Widespread health hazards posed by low levels of chemicals in environment
Expert calls for '21st-century epidemiology' to combat threat
BY SHAWNA WILLIAMS
Movies such as “Erin Brockovich” and “A Civil Action” depict the major health problems that chemicals can cause. But the effects of chemicals are often far more subtle than such well-known cases would lead us to expect, argued Shanna Swan, PhD, a research professor in family and community medicine (and adjunct in statistics) at the University of Missouri-Columbia.
Speaking at Stanford on July 26, Swan said the low levels of chemicals present in our environment affect us in ways that can be serious, yet are undetectable through conventional means. “These problems are new and we desperately need new methods to deal with these challenges,” she said.
In her presentation, sponsored by the Center for Research on Women’s Health & Reproduction, she outlined the limitations of classical methods of epidemiology and showed what “21st-century epidemiology” can accomplish.
Classical epidemiology works best when dose levels are high, few people are exposed, the effects increase steadily with the dosage and only one chemical is the culprit, Swan said. An example was the quick detection in 1971 of the link between birth defects and the pregnancy drug DES. The symptoms were dramatic and it was easy to determine who had taken the drug. But, “these kinds of black-and-white classifications don’t serve us very well when looking at low-level or ubiquitous exposures,” she said.
In these “21st-century exposures,” Swan said, exposure is widespread and most people are unaware of exactly what they’ve been exposed to, making it necessary to analyze body fluids instead of simply using questionnaires. Further complicating matters, substances may act differently when combined with other chemicals or have a strong effect at a certain dosage level but not at others. And because the effects are usually subtle and widespread, researchers need to look at new measures in whole populations – not just effects on individuals – to detect them.
Swan illustrated this with a study she designed to compare the number of moving sperm in 658 men in four U.S. cities as well as the levels of agricultural chemicals in their urine. To her surprise, men in the most rural of the areas – Columbia, Mo., – had less than half the viable sperm count of men in Minneapolis (counts for men in Los Angeles and New York were also significantly higher than in Missouri). Swan and her colleagues found low sperm quality was linked to the presence of six agricultural pesticides in the men’s urine, though none of the subjects was a farmer.
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