Physics & Chemistry

DDT

 

caption Chemical composition of DDT.

DDT is the abbreviation for the chemical 1,1,1-trichloro-2,2-bis-(p-chlorophenyl) ethane. DDT is used as an insecticide. It has the appearance of colorless needles or of a white to slightly off-white powder. It is insoluble in water and is slightly soluble in alcohol. Since it had not been shown to be particularly lethal to humans, it was used extensively in Word War II to reduce and manage mosquito populations and to control malaria for the protection of U.S. troops. Also, it was used on civilian populations in Europe to prevent the spread of lice and the diseases they carried. DDT became popular as the first modern pesticide, hailed as a miraculous advance in pest control. Its developer, Paul Müller of Switzerland, won the Nobel Prize in 1948). Peak usage in the U.S. occurred in 1962 when about 80 million kilograms of DDT were used of about 82 million kilograms produced. Although it is no longer used in the U.S., it is still used in some countries to control mosquitoes that may carry malaria.

The chemical compound DDT is referred to as an organochlorine or a chlorinated hydrocarbon. That is, it is a carbon-based chemical with hydrogen and chlorine atoms attached to the carbon atoms. Organochlorines are relatively insoluble in water; but they are highly soluble in fats or lipids. They are often referred to as lipophilic or fat-loving chemicals. Due to their lipophilic character they bioaccumulate in fat. Additionally, organochlorines tend to be very persistant in soils and sediments. A key study done in 1967 in an estuary on Long Island Sound showed a biomagnification factor for DDT of more than 200,000 times.

Most agricultural chemicals were not used directly in aquatic environments, but entered them through runoff from land. DDT was sprayed, however, extensively in salt marshes to control biting flies and mosquitoes. In the late 1940s and 1950s DDT was considered a panacea, as it could be applied as a powdered dust on the water in relatively small amounts per acre and could keep killing mosquito larvae for many months with a single application. It is a wide spectrum insecticide that could be used to kill all kinds of insects; but it was not very toxic to humans.

DDT enjoyed success until the development of chemically-induced resistance by mosquitoes. Resistance developed in insect populations because all of the insects exposed to DDT were not killed by the chemical. A few resistant individuals remained, they bred, and their offspring proved more resistant to the effects of DDT—this is evolution at work. In following applications, more DDT was sprayed. Eventually, the insects became so resistant that it became impractical to try to control them with DDT. This necessitated substitutions with other chemicals, to which the insects eventually became resistant. Pesticide—and antibiotic—resistance remains a serious problem.

Generally through the action of microorganisms, most DDT breaks down slowly into DDE(1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene)and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane). DDD was also manufactured and used as an insecticide, but to a much lesser extent than DDT. DDE has no commercial use, but is commonly detected along with DDT at concentrations in the environment that often exceed those measured for DDT.

Large amounts of DDT were released into the air and on soil or water when it was sprayed on crops and forests to control mosquitos and other insects. Although the use of DDT is no longer permitted in the U.S., it may be released into the atmosphere in other countries (for example, Mexico and China) where it is still manufactured and used. Also, DDT, DDE and DDD may enter the atmosphere when they evaporate from contaminated water and soil. DDT, DDE, and DDD in the air may then be deposited on land or surface waters. This cycle of evaporation and deposition may be repeated many times. As a result, DDT, DDE, and DDD may be carried long distances in the atmosphere. These chemicals have been found in bogs, snow, and in animals in the polar—Arctic and Antarctic—regions, far from where they have been used. Some DDT may have entered the soil from waste sites. DDT, DDE, and DDD may occur in the atmosphere as a vapor or be attached to solids in air. Also, vapor phase DDT, DDE, and DDD may break down in the atmosphere due to reactions caused by the sun.

DDT, DDE, and DDD last in the soil for a very long time, potentially for hundreds of years. They stick strongly to soil, and therefore generally remain in the surface layers of soil. Some soil particles with attached DDT, DDE, or DDD may get into rivers and lakes through surface runoff. Only a very small amount, if any, appears to seep into the ground and get into groundwater.

In surface waters, DDT will bind to particles in the water, settle, and be deposited in the sediment. DDT is taken up by small organisms and fish in the water. It accumulates to high levels in fish and such marine mammals as seals and whales. In these animals, its concentration can reach levels many thousands of times higher than in water. In these animals, the highest levels of DDT are found in their adipose (or fat) tissue. DDT in soil can also be absorbed by some plants and by the animals or people who eat those crops.

DDT's effects are not specific solely for insects. While it kills flies and mosquitoes, it also kills crabs and other crustaceans, and accumulates in tissues of all marsh animals. DDT acts on the nervous system, causing nervousness and hyperactivity. DDT and its metabolite DDE alter hormones, that affect calcium metabolism and can impact reproduction and development of exposed animals. Birds were major victims of the chemicals' effects. They experience reproductive failure as their eggs lacked sufficient calcium, were extremely thin, and shells broke when sat upon by the nesting adult birds. DDT and its metabolites accumulated to the greatest extent in raptors (that is, carnivorous birds that hunt and kill other animals), birds that eat carrion (that is, dead animals) and fish-eating birds. There were drastic declines in populations of these birds that feed at the tops of food chains. Many carnivorous birds (for example, bald eagles, ospreys, and pelicans) were on the brink of extinction before action was taken at the federal level to ban the use of DDT—and eventually other chlorinated hydrocarbons.

Because of its persistance, DDT remains an important environmental contaminant. The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for long-term federal cleanup activities. DDT, DDE, and DDD have been found in at least 442 of the 1,613 current or former NPL sites. The total number of NPL sites evaluated for these substances is not known. As more sites are evaluated, those at which DDT, DDE, and DDD are found may increase. This information is important as exposure to these substances may be harmful to humans and wildlife.

There are many factors that determine the potential for DDT to cause harm in humans and in wildlife. These factors include the dose (how much), the duration (how long), and how one comes in contact with them. When considering toxicity and absorption of chemical contaminants, one must also consider exposure to other chemicals along with such factors as age, sex, diet, family traits, lifestyle, and state of health.

In her 1962 book, Silent Spring, Rachel Carson sounded the initial alarm against DDT, and it was eventually banned in the U.S and Sweden in the early 1970s. Some observers maintain that DDT was “demagogued” out of use, with environmentalist groups exaggerating its risk and misrepresenting the scientific research as a means to increase their political power.

Further Reading

 

 

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Agency for Toxic Substances and Disease Registry. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Agency for Toxic Substances and Disease Registry should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

Cleveland, C., & Weis, J. (2012). DDT. Retrieved from http://www.eoearth.org/view/article/151631

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