A study released yesterday indicates that human breast milk may expose nursing babies in one year to 27 times the recommended lifetime limit of cancer-causing dioxin.

The study by Dr. Arnold Schecter, professor of preventive medicine at the State University of New York, supports earlier projections of dioxin levels in human breast milk based on studies of fatty tissue samples.

Although it raises previous estimates of the risk to nursing infants, the evidence is not strong enough that mothers should stop breast feeding, Schecter said.

Dioxin, a byproduct of certain chemical mixtures, causes liver and lung cancer, reproductive problems and immune system damage in laboratory animals in very low doses. It is found in fish and water downstream from bleached pulp mills, foods treated with certain herbicides and air, soil and water near municipal waste incinerators.

Critics said Schecter's study overstates the risk to nursing infants because it is based on an outdated and overly conservative measure of dioxin's risk.

"The benefits of breast feeding far outweigh any sort of theoretical risks that have been calculated," said Dr. Renate Kimbrough, of the Environmental Protection Agency, who has studied the effects of chemicals on lactating mothers. "Those levels most likely have been present in human milk for a long time."

"It's like the risk of smoking," Schecter told reporters. Although smoking increases the risk of lung cancer, "Not everyone who smokes is going to develop lung cancer."

Four years ago, low levels of dioxin were first detected in average Americans. Previously, only people living near toxic waste dumps or exposed to Agent Orange, a defoliant used during the Vietnam war, were believed to carry the substance.

Schecter, part of an international team of scientists studying the effects of dioxin, reported in April 1986 that, based on findings in fatty tissue, the chemical probably resided in human breast milk, exposing infants in their first year to 18 times more dioxin than the EPA considered safe in a lifetime.

At a news conference yesterday, Schecter disclosed the results of a study of breast milk samples taken from 50 American mothers over the last two years. The average dioxin level in their milk was 4.5 parts per trillion, he said.

Schecter said nursing infants exposed to such concentrations absorb 27 times more dioxin in a year than the EPA considers acceptable in a lifetime, under its most stringent assessment of the carcinogen -- to limit the risk to one cancer case in 1 million people exposed.

Kimbrough and Dr. Alan Poland, professor of oncology at the University of Wisconsin, said that Schecter exaggerated the risk by basing his calculations on the EPA's assessment of dioxin.

"If it was my kid and wife nursing, I wouldn't do anything different," Poland said, noting that there has been no evidence that dioxin causes human cancer at the exposure rate in Schecter's study.