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CDC's surveys can be an important first step in demystifying medical trends

by Bette Hileman
August 15, 2005 | A version of this story appeared in Volume 83, Issue 33

Credit: Photodisc
Credit: Photodisc

The Centers for Disease Control & Prevention’s biomonitoring studies, issued every two years, give researchers new opportunities to try to learn whether low levels of industrial chemicals might be responsible for certain disturbing health trends. Incidences of testicular cancer, autism, attention deficit hyperactivity disorder, and male birth defects have all increased dramatically over the past few decades. The most recent CDC study, the largest-ever survey of blood and urine levels of environmental substances, sampled 2,400 people across the U.S. for 148 chemicals.

Over the past 30 years, the number of male babies born with hypospadias-a condition in which the urethra opens at the side rather than the end of the penis-has increased from a very rare condition to one affecting one in 250 boys. Another birth defect that has become more common recently is undescended testicles. Males with this condition are likely to develop low sperm counts and have an 11-fold increased risk of testicular cancer in early adulthood. The incidence of testicular cancer has doubled since the 1930s.

Although there were many doubts that male fertility was declining when it was first announced in 1993, new research shows that this has become a clear trend in the industrialized world. New studies of 19-year-old men in Denmark, for example, show that about 25% have very low sperm counts-so low that they are considered essentially infertile. Sperm counts in the U.S. have declined, but not consistently across all regions.

Many researchers speculate that these male problems are caused by fetal exposures to environmental hormones-endocrine-disrupting chemicals that mimic or block the action of the human sex hormones estrogen and testosterone. One study published in July shows a strong link between phthalate exposure in pregnant women and damage to their male babies' reproductive tracts. Phthalates are used in cosmetics and perfumes and as plasticizers in polyvinyl chloride products. Studies on rodents show that some phthalates block the action of testosterone.

Other chemicals suspected of disturbing the male reproductive tract are bisphenol A and polybrominated diphenyl ethers (PBDEs), used as flame retardants, and perfluorooctanoic acids (PFOAs), used as surfactants in manufacturing DuPont's Teflon. These substances will be measured by CDC in the upcoming 2007 biomonitoring survey.

There are other worrying trends. According to CDC, reported cases of autism rose from one in 5,000 children in the late 1980s to between one in 500 and one in 166 today. Some experts say thimerosal-the mercury-containing preservative that was used until recently in most childhood vaccines-may be responsible for the rising incidence. But a 2004 Institute of Medicine study concludes that there is little evidence linking thimerosal to autism.

And about one-third of the children in some schools now have asthma. Air pollution increases the number of severe asthmatic episodes that require emergency room treatment. But the primary cause for the escalating incidence of asthma is not known. Some studies show a link to phthalate or pesticide exposure.

CDC's biomonitoring studies alone will not give us answers to these medical mysteries, but they can provide a starting point. First, they can eliminate certain chemicals as likely suspects. For example, the latest survey shows that mirex, dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin), and dieldrin are undetectable in almost everyone.

Substances that have declined sharply in blood and urine, though not completely off the hook, are probably not responsible for current disturbing trends. Lead levels, for example, have dropped precipitously. In 1976–80, 88% of children aged one to five had blood lead levels above 10 µg/dL-considered a toxic level. In the new survey, only 1.6% of children have dangerous levels.

Other chemicals, however, appear in concentrations near thresholds of concern and should raise a red flag, prompting further study. One troubling finding in the recent survey is that about 5% of the population 20 years and older have cadmium levels in their urine that may be associated with kidney dysfunction.

Another disturbing discovery is that 5.7% of women have levels of mercury within a factor of 10 of what has been defined as the health threshold effect, which means that their levels fall between 5.8 and 58 µg/L. Mercury levels above 58 µg/L are associated with neurodevelopmental effects on the fetus. Unfortunately, the current survey measures only total mercury, so it can't distinguish between the methylmercury from fish and the ethylmercury from thimerosal. The upcoming 2007 study will use newly developed analytic techniques to isolate the two forms.

To enable more rapid progress in discovering what is responsible for the disturbing medical trends, Congress should fully fund the National Children's Study. It is designed to correlate children's development from conception to age 21 with exposure to environmental chemicals. In fiscal 2004 and 2005, it was funded at only $12 million, when $26 million is needed annually.

For 2006, the organizers at the National Institutes of Health have requested a total of $69 million to make up for the shortfalls in previous years and to implement the plans developed over several years. Only from such a thorough project as the children’s study will researchers ever be able to learn if environmental exposures, or some other still-unrecognized problems, are the reasons for our health concerns.



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