Pesticide Risk Assessment
Determining how likely a person is to be harmed by a pesticide
is called risk assessment. Because it isnt possible to use
humans in toxicity studies, determining the likeliness of harm
occurring is an indirect process. Toxicity is determined by exposing
other mammals, such as rats and mice, to a pesticide and observing
the toxic effects. These dosages are measured in milligrams of
pesticide per kilogram of test animal body weight. This measurement
is the same as parts of pesticide per million parts of test animal
body weight, parts per million (ppm). This ppm methodology allows
data obtained by using small mammals to be more meaningful when
applied to large mammals such as humans.
The highest dose that will cause adverse effects, but not death,
is called the Maximum Tolerated Dose (MTD). The highest pesticide
dose that does not cause any observable harm or side effects to
experimental animals is called the No Observable Effect Level
(NOEL). This NOEL is typically divided by a safety factor of 100
or more to obtain the Reference Dose (RFD). The RFD is the toxicity
level normally used in estimating the human toxicity of a pesticide.
The safety factor allows leeway for very sensitive individuals
and because the data has been extrapolated from animal studies.
The RFD is the amount of a pesticide residue that could be ingested
daily over a 70-year lifetime without any ill effects.
Acute effects, those occurring after a short exposure, are more
easily determined than are chronic effects. Chronic effects are
those that may occur over a human lifetime of exposure. To determine
the chronic effects of a pesticide, it is assumed that exposure
to large amounts of pesticide over a short time period is equivalent
to small exposures over a very long time period. Thus, chronic
exposure studies usually involve laboratory mammals that receive
a relatively high dose of pesticide over two or three years. There
is a considerable amount of debate as to whether this comparison
of short-term, high exposure to long-term, low exposure is valid.
However, it is impractical to use chronic studies that involve
years of low-level exposure.
Once a pesticide is on the market, USEPA continues monitoring
health effects from pesticide use to validate the chronic exposure
studies conducted before the pesticide was registered. The health
records of pesticide applicators, as well as those working in
pesticide production facilities, are monitored. However, most
of the pesticides used today were developed after World War II,
with many developed during the last 20 years. Thus, a 70-year
lifetime exposure to these pesticides has yet to occur, making
complete validation of the risk assessment model incomplete in
some peoples minds.
The uncertainty of the causes of various cancers, other tumors,
birth defects, and fertility problems makes pesticides and many
other chemicals suspected causes. It is thought that even relatively
small exposures over a very long time period may cause some of
these problems. However, we are exposed to many similar chemicals
that occur naturally in plants, and we consume them with our food.
In addition to the uncertainty of risk, members of the general
public feel that they have little or no control over their pesticide
exposure. The average person accepts risk much more readily if
they feel that they have some control over that risk. For example,
people accept the risk of having a fatal car crash much more easily
than that of an airplane crash. However, the actual risk of dying
in a car is much higher than in a plane. One of the differences
is that people feel that they have control of the situation when
they are driving the car but have no control of the airplane.
Similarly, many people in this country apply insecticides inside
their own home to control pests but are concerned about the pesticides
applied by farmers and commercial applicators. These people feel
that they have little or no control over what is applied to their
food or to the environment. As one becomes more remote from the
situation, this feeling of lack of control appears to increase.
Thus, a person living in a city tends to be more suspicious of
farm-applied chemicals than does a rural person. Although risk
assessment is based on scientific principles and the uncertainties
are protected with safety factors, it is one of the most debated
principles of pesticide use.