During recent years there has been much discussion about the risks involved in using these poisonous substances. In order to avoid misunderstanding it is important to differentiate between three main kinds of hazard. There is, first, the risk of acute poisoning. A substance may just be so poisonous that a small dose taken by mistake or as an oversight may prove dangerous. Secondly, there is the risk of chronic poisoning. Some substances are not particularly poisonous in small doses but are only broken down or excreted very slowly, so that a small daily dose may accumulate in the organism and eventually reach a level where it becomes injurious.

Thirdly, the excessive use of substances which break down slowly has given rise to fears that they may accumulate in the soil and in water and have an injurious effect on animal and plant life. Fortunately this problem is not serious in the case of pest control in the home.

The danger of acute poisoning is very small with the substances supplied for indoor use. However with insecticides being used in farm buildings, silos and grain stores, in the food manufacturing industry and in private households there is always a risk that small daily doses will add up to a dangerous level.