Pesticides
The term ‘pesticide’ encompasses a wide variety of substances used to destroy unwanted life forms. More than a thousand pesticides are available and widely used in the world today. In addition, several hundred compounds that are no longer manufactured or marketed for crop protection use still remain in people’s houses. Both sources play an important role in clinical and forensic toxicology as causes of suicidal, homicidal and accidental poisonings. Pesticides are applied in agriculture for crop protection and pest control, and in human and animal hygiene. The Compendium of Pesticide Common Names, a website listing information on pesticides, lists twenty-three classes based on their field of use. Examples include insecticides, herbicides, rodenticides, fungicides, nemato cides, molluscicides and acaricides. Most pesticides have common names agreed by the International Organization for Standardization (ISO). These common names are used throughout this chapter for convenience and brevity, but their equivalent systematic chemical names can be ascertained easily according to the rules of the International Union of Pure and Applied Chemistry (IUPAC) and the Chemical Abstracts Service Registry Number (CAS RN; O’Neil et al. 2006; Tomlin 2006). Pesticides are also referred to in terms of chemical class. For example, substances used as insecticides include organophosphorus (OP)compounds, carbamates, chlorinated hydrocarbons, pyrethroids, organotin compounds and heterocyclic compounds. Some chemical classes have several uses. For example, OP compounds can be used as acari cides, insecticides, nematocides, fungicides, herbicides and rodenticides. Often the type of chemical is also indicated by a stem in the common name (e.g. ‘uron’ for ureas, and ‘carb’ for carbamates). Commercial formulations can be mixtures of pesticides from different classes, thus complicating the issue even further. The major classes of pesticides are shown in Table 4.4 together with the major chemical groups associated with these classes and examples of substances. It should be recognised that the information contained in this table is a very simplistic overview of all the classes, associated chemical groups and substances that fall within the category of pesticides, and that a far wider variety of chemicals could be encountered in clinical and forensic toxicology. Structures of various pesticides are given in Figure 1. Table 4.5 shows data on pesticide poisonings in various countries. While the data in Table 4.5 should not be taken as applicable to all countries, it does illustrate the fact that the majority of pesticide poisonings arise from a few chemical groups including OP compounds, carbamates and herbicides such as paraquat and 2,4-D. It also shows that a wide variety of pesticides from other chemical groups also feature in poisonings. Hence the clinician must be ready to spot symptoms of poisoning by a wide variety of pesticides and the toxicologist should have analytical methodologies in place to identify these poisons. The data also show that certain pesticides are more commonly encountered in some locations than others, presumably reflecting the agricultural crops grown in an area, the infestations which affect those crops, the preference for certain pesticides together with other factors such as pesticide availability, cost, legal restrictions, etc.
Figure 1 Structures of pesticides: 1, general structure for organophosphates (see Table 4.4) where R1=alkyl, O-alkyl or S-alkyl, R2 O-alkyl, R3 =alkyl, aryl, O-alkyl, O-aryl, S-alkyl, S-aryl, amine, X O or S; 2, general structure for carbamates where R1=methyl, R2=H or methyl, and R3=aryl, heterocyclic or oxime groups; 3, aldicarb (an oxime carbamate insecticide, acaricide and nematocide); 4, endosulfan (a chlorinated cyclodiene insecticide and an organochlorine acaricide); 5, pyrethrin II (a natural pyrethroid insecticide); 6, deltamethrin (a synthetic pyrethroid insecticide); 7, diflubenzuron (a substituted urea insecticide); 8, cyhexatin (an organotin acaricide); 9, dazomet (a cyclic dithiocarbamate herbicide and fungicide); 10, 2,4-D (a chlorinated phenoxyacetic acid herbicide); 11, metobromuron (a substituted urea herbicide); 12, atrazine (a triazine herbicide); 13, lenacil (a uracil herbicide); 14, paraquat (a quaternary ammonium herbicide); 15, tri-allate (a thiocarbamate herbicide); 16, thiram (a dithiocarbamate fungicide).
