Winter 2000 Issue
Tracking the Global Path of Chemicals
A recently released report from a CDC Foundation workshop charts a course for new research on what disrupts the human endocrine system
In a hot, dusty Mexican village, a farmer sprays DDT on his crops, hoping to fend off mosquitoes and the diseases they carry to his family and neighbors.
Thousands of miles away, an Alaskan woman looks out at the white-on-white blend of snow, ice, igloos and polar bears that her ancestors have lived among for generations.
What connects these two people in such different parts of the world? To scientists at CDC’s National Center for Environmental Health (NCEH), the link between them is chemical. Powerful chemicals such as DDT are vaporized into the air, traveling through the ozone layer and settling in the Arctic Ocean. There, the chemicals become concentrated in plankton, which are consumed by fish, which in turn are eaten by polar bears, whose fat is a delicacy for the unsuspecting humans who have now ingested a potentially harmful chemical – thousands of miles away from its source.
With support from the Turner Foundation and the CDC Foundation, CDC scientists are trying to unlock the puzzle of how particular chemicals affect humans, and which types of exposure are especially dangerous. Their focus is a group of chemicals known as endocrine disrupters, named for their ability to disrupt the delicate functions of the human endocrine system, which releases the hormones that regulate growth, development, metabolism and other functions.
With the CDC Foundation’s help, CDC convened a workshop in December 1998 that brought together experts from the many fields interested in learning more about endocrine-disrupting chemicals. Its purpose was to discuss next steps in research. “The CDC Foundation grant allowed us to bring in the right people to a first-class workshop to discuss where we stood, where we needed to go, and how to get there,” said Michael A. McGeehin, acting director of CDC’s Division of Environmental Hazards and Health Effects.
Scientists have known for decades that some man-made chemicals disrupt the endocrine systems of wildlife and laboratory animals. Although they suspect that the same chemicals that affect animals can disrupt the human endocrine system, evidence of a specific link between a chemical cause and a particular health effect has been elusive. That’s partly because both hormones and the chemicals that disrupt them work in very small concentrations – as small as parts per billion or trillion, a size equivalent to the width of this page in proportion to the distance to the sun.
Recent technical advances, however, have allowed NCEH’s laboratory, one of the most sophisticated in the world, to develop techniques that can detect minute traces of chemicals present in human blood or urine. Being able to detect traces of chemicals in humans is a step forward, but it’s not enough to solve the puzzle. The other big question marks are the timing of a person’s exposure to chemicals – for example, during stages of pregnancy – and the dosage, or level of exposure.
Even after that challenge is solved, the exposure still must be compared to health outcomes.The final report of the 1998 workshop, released recently, calls for the design of a large population-based study to track a cohort of women of childbearing age, since the effects of endocrine disrupters are most readily apparent in fertility and pregnancy outcomes. The proposed study – which would be a major undertaking – would follow these women throughout their pregnancies and their children’s development.
Before a full-scale study is launched, the CDC scientists and their partners in federal and state agencies have proposed several smaller pilot studies. Some of these preliminary studies are already under way in areas where exposure levels are known to be high, such as along the U.S.-Mexico border and in northern Alaska.
These pilot studies will help epidemiologists test the most effective methods for collecting blood and urine samples and matching these samples with information about women’s exposure to potentially harmful chemicals. One of the pilot studies will try to determine whether endocrine-disrupting chemicals are present during the earliest days of pregnancy among women who live and work on farms along the Mexican border. Another will examine how chemical exposure affects pregnant women and their infants in Alaska.
As Carol Rubin, chief of NCEH’s Health Studies Branch, points out, the endocrine disrupters issue truly represents global health. “The chemicals start at one end of the globe,” she explains, “and end up at the other – but they affect all of us who live in between.”
- Nicole Lezin