While the dangers of lead poisoning have been known for decades, the health risks from exposure to even small amounts of lead are much more serious than scientists previously thought.

Last week, growing concern over lead reached the U.S. Supreme Court, where lawyers debated a company policy that bans all fertile women from lead-exposed jobs in an effort to protect the fetus. In this case, Johnson Controls Inc. of Milwaukee, a car battery manufacturer, says the ban is justified to protect the fetus from known harm; the United Auto Workers says the 1982 policy is a form of sex discrimination that keeps women out of the highest-paying factory jobs.

The case raises numerous medical and legal questions about the harmful effects of lead exposure and the responsibility of companies to protect employees of both sexes from harmful substances. Central to arguments on both sides is what scientists now understand to be the risks of exposure to low levels of lead.

Of special concern is the risk to young children whose brains are developing rapidly and are particularly vulnerable to the effects of lead. Dozens of studies detailed in a 1988 report to Congress by the federal Agency for Toxic Substances and Disease Registry describe the impact of environmental exposure ranging from inhaling fumes of leaded gasoline to eating dust or chips of leaded paint common in older housing.

Among the harmful effects from even low levels of lead exposure are mental retardation and learning disabilities; high doses can cause convulsions, coma and even death.

Meanwhile, a number of studies of adults have indicated that the current workplace lead exposure standard set in 1978 by the Occupational Safety and Health Administration (OSHA) probably does not provide adequate protection.

The Johnson case, however, focuses on the risks of lead to the fetus. In 1987, Harvard Medical School researchers published a study showing a decline in a child's cognitive ability following exposure to lead while in the womb.

The Harvard scientists used the concentration of lead in the umbilical cord at the time of birth as a measure of the amount of lead absorbed by the fetus during a pregnancy. The 249 children from middle-income and upper-middle-income homes included in the study were tested for blood lead levels until they were 2 years old. The infants were tested every six months for cognitive functions that included memory, learning, the ability to recognize objects, the beginnings of verbal communication and problem-solving.

Results showed that children whose levels of lead exceeded 10 micrograms per 100 milliliters of blood experienced a 10-point drop in intelligence compared with those who had lead blood levels of less than 3 micrograms, according to Herbert Needleman of the University of Pittsburgh, a prominent lead researcher and member of the Harvard team.

University of Cincinnati researchers repeated the Harvard study in low-income women and found that a blood lead level of around 10 micrograms measured in the umbilical cord was associated with low birth weight and premature delivery. Follow-up studies of the children showed that their blood lead levels continued to rise after birth, primarily from exposure to lead paint dust, and that they had slower mental and physical development than did children who were not exposed to lead.

In both studies, the harmful effects occurred at blood lead levels well below the 1985 standard of 25 micrograms set by the federal Centers for Disease Control as safe for children. In the workplace, employees' blood lead levels can legally rise as high as 50 micrograms, twice what CDC deemed safe for children and five times higher than what the Harvard study showed could harm an infant's mental development.

Although much of the debate in Auto Workers Union v. Johnson Controls centers on the risks to a fetus from the mother's exposure to lead, the case also raises questions about the possible impact on children if the father is exposed to lead in the workplace. Several studies of men exposed to high levels of lead at work show that the element can affect reproduction by causing genetic mutations and changes in the shape of the sperm cells and a drastic reduction in sperm count, Needleman said.

No studies, however, have shown that these sperm changes lead to birth defects. In addition, there have been no reports of a group of children whose lower-than-average mental development scores could be traced to paternal lead exposure.

In animal research, a recently reported University of Maryland study showed that exposing male rats to low levels of lead caused abnormal brain development in their offspring. The study suggests that lead-caused genetic damage to the sperm from a man's occupational exposure could lead to similar developmental problems for his children, said study author Ellen Silbergeld. But additional studies are needed to show whether this occurs in humans.

The focus on future offspring and the risks to children obscures the fact that exposure to small amounts of lead can lead to significant health problems for adults -- both men and women. These chronic effects range from high blood pressure to hearing loss to mood disorders.

"We are seeing increases in blood pressure and increases in heart disease that are associated with blood lead levels in the 10-to-20 range {micrograms of lead per 100 milliliters of blood} and no evidence of a threshold {of a safe dose}," said Joel Schwartz, a scientist at the federal Environmental Protection Agency who used a national health survey conducted between 1976 and 1980 to measure health effects linked to lead. He calculated that during that period, the lead-caused rise in blood pressure alone is responsible for 20,000 deaths from heart attack and stroke each year.

The newly discovered low-level effects of lead tend to bolster both sides of the case now before the Supreme Court. In fact, the company and the union often use the same scientific data to make different points, and that has sometimes upset the scientists.

"Our work has been used as evidence that it is okay to bar women from the workplace," said Needleman. "We think our work has been misquoted and misused. The industries have used the science to avoid paying money to clean up the plants."

Industry officials have a different view. According to a Johnson Controls statement: "There is no known technology available at any price that can make all work areas safe for unborn children."

Whatever the Supreme Court decides, the case has focused new attention on studies that raise concerns about the existing worker-protection standard of 50 micrograms per 100 milliliters of blood.

"The OSHA standard was set in 1978, and it is woefully inadequate today," said Schwartz. "We see blood pressure effects at levels much lower than the OSHA standards."

"Maybe there is a need to re-examine the standard," said John Martonik, deputy director of the OSHA office that sets health standards. But, he said, OSHA has had a difficult time enforcing the current standard, which reduced previous occupational levels of lead exposure. Prior to the OSHA standard of 50 micrograms, blood lead levels in workers commonly exceeded 90, though some companies had programs to remove workers from the plant when their blood levels reached 80.

While the current lead standard does not completely protect workers, "it provides a substantial amount of protection, though there may be some marginal risk remaining," said Martonik. "How do you decide what is good enough?"