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chemistry of pesticides, including characteristics and their effects on the environment. What are pesticides? They are substances, including chemical, biological, antimicrobial, or disinfectant, or some type of mixtures of these substances, that are used to control pests like weeds, insects, microbes, and other pests. The word itself comes from the Latin word "cida," which means "killer" (1). For most people, the word "pesticide" really means some kind of poison used to control indoor and outdoor pests like insects. The use of pesticides really took off around the world after World War II ended in 1945, but humans have used organic substances to control pests for thousands of years, such as arsenic, sulfur, and other compounds. By the late nineteenth century, many compounds were commonly used on farms and in cities, and inorganic or synthetic pesticides began to develop in the early twentieth century. One of the most famous of those is dichlorodiphenyltrichloroethane (DDT), which a Swiss scientist named Paul Muller created in 1939. At first, DDT seemed like a miracle chemical, because it was not water soluble, it did not need to be reapplied often, and it was toxic to insects and pests but did not appear to be harmful to mammals (2). However, DDT became one of the most controversial pesticides, and it was used heavily around the world.
There are several different types of pesticides, and their toxic effects and mode of action all differ. First are the Organophosphate pesticides, which are compounds that act on the enzyme acetylcholinesterase. They contain phosphorous, and they act on the nerve actions of the insect. They attack the enzyme, destroying nerve function, which is necessary for life. Eventually, the pest dies from respiratory failure. One example of an Organophosphate pesticide is Malathion, which is used to control mosquito and Mediterranean fruit fly.
Carbamates are another type of pesticides that cause cholinesterase inhibition poisoning by affecting the same enzyme acetylcholinesterase that Organophosphates effect. However, they inhibit the enzyme, rather than attack it, and they are less powerful than the previous group of chemicals. It is interesting to note that Carbamate compounds also exist in polyurethanes, although in far fewer numbers. One type of Carbamate is Sevin, used in agriculture and other applications around the world. It is also banned in several countries because it is highly toxic to a number of species.
Organochlorines are pesticides that include chlorine as an ingredient, along with carbon and hydrogen. These pesticides attack the neurons by opening sodium ion channels in them, which causes them to fire continually, which causes spasms and then death. One of the most well-known of the Organochlorines is DDT, which has been found to be extremely toxic to humans and other animals besides pests, and is known to linger in the environment for years, leading to continuing poisoning of other animals, which led to its ban in the United States in 1972. Many other Organochlorines have been banned, as well.
Many pesticides have a dynamic and often detrimental effect on the environment. A University Agricultural Extension handout notes, "Once a pesticide is introduced into the environment, whether through an application, a disposal or a spill, it is influenced by many processes. These processes determine a pesticide's persistence and movement, if any, and its ultimate fate" (3). This "pesticide fate" has been highly studied, and many pesticides, such as DDT, have been shown to be highly toxic and linger in the environment for years on end. There are three major fate processes of pesticides in the environment: adsorption, transfer, and degradation. Some of the fate processes can be beneficial, such as moving (adsorption) a pesticide into the soil to control certain weeds or pests, while others can be extremely harmful. In addition, considerations like how long the pesticide lasts in the environment can have harmful effects for other plants and animals in the environment.
In the terrestrial environment, pesticides can cause a number of effects. Some pesticides linger in the soil, and can leach into the groundwater. First, pesticides and their residue can adsorb into the soil where pesticides are applied. The handout continues, "Positively charged pesticide molecules, for example, are attracted to and can bind to negatively charged clay particles" (3). Wet soils also adsorb less pesticides, and different pesticides adsorb differently. Many pesticides dissipate quickly, which makes them less harmful but also less effective in controlling pests. Pesticides that linger too long can "transfer" to other plants or animals in the soil, as well. The handout notes, "Injury can result when a pesticide used for one crop is later released from the soil particles in amounts great enough to cause injury to a sensitive rotational crop. This pesticide 'carry-over; can also lead to the presence of illegal residues on rotational food or feed crops" (3). Pesticides can also transfer to other, non-targets, such as animals, humans, and other crops through a variety of ways, from blowing in the wind (volatilization) to leaching through the soil into other plants or the groundwater. This can lead to a variety of problems, from adding chemicals and toxins to water humans consume, to creating soils that are too toxic to use for agriculture or other uses.
In the aquatic environment, pesticides can spread downriver into rivers, streams, and eventually into the sea. Some of them can enter animals like fish and birds who feed off animals that live in the rivers and streams and come in contact with pesticide residues. This is what happened with DDT, and why residues from DDT were found in whales far out at see, and in areas of the world that never used DDT directly.
Pesticides can have very detrimental effects on the human population. Another author notes, "Misuse and overexposure to pesticides have been shown to be harmful to humans. Children appear to be especially susceptible to the effects of pesticides, and misuse or use at greater-than-recommended levels can lead to exposures that can cause serious acute or chronic illness or even death" (4). One reason they can be so harmful is that they can lead to neurological and organ damage that takes time to show up. Children are at a greater risk because their young bodies are still developing, and they tend to come in more physical contact with areas where pesticides may have been used, such as playing on the floor, and putting toys into their mouths (4).
In addition, even if pesticides are not used inside the home, they can transfer into the home by many other ways. Another writer notes, "Engineered to be absorbed by, or adsorbed onto, their targets and to provide lasting effects, today's outdoor pesticides can find their way indoors on shoes, pets, and airborne dust" (5). Thus, pesticides used outside, or even in other areas, can still show up in the home, and can still have an adverse effect on the human population. For example, studies indicate that certain pesticides can congregate in certain organs or areas of the body, and can even be present in mother's breast milk. Researchers note, "Our study suggests an association between congenital cryptorchidism and some persistent organochlorine pesticides present in mothers' breast milk" (6). Other studies show links to caner. Another author notes, "In the United States, epidemiologic studies of farmers, farm workers, and pesticides applicators suggested a possible link between pesticides exposure and cancer" (8). There are also studies that indicate pesticides can cause complications in Parkinson's disease patients and others, and this is just the residue from pesticides, often it does not even require direct contact with the pesticides for the health concerns to occur.
One of the first synthetic pesticides, DDT was in wide use by the 1960s, and then studies began to show real problems with the pesticide. As noted, it was discovered in 1939, and the scientist who discovered it won a Nobel Prize for the discovery in 1949. It was seen as a real wonder chemical, and it was used around the world for all types of pesticide applications. World War II soldiers were even de-loused with it. An EPA Web site notes, "It was initially used with great effect to combat malaria, typhus, and the other insect-borne human diseases among both military and civilian populations and for insect control in crop and livestock production, institutions, homes, and gardens" (7). In the late 1950s and early 1960s, studies began to show that DDT might not be as harmless as people believed, and the U.S. Department of Agriculture (prior to the EPA taking over the job of regulation), began to study the effects of the chemical on animals and humans. Then, in 1962, Rachael Carson's book "The Silent Spring" warned about the dangers of pesticides and the declining bird and animal populations, and the public became much more concerned.
In 1972, the EPA issued a cancellation order that banned DDT, and their studies show that it can transfer great distances in the upper atmosphere, it can accumulate in body tissues that are fatty, such as the liver,…[continue]
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