Pyrethrins and Pyrethroids 2. Relevance to Public Health 2.1 Background and Environmental Exposures to Pyrethrins and Pyrethroids in the United States

Pyrethrum is the natural extract that occurs in the flowers of Chrysanthemum cinerariaefolium and Chrysanthemum cineum. Pyrethrum has long been recognized as possessing insecticidal properties; over the years, first the chemical extracts of pyrethrum, and then more recently, the specific synthetic chemical analogs have been produced. The six active insecticidal compounds of pyrethrum are called pyrethrins. The six individual pyrethrins are pyrethrin I, pyrethrin II, cinerin I, cinerin II, jasmolin I, and jasmolin II. The pyrethroids are synthetic analogs and derivatives of the original pyrethrins and represent a diverse group of over 1,000 powerful insecticides. Although they are based on the chemical structure and biological activity of the pyrethrins, the development of synthetic pyrethroids has involved extensive chemical modifications that make these compounds more toxic and less degradable in the environment. A list of the pyrethrins and some of the more common pyrethroids is presented in Table 2-1. While many pyrethroids have been developed, only about a dozen or so are frequently used in the United States. The individual pyrethroids are typically grouped into two general classes, called Type I and Type II, based on a combination of toxicological and physical properties, which is further described in Chapters 3 and 4. Also, the individual pyrethroid substances, due to a complex chemical structure, are often composed of two, four, or eight isomers, and their commercially manufactured products routinely contain a mixture of these various isomers. Thus, the production of individual pyrethroids with slightly varying isomeric ratios can often be the reason for the differences in the reported toxicities of the same compound. See Chapter 4 for the structures and explanation of the isomerism, as well as more information on the chemical and physical properties of the pyrethrins and pyrethroids.