from 50 to 500 mg/kg. Exposure to just one teaspoon to one ounce could be fatal to
a 150-pound person. Pesticide products classified as either ‘‘slightly toxic’’ or ‘‘rela-
tively nontoxic’’ are required to have the signal word CAUTION (PRUDENCIA) on
their labels. Acute oral LD 50 values are greater than 500 mg/kg.^41Pesticide Synergy: The Whole Is Greater Than the Sum of the Parts
(or When 1þ 1 ¼10)A study published inScienceshowed that pesticides may be thousands of times
more potent than previously thought. It demonstrated that, when tested alone, each
of two particular organochlorine pesticides had to be at concentrations on the order
of 100,000 times greater than natural estrogen to cause responses in yeast cells that
reacted to estrogen. Yet the same two organochlorines mixed together required con-
centrations of only between ten and 100 times more than natural estrogen to induce
the same response. Thus, exposure to multiple pesticides may be thousands of times
more potent in mimicking estrogen than was previously thought. Other research
found that 30 percent of apples contain at least three pesticide residues. These find-
ings sent a chill through the EPA, which suddenly faced the possibility that all their
safety tests of single chemicals were suspect.
Considering the diversity of pesticides found in our environment, the increased po-
tency of combined pesticides raises many questions. Does this mean that current tol-
erance levels set for individual pesticide residues are actually far above dangerous
limits when combined? What are the implications for the unborn, young children,
and adolescents? In light of these findings, how should pesticide use be assessed?
How should new chemicals be evaluated? In addition, what are the implications for
future pesticide policy formulation?^42
It appears, therefore, that synergistic effects between multiple pesticides and/or
other chemicals represent one of the greatest gaps in the EPA’s ability to protect the
public from adverse health effects associated with pesticide use and exposure. The
government recognizes that pesticide exposures occur in combinations and as unique
events, yet its rules and regulations test only a limited number of possible interac-
tions. Given that there are more than 875 active ingredients currently registered for
use, it would be impossible to examine all possible combinations, but we must start
somewhere. One approach would be to prioritize analyzing pesticides most likely to
act in combination. This would include testing groups of pesticides that are fre-
quently used on the same crops, such as atrazine and chlorpyrifos, which are among
the most common herbicides and insecticides applied to corn.^43Pest ResistanceGenetic variations within pest populations leave some naturally resistant to pesti-
cides. If pests have not been previously exposed to a new pesticide, most individual
pests in the population are susceptible, but some individuals are resistant. Pesticides
used to control the pest will kill most susceptible individuals, but the few resistant46 | Pesticides