ANAHEIM, CalifAlthough toxicology and epidemiology have
both contributed importantly to our understanding of cancer hazards,
researchers have now gone as far as we can go using each
discipline separately, Christopher Schonwalder, PhD, said at a
symposium on mixed environmental hazards and cancer at the annual
meeting of the American Association for the Advancement of Science
Dr. Schonwalder, director of the Office of International Programs at
the National Institute of Environmental Health Sciences Office
of Hazard Assessment, Research Triangle Park, North Carolina, said
that each of these disciplines has been useful, but that further
progress now demands that researchers bring them together.
Billions Spent on Compliance
The nation now spends $150 to $400 billion each year on compliance
with environmental laws, Dr. Schonwalder said, but we know
were not getting it right. Current policies overcontrol
some hazards, accruing unnecessary costs, but undercontrol others,
permitting needless disease.
Congress expects science to determine what is safe, he said, and the
law requires action based on scientific findings. Existing scientific
knowledge, however, does not allow a more precise or accurate
application of environmental controls.
Safe is subjective, he said, and important
public policy decisions now must be made on the basis of limited
information. In the absence of better knowledge, he said,
policy makers are forced to use defaults or to apply the
precautionary principle, which can result in tremendous overcontrol
of certain factors, while some important hazards are undoubtedly
Better Information Required
With one-third of cancers now attributed to known environmental
factors, one-third considered environmentally related, and one-third
attributed to genetics or unknown factors, sound public health policy
will require better information, Dr. Schonwalder said. Both data and
understanding are needed to reach the right balance, and that will
require a more comprehensive approach to the study of hazards than
has been used in the past, he said.
The public health approach to hazards emphasizes prevention in
populations and depends on measures of effectiveness, which are
difficult to devise for cancer, he said. Public health approaches to
existing problems seek to identify the greatest risks and to discern
problems and solutions in context.
Potential problems, on the other hand, such as chemicals newly
introduced into the environment, require a bottom-up approach to
hazard analysis, beginning with toxicology, proceeding to an
understanding of the cause and effect of disease processes, and
culminating in a determination of whether the new exposure will lead
to health problems in the future.
This approach, he noted, allows better predictions. Twenty years ago,
toxicologists knew very little about the mechanisms that turn
exposure into disease. But, more recently, considerable progress has
been made in understanding a number of specific hazards.
Role of Epidemiology
Epidemiology provides different but important elements of the hazard
equation, Dr. Schonwalder said. An observational science, it
identifies risk factors by studying the occurrence of diseases in
populations, as opposed to toxicology, a lab science that measures
dose-response interactions within individuals.
Although useful in isolating potential hazards, epidemiology is not
sufficient to understand the hazard-disease relationship, he said,
because it often provides relatively weak associations and
conflicting results, while being sensitive to large confounders.
Exposure assessment, which is often technically difficult or
impossible to do accurately, forms a very weak link in many
The use of biomarkers in epidemiologic studies, he suggested, could
solve this problem.