Common Industrial Chemicals In Tiny Doses Raise Health Issue
Levels of Risk
Common Industrial Chemicals In Tiny Doses Raise Health Issue
Advanced Tests Often Detect Subtle Biological Effects; Are Standards Too Lax?
By Peter Waldman
For years, scientists have struggled to explain rising rates of some cancers and childhood brain disorders. Something about modern living has driven a steady rise of certain maladies, from breast and prostate cancer to autism and learning disabilities.
One suspect now is drawing intense scrutiny: the prevalence in the environment of certain industrial chemicals at extremely low levels. A growing body of animal research suggests to some scientists that even minute traces of some chemicals, always assumed to be biologically insignificant, can affect such processes as gene activation and the brain development of newborns.
An especially striking finding: It appears that some substances may have effects at the very lowest exposures that are absent at higher levels.
... What if it turned out some common substances have essentially no safe exposure levels at all? That was ultimately what the U.S. Environmental Protection Agency concluded about lead after studying its effects on children for decades. Indications some other chemicals may have no safe limits have led regulators in Europe and Japan to bar the use of certain compounds in toys and in objects used to serve food.
Using advanced lab techniques, scientists have found that with some chemicals, traces as minute as mere parts per trillion have biological effects. That's one-millionth of the smallest traces even measurable three decades ago, when many of today's environmental laws were written. With some of these chemicals, such trace levels exist in the blood and urine of the general population.
Some chemical traces appear to have greater effects in combination than singly, another challenge to traditional toxicology, which tests things individually.
For their part, companies and industry groups have attacked low-dose research as alarmist and are challenging the findings with scientific studies of their own.
With so much still unknown, regulators are proceeding on different tracks in different countries. Japan's government designates about 70 chemicals as potential "endocrine disruptors" -- substances that may, at tiny doses, interfere with hormonal signals that regulate human organ development, metabolism and other functions. ... The Japanese government also has banned certain phthalates in food handlers' gloves and containers, after detecting them in food.
The European Union has banned some kinds of phthalates in cosmetics and toys, and it is considering a ban on nearly all phthalates in household goods and medical devices.
The White House plays down the issue, saying the low-dose hypothesis is unproved. But many federal scientists and regulators at the EPA and Health and Human Services Department are forging ahead with new methods for assessing possible low-dose dangers.
Dr. Colborn and colleagues popularized low-dose concerns in a series of conferences, articles and a best-selling 1996 book called "Our Stolen Future."
In 2000, a separate EPA-organized panel, after reviewing 49 studies, said some hormonally active chemicals affect animals at doses as low as the "background levels" to which the general human population is subject.
The panel said the health implications weren't clear but urged the EPA to revisit its regulatory procedures to make sure such chemicals are tested in animals at appropriately small doses.
The EPA hesitated. It responded in 2002 that "until there is an improved scientific understanding of the low-dose hypothesis, EPA believes that it would be premature to require routine testing of substances for low-dose effects."
... some hormonally active chemicals seem to have more effects at extremely low exposures than at higher ones. ... researchers have found chemicals that have hormonal effects on lab animals and on human cells in much tinier amounts than their standard no-observable-effect levels. And with some of these chemicals, as the tiny doses given to animals are increased, the effects recede. Then, at much higher levels, broad systemic impacts appear, such as reduced body weight.
An example is bisphenol A, or BPA, the ingredient in polycarbonate baby bottles and food-can linings. It evidently is widespread in the environment. In the U.S., the CDC has found traces of it in 95% of urine samples tested. In Japan, researchers have detected BPA in fetal amniotic fluid and the umbilical cords of newborns.
Studying BPA in rats in 1988, the EPA concluded the lowest exposure with an "observed adverse effect" was 50 milligrams a day per kilogram of body weight ... the agency set a daily safe limit for humans of 0.05 milligrams of BPA per kilogram of body weight.
Since then, however, academic scientists in several countries have done more than 90 studies that have found BPA effects on animals and human cell cultures from exposures well below this level.
The EPA used a relatively crude measure of the chemical's effects: changes in rodents' body weights. The new studies looked at subtler, hormone-related effects. Some studies found changes in rodents' reproductive organs and brains at doses as low as 0.002 milligram per kilogram of body weight per day. That is just one-25,000th the dose that the EPA said was the lowest exposure having an observable adverse effect.
Seeking to explain this pattern, scientists cite the endocrine system's exquisite sensitivity. Animals and humans secrete infinitesimal amounts of various hormones, such as estrogen, that trigger responses when they occupy special receptors on the cells of various organs. BPA is among numerous chemicals that can mimic estrogen by occupying cells' estrogen receptors. When they do this at critical phases of development, the chemicals can trigger unnatural biological responses, such as brain and reproductive abnormalities.
At higher doses, however, BPA and other endocrine disruptors -- instead of triggering the unnatural responses -- appear to overwhelm the receptors. That explains, scientists say, why some chemicals seem to have more potent hormonal effects at very low doses than at higher ones.
Environmental chemicals don't exist in isolation. People are exposed to many different ones in trace amounts. So scientists at the University of London checked a mixture. They tested the hormonal strength of a blend of 11 common chemicals that can mimic estrogen.
Alone, each was very weak. But when scientists mixed low doses of all 11 in a solution with natural estrogen -- thus simulating the chemical cocktail that's inside the human body today -- they found the hormonal strength of natural estrogen was doubled. Such an effect inside the body could disrupt hormonal action.
"In isolation ... the concentrations found in wildlife and human tissues will always be small" wrote the scientists, ... but because such compounds are so widespread in the environment, the researchers concluded, the cumulative effect on the human endocrine system is "likely to be very large."
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