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
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