What is a pesticide?

A pesticide is any substance or mixture of substances used to kill or repel unwanted plants, insects, animals, or other organisms. In general, pesticides are categorized by the type of pest they control: insecticides (bug killers), herbicides (weed killers), fungicides (fungus killers), antimicrobials (bacteria and virus killers), rodenticides (mouse and rat poisons), and so forth. Mosquito repellants, wood preservatives formulated to deter termites, and even the flea and tick powders used on pets are also considered pesticides.

A pesticide can also be defined by the type of chemical ingredient it contains. The four most common categories of synthetic chemical pesticides are organophosphates, chlorinated organics, carbamates, and pyrethroids. Biopesticides, another category, include compounds derived from natural sources such as plants, animals, and minerals. Both baking soda and genetically engineered plants fall into this last category.

Why are pesticides used?
Pesticides are used for three primary reasons: to protect public health, to facilitate crop growth, and to safeguard managed forests. Pesticides are frequently used, for example, to prevent a number of illnesses, including malaria and West Nile virus (which are spread by mosquitoes) and river blindness (spread by black flies). The use of chlorine to disinfect drinking water and wastewater is probably the one application of pesticides that most Americans take for granted.

Pesticides have also transformed commercial agriculture. After World War II, synthetic chemicals used to kill mosquitoes and body lice gradually found new applications in the defense of food crops from pests. For better or worse, the use of chemical pesticides has greatly altered traditional farming practices and food processing, and their use has contributed to a worldwide rise in food production.

Trees, for their part, are subject to a variety of threats from pests, mainly insects. Pesticides are used to stem the loss of forests, which have numerous (and obvious) economic, environmental, and recreational benefits.

Are all pests really pests?
It may seem as if we'd all be better off without the ants and bees that spoil our outdoor fun, or the termites that eat away at our homes. But the truth is that so-called pest organisms-including insects, rodents, and microorganisms-often have a significant beneficial role to play in the ecosystem they inhabit. Many insects serve as food for other insects, birds, and other wildlife; termites help break down the wood of fallen trees; bees are needed to pollinate flowers. In other words, what might be narrowly defined as a pest to humans is actually of great use to nature in a larger sense.

How do pesticides work?
Pesticides act in a variety of ways, but for the most part they disrupt some key biological process, resulting in the death of the intended target organism. For example, many herbicides interfere with a weed's ability to photosynthesize sunlight into energy. Similarly, organophosphates and carbamates interfere with the production of a crucial enzyme that regulates acetylcholine, a vital chemical that functions as a signaling molecule in the nervous systems of insects and animals. Many species (including humans) use acetylcholine to regulate essential functions like muscle control and thus are potentially susceptible to the effects of these types of pesticides. Some organochlorines work by over stimulating the nervous system, while others hamper a cell's ability to convert food into energy. Pyrethrins, which are derived from a naturally occurring chemical found in chrysanthemum plants, and their synthetic chemical cousins the pyrethroids, also interfere with the nervous system of insects.

How can pesticides harm the environment?
Anyone who has ever sprayed insecticide to kill the aphids feasting on her tomato plants or spritzed some weed killer on his lawn knows that pesticides don't zero in on an intended target; rather, they blanket the entire area where they are applied. This has the unintended effect of exposing other organisms to the poison. Pesticides may inadvertently harm beneficial organisms, including bees, worms, and insects such as ladybugs, which eat pest insects. The unintentional death or sickening of non-target wildlife is the primary way in which pesticides harm the environment.

Wind and water can also spread pesticides far beyond the area where they are applied, thereby increasing the chance of contact with people and wildlife. In fact, the U.S. Geological Survey reports that low levels of pesticides have been present in the nation's streams, rivers, and lakes for decades because of runoff after rainfalls. Depending on the amount of rain, concentrations of pesticides in natural water can increase seasonally during those times when pesticides are applied to farmlands.

One of most famous examples of the inadvertent harm caused by pesticides involves the use of the organochlorine pesticide DDT in the 1950s and '60s. Use of this environmental toxin was strongly associated with declining fish and bird populations. For this reason, and because of the fact that DDT persists in the environment for many years or even decades after use, it was banned in the United States and many other (but not all) countries in the early 1970s.

How might you be exposed to pesticides?
Ingestion, inhalation, and absorption are the three main routes through which people can be exposed to pesticides. Ingestion occurs primarily when pesticide residues remain on fruits and vegetables or when contaminated meat, fish, or drinking water is consumed. Inhalation of pesticides is possible anywhere they are sprayed, such as near farms, as well as in and around schools, office buildings, and the home. The absorption of pesticides may occur when they come into contact with skin.

Do pesticides cause health problems?
Given that more than 1,055 active ingredients are registered as pesticides with the U.S. Environmental Protection Agency (EPA), and that thousands of commercial products contain those ingredients, trying to determine (and explain) the possible impact on human health is tough. Still, it's fair to say that many pesticides do pose some level of health risk to people for the same reasons that make these substances poisonous to pests. Laboratory studies have shown that pesticides can cause short-term problems like eye and skin irritation and longer-term consequences like birth defects, cancer, and possibly even neurological problems.

The risks involved in pesticide exposure depend on the toxicity of the ingredients and the amount of time a person is exposed to the pesticide. People most at risk are those who work in close contact with pesticides, such as farmers and exterminators. Even so, long-term, chronic exposure may have adverse health effects for anyone. Currently, the EPA evaluates pesticides for a wide range of health effects, and lists human health risk assessments for many of these substances on its website. Though recent legislation requires manufacturers to periodically re-register their pesticides with the EPA to ensure that they conform to the latest standards and policies, the applicable time frame is 15 years.

Are children more sensitive to pesticides?
Yes, compared with adults, children are at increased risk when exposed to some pesticides, for a variety of reasons. For starters, children are physically much smaller, so concentrations of pesticides are higher per pound of body weight. Also, the organs and immune systems of infants and young children are still developing and so may not yet offer the same level of protection-or any protection at all-from certain chemicals. Finally, kids spend a lot more time playing outdoors than adults and thus can come into contact with chemically treated lawns or other areas where pesticides are commonly applied.

How can you limit your exposure to pesticides?
The first step is to identify the ways in which you might be exposed to pesticides. Assess your own use of pesticides in and around your home. Do you use bug sprays or lawn and garden pesticides? If so, be sure to read the labels and make certain you are using and storing the products according to the manufacturers' instructions. For example, it may be a good idea to wear gloves or a face mask to minimize exposure. Washing fruits and vegetables before eating them will reduce pesticide residues. You can also purchase organic food products that are grown and processed without the use of synthetic chemicals and fertilizers. Environmentalists also recommend learning about pesticide use in your community. Your local municipal drinking water treatment plant can inform you about any possible contaminants in your water supply (some home water-filtration systems may remove pesticides). If you rely on well water, you should consider having it tested regularly for the presence of various contaminants, including pesticides.

Are there alternatives to synthetic pesticides?
Currently, the most popular movement afoot aimed at reducing reliance on synthetic chemical pesticides in commercial agriculture is a program called Integrated Pest Management (IPM). The goal is to use pest-suppression techniques that have minimal impact on human health and the environment. One way to accomplish this is by using the least toxic approach to pest control first before moving on to more aggressive measures. For example, incorporating biological controls-such as border crops that attract beneficial insects-may be one way to reduce pesticide use. Those interested in reducing the use of pesticides on their lawns and gardens should contact the nearest cooperative extension. Publications that offer information on alternatives to synthetic pesticides are listed in Where to learn more.

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