Whether in their homes or outdoors -- in urban or rural areas -- Americans are exposed to thousands of chemicals in the environment. A new survey released by the Centers for Disease Control and Prevention examines exposure to dozens of these toxins. It's part of an effort to help citizens and government officials make more informed public health decisions.

Scientists used a process called biomonitoring to detect the way pollutants make their way through the human body. The CDC Environmental Health Laboratory tested blood and urine samples from 2,200 Americans for the presence of 148 chemicals.

The results provide some good news, says report co-author John Osterloh. For example, levels of cotinine, a chemical marker for secondhand smoke among non-smokers, have dropped significantly over time. "What we have shown compared to the late 1980s and early 1990s (is) that there has been a decrease in those numbers representing a fall in cotinine of about 68% in children, 69% in adolescents and about 75% in adults," he says. The finding suggests that laws limiting smoking in public buildings and restaurants have made a difference.

Another piece of good news: Children's blood lead levels are down. Lead is known to cause learning disabilities and behavior problems in children. The CDC survey finds levels declined from 4.4% in the early 1990s to 1.6% a decade later. The trend follows the removal of lead from gasoline over the same time period. John Osterloh says lead exposure remains a concern, especially from paint in old houses and deteriorating buildings. "There are localities and continued monitoring outside of what the exposure report does," he says. "In fact there are approximately 1 million children that get lead levels (tested) through their state health departments."

The report also includes extensive data for chemicals such as mercury, cadmium and other metals, and 43 pesticides. However that is only a fraction of the chemical toxins in the environment according to the Pesticide Action Network. The advocacy group - which promotes alternatives to chemical pesticides - points out that 90% of the people tested in the study had a mixture of pesticides in their bodies. Network spokeswoman Kristin Schafer says the list must be expanded, and detail added. "For example it would be very useful to know where this sampling was done," she says. "This data set they are releasing doesn't have regional or state-by-state breakdown of the sampling. With an eye to looking at the impact of policies already in place that go beyond some of the federal policies, it would be very interesting to see whether chemicals are lower in those states that have controlled those chemicals in their policies."

The report does provide exposure data on the U.S. population by age, sex and race or ethnicity. John Osterloh says the decision to include a chemical in the study depends on the threat it poses, methods for studying it and funding to do so. "We are attacking chemicals that are of concern to many people," he says. "Hopefully the expansion to the next report will include a few more. It is probably going to be around 300 chemicals."

That number is expected to increase to more than 450 by the end of the decade. But, John Osterloh says - the presence of a chemical does not necessarily imply disease or health risk. The purpose of the study is to find which chemicals do pose a threat and follow them over time.

"By showing that these chemicals are present in humans we hope to drive the scientific research process to say we should look at these chemicals because they are present," he says. "We should look at these levels because these are the amounts that are in people. And, we should look for effects that are associated with these kinds of levels. And, as we add more chemicals we hope to drive more research to a more focused area -- again the chemical and the level with respect whatever effect might be discovered."

John Osterloh says he expects the exposure survey to help raise public awareness. Kristin Schafer with the Pesticide Action Network hopes it does more. She recommends that based on its findings consumers turn to organic foods and pesticide-free household and hygiene products.