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Chapter 115

An herbicide is any chemical that regulates the growth of a plant, which 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.

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 and Exposure Database of the American Association of Poison Control Centers describes approximately 10,000 herbicide exposures each year. Over the last 12 years, there were only approximately five deaths per year and 20 patients per year with clinical outcomes categorized as "major" that were attributed to herbicide poisoning (see Chap. 135). Most deaths were due to paraquat and diquat, although more recently glyphosate and phenoxy acid compounds are more commonly implicated. Cases of severe poisoning that required hospitalization usually occurred following intentional self-poisoning. Significant toxicity may also occur with unintentional (eg, storage of a pesticide in food or drink containers) or criminal exposures.

This chapter focuses on the most widely used herbicides and also those associated with significant clinical toxicity. In particular, it discusses risk assessment and the management of patients with a history of acute herbicide poisoning.

History

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 was 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 initially marketed in the early 1960s and was followed by the benzoic acid compound dicamba later that decade. Since then there has been a progressive increase in the use and development of herbicides, which remains an active area of research. Increasing numbers of herbicide formulations are marketed every year including 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 improve 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.

Herbicides are the most widely sold pesticides in the world, accounting for more than 35% of the total world pesticide market and for more than 45% 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 consumed by agriculture, government ...

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