The dose and extent to which non-target organisms are exposed to pesticides is associated with the release, distribution and fate of these compounds in the environment. Selectivity, in addition to being conferred by the mode and biochemical site of action of pesticides, may also be influenced by the delivery of the compound to the target organism. The most desirable method of delivery is directly to the problem pest, disease or weed; here entry to the environment may be localised and effects on non-target species kept to a minimum. Examples of such targetted applications mainly come from human health and veterinary practice, where individual patients or animals may be treated. In such situations, cast skin or hair, washings and excreta may be the only source of pesticides entering the environment.
Careful positioning of pesticides may ensure selectivity to the target organisms. Application of compounds to containers in which high-value ornamental plant species are growing is one such example. Closed systems of growing with recycling of irrigation water are becoming accepted in glasshouse complexes in Europe, and pesticides applied in the irrigation water of hydroponically raised crops are not likely to enter the external environment. Rodent pests in catering and domestic premises, farms and elsewhere are usually controlled by careful positioning of poisoned baits, such that these are not accessible to non-target species. However, careless placement of these baits, many of which have a high mammalian toxicity, has on occasions resulted in deaths to pets and domesticated stock.