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HISTORY, CLASSIFICATION, AND EPIDEMIOLOGY

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An herbicide, any chemical that regulates the growth of a plant, encompasses a large number of xenobiotics of varying characteristics. Herbicides are used around the world for the destruction of plants in the home environment and also in agriculture where weeds are particularly targeted. Poisoning may occur following acute (intentional or unintentional poisoning) or chronic (such as occupational) exposures. Depending on the herbicide and the characteristics of the exposure, this may lead to clinically significant poisoning, including death. This chapter focuses on the most widely used herbicides and also those associated with significant clinical toxicity. In particular, risk assessment and the management of patients with a history of acute herbicide poisoning are emphasized.

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Prior to the 1940s, the main method of weed control and field clearance was manual labor, which was time consuming and expensive. A range of xenobiotics were tested, including metals and inorganic compounds; however, their efficacy was limited. The first herbicide marketed was 2,4-dichlorphenoxyacetic acid during the 1940s, followed by other phenoxy acid compounds. Paraquat was marketed in the early 1960s, followed by dicamba later that decade. The development of new herbicides is an active area of research and new herbicides and formulations are frequently released into the market. This includes a number of novel structural compounds for which clinical toxicology data are unavailable. Hundreds of xenobiotics are classified as herbicides and a much larger number of commercial preparations are marketed. Some commercial preparations contain more than one herbicide to potentiate plant destruction. From another perspective, crops are being developed that are resistant to particular herbicides to maximize the selective destruction of weeds without reducing crop production.

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Herbicides are the most widely sold pesticides in the world, where in 2007 they accounted for approximately 40% of the total world pesticide market and 48% of the pesticide market in the United States. Home and garden domestic use accounts for 13% of the overall herbicide use in the United States, while the remainder is used in agriculture, government (eg, vegetation control on highways and railways), and industry. In each of the US market sectors, herbicides were four of the top five used pesticides in 2007, including glyphosate, atrazine, acetochlor, and 2,4-dichlorophenoxyacetic acid.33

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Not all herbicide exposures are clinically significant. In developed countries, most acute herbicide exposures are unintentional and the majority of patients do not require admission to hospital. The National Poisoning Database System (NPDS) of the American Association of Poison Control Centers (AAPCC) describes approximately 10,000 herbicide exposures each year. Over the last 12 years, there were approximately five deaths per year and 20 patients per year with clinical outcomes categorized as “major” that were attributed to herbicide poisoning (Chap. 136). Most deaths were due to paraquat and diquat, although recently glyphosate and phenoxy acid compounds are more commonly implicated. Cases of severe poisoning that required hospitalization usually followed intentional self-poisoning. Significant toxicity may also ...

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