When Denise Wadleigh was expecting her fourth child in 1980, the family bought a house in a middle-class section of Jersey City, N.J., so that the youngsters could make mud pies in their backyard.

Less than a year after they moved in, the family's 2-year-old son had lead poisoning, the new baby had high levels of lead in his blood and the family's cocker spaniel was fatally poisoned, all casualties of the cherished backyard, whose lead content turned out to be 25 times higher than average naturally occurring levels of the metal.

Defying more than 15 years of government controls, lead has emerged as one of the nation's most dangerous and pervasive pollutants, penetrating U.S. society far beyond the lead factories and ghetto tenements of chipping lead paint where the problem was once thought to be confined.

Lead is now known to reach into average American home, contaminating backyard dirt, the tap water of an estimated 42 million people, paint, food, dust -- even imported ceramic dishes.

At the same time, the metal is considered a more insidious health threat, posing a risk not only at the heavy doses of traditional victims but at the low absorption rates of the general population, including 17 percent of preschool children in metropolitan areas pinpointed in new Public Health Service estimates.

"We caught hold of the tail of the tiger," Ellen Silbergeld, chief toxics scientist for the Environmental Defense Fund and a member of the EPA's Clear Air Scientific Advisory Committee, said of earlier efforts to reduce lead in the environment.

"But now we're starting to see the rest of the beast," she added. "And it's bigger than we thought."

Recent scientific research into low-level effects of lead has provided new clues to some of the oldest and costliest diseases, including hypertension in middle-aged men and miscarriages, stillbirths and other injury to the fetuses of pregnant women.

The American Academy of Pediatrics regards the metal as the most serious toxicological danger to children, more threatening than asbestos in schools and passive smoking. The widespread, subtle consequences for youngsters even at low levels of absorption include partial loss of hearing and IQ, growth retardation, inhibited metabolism of Vitamin D and disturbances in blood formation. These low-level effects, often causing no obvious symptoms, are known as the "silent epidemic."

Two scientists recently resigned as authors of a PHS lead study being prepared for Congress, protesting editing decisions and questioning whether the Reagan administration has the commitment to reveal the full scope and details of a public health problem that requires far-reaching, costly remedies.

But even an administration that was initially eager to relax lead controls now acknowledges that the problem far exceeds the limited solutions of the 1970s.

"If we take into account the variety of individuals at risk, we have a pollutant that has a potential for affecting a very, very large proportion of the population, and the nature of health effects associated with it are enough to be of great concern," said Lester D. Grant, director of the EPA's Environmental Criteria and Assessment Office in Research Triangle Park, N.C. He predicted that lead will be a target of intense government control for the rest of the 1980s.

The lead industry considers such concerns exaggerated, noting that natural occurrences have released far more lead into the environment than man-made sources and that no conclusive evidence points to "harmful" effects from low levels of the metal.

"Lead was on earth before we people were," said Werner T. Meyer, president of the Lead Industries Association trade group. "So, a certain tolerance for lead must exist in the human body."

Lead has long been considered a health danger, but until recently the government avoided regulating a metal of such military and commercial value. The industry employed 50,000 workers as late as the 1960s, and lead still ranks fifth among the metals most consumed in the United States.

Unlike such naturally occurring metals as iron, lead is not considered essential for life functions. Yet everyone is said to contain some of it, consumed in tiny particles from the air, soil, dust, food and water.

Efforts to reduce exposure to lead began in the 1970s. After large numbers of inner city children were diagnosed with lead poisoning from eating paint chips, Congress banned sale and manufacture of lead-based paint. EPA regulations to phase out lead additives in gasoline have removed 80 percent of the lead in the air since 1977. And the food industry replaced lead-soldered cans for baby food, cutting by half the lead content of food for infants.

The impact of those reforms was reflected in the National Health and Nutrition Examination Survey (NHANES), which showed a 37 percent drop in the average blood lead level of Americans from 1976 to 1980.

But the NHANES results published in 1982 also suggested that earlier controls had just scratched the surface. Nine percent of preschool children met current standards of lead poisoning while 88 percent were at or above the maximum safety level recommended last year by the EPA's Clear Air Scientific Advisory Committee. About half of the adult U.S. population surpassed the safety guideline.

The study indicated how far the lead problem extends beyond the factory line and inner city ghetto, a point underlined in the PHS study. It shows that more than a third of the children with worrisome lead levels live in the suburbs.

A search for sources of pollution of such social reach has spotlighted old problems. An estimated 20 million housing units still contain lead paint. As of 1985, cars were still spewing 21,000 metric tons of lead into the air each year.

But the search also has led to places integral to daily life and once regarded as safe.

The EPA reported six months ago that one of every five Americans served by public water systems consumes unsafe levels of lead. The metal comes from corrosion of lead pipes and lead solder in the residential plumbing system.

Excess lead in drinking water alone, according to the EPA, annually causes lower IQ scores for 240,000 children and hypertension for 130,000 middle-aged males while increasing the risk of miscarriages, lower birth heights and weights, premature birth and stillbirths for 680,000 pregnant woman.

Ronnie Levin, an EPA economic analyst who wrote the report, said drinking water represents 15 to 40 percent of the lead to which Americans are exposed, depending on local conditons. Only recently recognized as a conduit for lead, household tap water may have been a hidden source of lead exposure among some poisoned children for whom no other source could be identified, Levin said.

It took Maurice Sanders and Judy Southerland nearly a year to identify the tap water at their Northwest Washington home as the source of lead poisoning discovered in their twin daughters at the age of 18 months.

After the diagnosis in October 1985, they paid $4,500 to repaint their house and replace its drywall ceiling to cover old lead paint. They threw out newspapers and magazines because of lead pigment in color pages. They asked the girls' live-in uncle to leave his construction boots and clothes outside for fear he had picked up lead on the job. They had the city check for lead in the water, but the tests showed no problem.

It was only after the high blood leads persisted that they had the water retested by a private firm. The results showed lead levels four to 10 times higher than the EPA considers safe.

"You assume your house is a safe place to live," said Sanders, 37, a computer planner for the Library of Congress.

Many assumptions have come under challenge in the search for lead sources.

Fields, playgrounds and backyards, such as the Wadleigh's, where children while away their days are often heavily contaminated by leaded-gasoline fallout from nearby highways and paint chips blown there from nearby buildings, the EPA said.

The Wadleighs live along a major truck route in a middle-income neighborhood of older homes probably once covered with lead paint. Still, Denise Wadleigh said, lead poisoning "you automatically thought was for the ghetto areas where the paint was actually falling off the walls and ceilings, not the Jersey City Heights where the middle class live."

Her lead-poisoned son was hospitalized for five days to have the lead purged from his blood, and the Wadleighs paid $6,000 to cover the backyard in concrete.

"I found out dirt can be dangerous," she said.

Household dust filled with fine soil and airborne particles of lead as well as lead paint fragments represents another major route of exposure for children who play on dusty rugs and lick surfaces and objects, the EPA said.

Food is considered a major route of exposure. The metal descends from the atmosphere onto crops, covers food in the dusty process of harvesting and transportation and contaminates products in the grinding, crushing and sieving with lead implements and the packaging in lead cans.

A Seattle woman nearly died after swallowing lead particles from imported ceramic dishes. Foreign-made plates, which account for most of U.S. dinnerware sales today, are dangerous if their lead glazes are not fired at the high temperatures legally required here to prevent the metal from getting into food and liquids.

Young professionals who renovate their homes have developed abdominal pains, dizziness, extreme fatigue and weight loss. Their symptoms were traced to lead dust or fumes they inhaled while removing old paint with scrapers, heat guns or sanders.

The lead industry's Meyer acknowledged danger only from paint and soil, noting, however, that soil problems are limited to "spots" easy to cover. Other sources, he said, generate insignificant amounts of lead compared with natural releases, such as volcanic eruptions, which humans have survived throughout millennia.

"Because measurable changes occur as lead enters the body," Meyer said, "there is no proof that these changes are harmful."

How much lead is harmful is the subject of intense scientific research and debate. The Centers for Disease Control considers 25 micrograms of lead per deciliter of blood as the threshold for lead poisoning in children, the level met or exceeded by 9 percent of 6-month to 6-year-olds in the NHANES. As lead absorption increases beyond that point, the child faces increased risk of acute diseases of the kidney, blood and gastrointestinal and central nervous systems.

Many specialists consider the CDC standard irrelevant as a public health guideline because of the wide range of nonacute but adverse health effects of lead detected at levels substantially lower than 25 micrograms -- and, in some cases, at the lowest measurable level.

A comprehensive analysis published by the EPA and reviewed by a panel of scientific advisers concluded last year that low levels of lead pose an especially high risk for preschool children, whose hand-to-mouth habits and contact with dust and dirt increase their exposure to lead. The study said that safe lead levels for children are "difficult to identify specifically."

In March 1986, the panel recommended a maximum safety level of 10 micrograms, the level matched or surpasssed by 88 percent of the preschool children in the NHANES.

A draft of the PHS study estimated that 2.4 million children who live in U.S. metropolitan areas exceed 15 micrograms, a level, it said, that "provides virtually no margin of safety for recognized or official toxicity levels."

"No {lead} level is probably optimal," said Paul Mushak, adjunct professor of environmental pathology at the University of North Carolina School of Medicine and senior author of the PHS study from which he and his coauthor resigned June 5.

Studies have traced the lowest IQ scores among children to those with the highest lead count. A loss of 1 to 2 points has been associated with 15 to 30 micrograms, with the loss rising to 5 points at 50 to 70 micrograms. Studies have traced intellectual impairment to the lowest detectable blood lead levels -- 6 to 10 micrograms.

The latest evidence indicates that lead's dangers begin for the unborn child, passed to the fetus by pregnant mothers exposed to even small doses of lead. A study published in April showed that children who absorbed the most lead while they were in the womb performed worst on physical and mental tests in their first years of life.

The most serious threat noted for males aged 40 to 59 is hypertension. A 1985 study by government and academic researchers found blood pressure rising at the lowest detectable levels of lead in men -- 5 micrograms.

Little or no association between blood pressure and lead in women was seen.

Compared with cigarette smoking and diet, lead is a "small component" of hypertension, said Joel Schwartz, an EPA epidemiologist and coauthor of the 1985 study. But because 35 percent of American adults suffer from the disease, he said, "a small percentage decrease has big public health implications."

The lead industry challenges such views for hypertension and other low level consequences of lead. Rosalind Volpe, manager of environmental health for the International Lead Zinc Research Organization, said the hypertension findings are inconclusive. For central nervous system and hormonal functions, she said, industry-sponsored studies show no low-level effects. Although effects on hemoglobin formation have been noted, she said, there is no evidence they are adverse.

"We are going after a mosquito with an atomic canon," Meyer said, arguing that lead's health effects are not dangerous enough to warrant costly preventive measures.

But, it is because those effects are preventable that public health specialists have intensified efforts to control the pollutant. The EPA has recommended a 60 percent reduction in the amount of lead permitted in drinking water and elimination of leaded gasoline. It is planning a study on the effectiveness of removing lead-contaminated soils. The Federal Drug Administration is pressing for further reductions of lead solder in cans. And, the Department of Housing and Urban Development plans to quantify the problem of lead paint in federally insured, owned and subsidized housing built before 1973.

Schwartz said those regulatory moves and earlier cutbacks are expected to lower blood pressure enough by the mid-1990s to save thousands of people from fatal strokes and heart attacks.

"Obesity has a much bigger impact on blood pressure," he said. "But it's a lot easier to lower lead in gasoline than to get people to lose weight."