WASHINGTON - Public health efforts aimed at curbing exposure to lead and secondhand smoke have met with success, but a study released Wednesday also suggests that greater than expected amounts of toxic chemicals, including mercury, are being retained in humans, especially women and children.

The study, released by the Centers for Disease Control, marked the start of the first attempt to track systematically the amount of chemicals in the human body. But the study also underlined how little scientists know about human exposure to chemicals.

``Our government knows more about the level of chemicals in the fish and the animals in our environment than we know about the children and the people in the population,'' said Dr. John Balbus a public health advocate and director of the Center for Risk Science and Public Health at George Washington University.

``We're seeing some data that we have never seen before,'' said Kimberly Thompson of the Harvard School of Public Health. ``We're having better information about human exposure to a number of different chemicals, so that's important.''

Dr. Jim Perkle of the Centers called the findings a ``quantum leap'' in terms of health information. ``It's a major advance for exposure information for the U.S. population,'' he said. ``The better exposure information we have, the better we can protect health.''

In the past, scientists have used statistical models based on the amount of chemicals in air, water and food to guess the amount present in humans, but this is the first look at the actual levels of accumulation. Public health officials will be able to refocus their research because they'll have a better idea of where potential problems lie.

The study used a technology called biomonitoring to measure the chemicals in urine and blood samples of 5,000 people around the country in 1999. The report tracked 27 chemicals, broken down into four broad groups: metals, such as lead and mercury; cotinine, which is a breakdown product of nicotine; the products of certain pesticides; and phthalates, chemicals that are used in plastics, lubricants and solvents. Future studies will expand to include over 100 substances.

Of the compounds tracked in this report, 24 had previously never been measured in humans.

``It's hard to put the actual magnitude of the numbers in perspective without knowing a little bit more about what the health effects are,'' Thompson said. ``The study is good in terms of giving us new data; now we have to figure out what it all means.''

Still, some trends are clear.

For example, cotinine, the byproduct of nicotine, is one of the chemicals that had previously been measured. The cotinine levels in nonsmokers has dropped 75 percent since 1991, according to the report.

``This decrease documents a dramatic reduction in exposure of the U.S. population to environmental tobacco smoke since 1991,'' said Dr. Jim Mirkle, a co-author of the report. ``However, environmental tobacco smoke remains a major public health concern since more than half of American youth continue to be exposed to this known human carcinogen.''

Lead levels, which are also dropping, have been tracked for 25 years.

``The good news is that blood lead levels continue to decline among children overall,'' said Dr. Eric Sampson, another co-author of the report. ``However, other data show that children living in environments placing them at high risk for lead exposure remain a major public health concern.''

And while it is hard to measure health risks without a baseline, environmental and public health advocates noted several trends that they say raise red flags, including higher than expected levels of mercury and phthalates in people, especially women and children.

Balbus pointed to a study that was released last year by the National Academy of Sciences that estimated that 60,000 children are born annually with dangerous levels of mercury.

``And yet the levels that CDC released today exceeded the academy's estimate,'' he said. ``Based on the results of this survey, levels of mercury among many women of childbearing age are high enough that with just a small increase in their exposure, and if they become pregnant, their children could have central nervous system damage.''

Phthalates are present in a wide range of consumer and other products, and public health advocates also expressed concern about the amount of pesticides present in a subset of people, but said that the number of people affected was too small to draw firm conclusions.

The scientists at the Centers and chemical industry representatives stressed that the mere presence of a chemical in someone's system does not automatically lead to their becoming ill.

``We believe it is essential that people understand the practical limitations of biomonitoring data,'' said Sandra Tirey of the American Chemistry Council, which represents chemical manufacturers. ``For example, biomonitoring data alone is not an indication that anybody's health is being harmed.''