implants.

In April of 1991, a congressional subcommittee told implant manufacturers they had 90 days to prove their products were safe or to pull them off the market. The FDA eventually found the manufacturers' reports unsatisfactory, and criticized companies for reassuring patients that the devices were perfectly safe.

Last month, the FDA requested that doctors and manufacturers participate in a voluntary moratorium, halting the use and distribution of silicone implants until further notice.

At Harvard Medical School, doctors are critical of the way the FDA has handled the situation.

"The FDA has been irresponsible. They've had jurisdiction since 1976 and they've been dragging their feet," said Dr. Nicholas E. O'Connor, a plastic surgeon at Brigham and Women's Hospital. "They're not releasing information on how risky it is, and they haven't said what [doctors] should do."