Extension Ag Update
January/February 2001
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Pesticide Risk Assessment

Determining how likely a person is to be harmed by a pesticide is called risk assessment. Because it isn’t 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 people’s 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.