Residents of a Missouri trailer park show immune system abnormalities as an apparent result of long-term exposure to the toxic chemical dioxin, raising the possibility that exposed persons could become ill in the future because of impaired defenses against infection or cancer, according to a study published today.

The study, which appears in the Journal of the American Medical Association, found no more illness in those exposed to dioxin than in an unexposed control group. However, researchers said the immune system findings suggest that those exposed may have less ability to fight disease over the long term.

"It is important new information . . . . We just don't know the impact it may have," said a spokeswoman for the Environmental Protection Agency.

Dioxin, an unwanted byproduct in the manufacture of herbicides and other chemicals, causes both cancer and immune abnormalities in laboratory animals but has not been proven to cause cancer in humans. It has been found in dozens of sites around the country and is considered a major public health problem.

As many as 5,000 Missouri residents were exposed to the chemical, most of them in areas of the eastern part of the state, where it was present in waste oil sprayed on roads in the 1970s.

Public health experts from Missouri and the federal Centers for Disease Control evaluated the health of 154 residents of Quail Run Mobile Home Park in Gray Summit, Mo., where soil levels of dioxin were among the highest found in the state. Medical tests of the participants, who lived in the contaminated area for an average of almost three years, were compared with tests on an unexposed control group.

The researchers found no difference between the groups in the frequency of miscarriages, birth defects, cancer and most other diseases. Exposed individuals had a higher incidence of skin problems, severe headaches and "pins and needles" sensations in their hands or feet.

The exposed group showed a tenfold increase in anergy, or failure of the immune system to react to proteins that usually cause a red bump when injected in the skin. They also showed more abnormalities in the function of T-cells, which are important in fighting infection.

The scientists also found a pattern of abnormal liver tests that was most marked in those exposed longest, suggesting liver damage from dioxin, according to Dr. Richard E. Hoffman, a CDC medical epidemiologist who was the principal author.

Hoffman said that his group is performing follow-up tests in those with anergy and is conducting studies to measure dioxin in the fat of exposed persons and to further explore reproductive effects.

He said that it may take decades to determine dioxin's impact on the cancer risk of those exposed, "plus the numbers [exposed] aren't very large."

The CDC currently considers health risks from dioxin contamination to be present when soil levels of the dioxin compound called 2,3,7,8-TCDD are above 1 part per billion. There is no plan to change that standard because of the study's findings, according to an EPA spokeswoman.