THURSDAY, July 21 (Health Day News) -- Americans' exposure to secondhand smoke has decreased dramatically, and lead blood levels in children continue to drop, according to a government report released Thursday.
On the other hand, that same report found the vast majority of people tested carried evidence of pesticides in their bodies, the health risks of which are largely unknown.
These were some of the main conclusions of the Third National Report on Human Exposure to Environmental Chemicals, which was conducted by the U.S. Centers for Disease Control and Prevention.
CDC Director Dr. Julie Gerberding characterized the document as the "largest and most comprehensive report of its kind ever released anywhere by anyone" and one which represents "a giant step forward in our ability to understand the relationship between exposure to various chemicals and potential health effects."
The report, weighing in at 475 pages, measured blood or urine levels of 148 chemicals and their breakdown components or metabolites in a nationally representative sample of the U.S. population. Thirty-eight of the chemicals had never been measured before. The report, culled from the National Health and Nutrition Examination Survey (NHANES) and covering the timeframe between 1999 and 2002, involved about 2,400 people.
But just because a particular chemical is present in the human body does not mean it causes a negative health effect. "The vast majority of compounds have no evidence of health effects," Gerberding noted.
On the other hand, some compounds may have health risks that scientists are unaware of. "The ubiquity of exposure to chemicals that have potential effects means there's some urgency to try to understand what this means," said Dr. Ted Schettler, science director of the Science and Environmental Health Network. "There are hundreds of thousands and millions of people who are exposed to some of these chemicals at far higher levels than average."
One of the most notable findings was that only 1.6 percent of children aged 1 to 5 years had elevated blood lead levels (10 micrograms per deciliter or higher). This compares to 4.4 percent in the early 1990s. In essence, this means that public-health efforts to eliminate lead paint health risks are succeeding, although there are still pockets of children at high risk for exposure.
"This doesn't mean that children with any detectable lead in their blood are safe from complications. We don't know what is a safe lead level," Gerberding said. "We continue to strive to make sure children are free from lead exposure but, nevertheless, it is an astonishing health achievement."
Exposure to secondhand smoke is also on the wane, as measured by levels of cotinine, a metabolite of nicotine. Median cotinine levels from 1999 through 2002 decreased 68 percent in children, 69 percent in adolescents, and about 75 percent in adults. Not all population segments saw the improvements, however. Non-Hispanic blacks had levels of cotinine twice as high as those of non-Hispanic whites or Mexican- Americans.
"While population levels have decreased, among African-Americans of any age, there is not this degree of reduction," Gerberding said. "We have a disparity."
On the flip side, about 5 percent of people aged 20 and over had urinary cadmium levels at or near levels that may be associated with kidney injury and an increased risk for low bone mineral density. Cadmium is primarily associated with smoking, as opposed to passive smoking. "We don't know that there is a direct effect on health, but there is a need for further research," Gerberding said.
All women in the sample aged 16 to 49 had mercury levels below those associated with neurodevelopmental effects in a fetus. Still, 5.7 percent of women of childbearing age had levels within a factor of 10 of this threshold.
"We have no conclusive information of harmful effects associated with this, but it shines a light on the need for specific information," Gerberding said. Most of the mercury in the blood comes from eating fish or shellfish that acquire methylmercury from their environment.
The report showed undetectable or very low levels of each of three organochlorine pesticides: aldrin, endrin and dieldrin. Although these pesticides are no longer used in the United States, they are used elsewhere in the world.
First-time data for 29 dioxins, furans and dioxin-like polychlorinated biphenyls were included in the report and are part of an "ongoing risk assessment," said Dr. Jim Pirkle, deputy director for science at the CDC's Environmental Health Laboratory.
Similarly, information on phthalates, a group of chemicals used in the manufacture of plastics and vinyl, should help to evaluate risk levels with more precision, Gerberding said.
The compounds may interfere with normal reproductive tract development, Schettler said, and there was a wide variation of exposure, meaning that some people were exposed to 10 times the average exposure.
The report also suggested widespread exposure to pyrethroid insecticides. Animal studies show that this class of pesticides is neurotoxic, and also interferes with brain development, Schettler said.
"What you're finding here is a very, very high prevalence of exposure throughout the population," Schettler said. "Here we have an entire population virtually exposed, and it's often the children who have the highest exposure levels. It's unlikely that this is good news, [but] we don't know what the health implications are."
Both Gerberding and Pirkle described the extensive review process that the document underwent.
"This report is a scientific report, and the clearance process is expected," Gerberding said. "We're very confident that nothing in this report has been altered for any political considerations. We're very proud of it because we know that we can stand by the science. It represents the absolute state-of-the-art science in the world."
More information
The full report is available online at the CDC.
