| MARCH 3, 2003:
Scientists call the accumulation of chemical contaminants--like
PCBs, mercury, and pesticides--within a person's body their
“body burden.”
Body burden is just a number--a concentration in parts per
billion or micrograms per liter. But, technical as it is,
the term calls forth an image too; an image of a body bent
over and struggling beneath a heavy load. When scientists
start taking about body burden, I think about real bodies--my
own and my children's.
Thanks to a new report from the Center for Disease Control
we have a better sense than ever before of the typical American's
body burden. In the Second National Report on Human Exposure
to Environmental Chemicals CDC scientists measured the levels
of 116 chemicals in the blood and urine of 2,500 volunteers.
The study found detectable levels of 89 chemicals, including
pesticides, phthalates, herbicides, pest repellents and disinfectants.
Chemical by chemical, the report documents the average concentration
of contaminants in the bodies of the people studied. But what
does all this data mean? At what concentration do these chemicals
become dangerous? For all but a handful of chemicals, nobody
knows the answer to this question. The report says so clearly,
in one understated sentence:
Research studies,
separate from the Report, are required to determine which
blood or urine levels are safe and which cause disease.
This is not easy research. The questions are complex. Is
the safe level different for children--who eat and respire
more per pound of body weight--than for adults? What do you
measure to determine safety? How relevant are animal studies
to questions about human health?
These have always been the questions of toxicology, but there
are new questions emerging too. The website associated with
the book Our Stolen Future, which introduced the idea
of endocrine receptors to the general public in 1996, collects
scientific papers and news stories that track discoveries
about the health effects of chemical exposures. The papers
collected there make it clear that we need to add three new
questions to the way we think about safe levels of chemicals.
- Could a given chemical
have health effects a long time after exposure?
For chemicals that interfere with cell signaling systems,
like hormone systems, subtle impacts during early development
can result in illness after a long latency. Traditional
tests for the safety of chemicals look for immediate effects,
not illness emerging years after exposure.
- Has a given chemical been
tested for low-dose effects? Traditionally,
lower and lower doses of a chemical are tested until a concentration
range that has no ill-effects is identified as the safe
dose. All lower doses are usually assumed to be harmless.
But for some chemicals the dose-response relationship is
not so simple. At lower concentrations than the "safe
dose" unexpected effects can appear as a biologically
active chemical "hijacks" cellular processes.
Traditional toxicology may miss this low-dose effect because
it is focused on testing for outright damage by toxic chemicals.
- Is a given chemical safe
when mixed with other chemicals? Most studies
of chemical safety examine the effects of one chemical at
a time, but people are exposed to complex mixtures of contaminants.
New studies--such as one on the impact of a commercially
available weed killer mixture on pregnant mice--suggest
that mixtures of chemicals can have effects that none of
the chemicals have on their own.
No wonder the CDC report can't say much about the safe levels
of the chemicals they measured. Looking for effects from very
low doses, over very long time periods is difficult enough.
Try to do that for all possible real-world combinations of
chemical exposure and the task grows exponentially.
Maybe some day our science will reach a level of sophistication
that can give us solid assurances about chemical safety. But
that's a distant goal, not a current reality. Until then,
we are all walking, breathing experiments in toxicology. Until
then we are all living with risk.
There was one definitive finding of the CDC study. For the
chemicals lead, DDT, PCBs and hexachlorobenzene--all chemicals
that have been banned or strictly regulated in the US--body
burdens have decreased since they were last measured. That's
great news. It means that the pollution of our bodies, like
the pollution of our rivers or our air, is reversible.
A river doesn't come back to life until the pollution is
cut off at the source, and the same will be true for our bodies.
If a lower body burden seems like a sensible, desirable thing,
then we are going to need to limit the chemicals to which
we expose ourselves.
The number of manmade chemicals in our environment is astounding.
The CDC tested for the presence of 116, but the EPA estimates
that at least 80,000 chemicals--690 times the number tested
by the CDC--are produced and used today.
If we keep assuming that all chemicals are harmless until
we uncover the exact doses, combinations, and lag-times that
will make us sick, our bodies are going to remain polluted
for a long, long time. Wouldn't it make more sense to stop
giving chemicals the benefit of the doubt until we are convinced
they are safe?
|
