Summary

In general, there is no pesticide that is completely "safe." However, the elimination of all synthetic pesticides would lead to an increase in the transmission of disease by insects and an increase in the cost of food, both of which would affect human health adversely. Any decision about discontinuing the production and use of a given pesticide must consider whether cheap, safer alternatives are available and, if not, what the consequences are of both action and inaction. The quandary about whether to ban the use of DDT in tropical developing countries is an excellent illustration.

When a new pesticide, or indeed any other synthetic chemical, is about to be introduced into the market, many environmental groups and some government agencies have proposed that we should err on the side of being too cautious and only allow its introduction if there are no signs that problems could arise. They propose that in such situations, to prevent possible harm to the health of humans and other organisms, we should employ what is now known as the precautionary principle. One definition of this principle was given at the 1992 U.N. Conference in Rio on Environment and Development: "Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation." Opponents of the use of this principle point out that it is impossible to anticipate all possible consequences, positive or negative, of introducing a new substance and that consequently we could become frozen into inaction. The best technique for predicting where a given pesticide will ultimately end up in the environment is through the calculations described in Box 10-3.

BOX 10-3

The Environmental Distribution of Pollutants

When a persistent chemical, such as DDT, is released into the environment, we find that later some of it has dissolved in natural bodies of water, some is in the air, some is present in soil and sediments, and some is located in living matter. A constant interchange of the chemical occurs among these various physical phases. It is possible to estimate the amount and concentration of the chemical in each phase once the release of the chemical has stopped and sufficient time has passed that equilibrium among the phases has been achieved. Even when equilibrium conditions are not yet in place, it is of value to determine the phases where the chemical will ultimately be concentrated.

Recall from your previous background in chemistry that in calculations involving substances participating in chemical reactions, we algebraically combine experimental values of equilibrium constants with information concerning initial concentrations in order to determine equilibrium concentrations. A somewhat analogous procedure can be applied to determine the distribution of a substance when by physical processes it has achieved equilibrium between several phases. The condition that equilibrium has been achieved in its distribution is that the fugacity, /, of the substance, which is defined as its tendency to escape from a given physical phase, is equal for all phases. Fugacity has units of pressure, e.g., atmospheres or kilopascals. Thus, for example, when all the DDT in the environment has distributed itself among air, water, sediment, biota, etc., the concentrations in each phase are such that its tendency to escape from any phase (and enter any other) has the same value for all phases.

As you might expect, the fugacity of a substance in a given phase is proportional to its concentration, C, in that phase:

Continue reading here: Fx cxzx

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