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Chemicals: DDT

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DDT (dichlorodiphenyltrichloroethane) is a commercial organochlorine insecticide that has been used in countries around the world. It has been used widely on agricultural crops as well as for "vector control" - the control of insects that carry such diseases as malaria and typhus.[1]

In the 1960s, scientists began to notice that DDT could cause damage to wildlife and had the potential to harm human health. That led to increased international attention to DDT's impact on wildlife and humans, which in turn triggered a number of international restrictions on the pesticide. It also led to increased awareness of the long-term effects of other pesticides and industrial chemicals. Nevertheless, DDT is still used in many parts of the world, and its persistence in the environment poses a danger for ecosystems and human beings.

DDT in the Body

When DDT is applied for agricultural or vector pest control, the parent compound (DDT) is used. Once it is released into the environment, though, it begins to degrade and can be found in two additional forms, DDE (dichlorodiphenylchloroethane) and DDD (dichlorodiphenyldichloroethane). DDE is DDT's main metabolite and also the most persistent form of the chemical. However, DDD also occurs as a breakdown product and in some instances is independently used as a pesticide.

Whether it is used in agriculture or for vector control, once DDT enters the environment, it can remain for many years. DDT can be transported in different ways. In air, DDT degrades very quickly, with breakdown occurring in less than five days. However, people and animals can be directly exposed by air at the time of initial application. DDT is far more persistent in water and soil. In water, it does not easily dissolve, but instead attaches to sediment particles or is broken down by microorganisms into DDE and some DDD. In soil, DDT lasts for a very long time because it binds strongly to soil particles. Once attached, DDT and its byproducts can persist for as long as 15 years. Moreover, when bound to soil particles, DDT can begin to bioaccumulate, building up in plants and in the fatty tissue of the fish, birds, and animals that eat the plants.[1]

The breakdown of DDT into the metabolites DDE and DDD depends on several factors, including climate - temperature and humidity, for example - and the presence in soil and water of microorganisms. Thus, country-by-country differences in DDT metabolite levels may emerge, depending on local geography and climate. In some wet, tropical areas, DDT has been found to have a shorter half-life than in dry regions.[2]

Humans can be exposed to DDT and its metabolites in several ways. The principle route of exposure is the consumption of foods, particularly through leafy and root vegetables, fatty meat, fish, and poultry.[1] The levels of chemicals absorbed in food usually reflect the contamination present in the country of production. Although DDT contamination can occur in a variety of food products, the most serious contamination usually occurs in fish and other organisms high on the food chain that themselves have bioaccumulated DDT. Other less common routes of exposure that are considered minor are breathing contaminated air or drinking contaminated water, especially near waste sites and landfills or in recently treated homes; and breathing or swallowing dust or soil particles near waste sites and landfills or in recently treated homes.[1]

DDT's elimination from the body can take some time; its half-life in humans has been estimated at four years. DDE's half-life is estimated at approximately six years.[3] Because of these varying breakdown rates, the proportion of DDT and DDE detected in human tissues can be used as an indication of the length of time since exposure. In areas where DDT exposure has been recent, the DDE/DDT ratio is low, while in areas where substantial time since exposure has passed, the DDE/DDT value is higher. Because DDE is attracted to fat, levels in breast milk are often six to seven times higher in a mother's milk than in her blood.[4]

Controlling Exposure: Bans and Restrictions

As of 1995, DDT had been banned for all uses in 49 countries and severely restricted in 23.[5] In countries with "restricted" use, DDT has often been designated for application in malaria control, even while limitations or bans on agricultural use may have been put in place. The effectiveness of bans and restrictions differs from country to country depending on enforcement efforts. Thus, relying on the official ban date as an indicator for decreasing use can be misleading. Worse still, DDT is often produced, imported, and/or exported to and from countries despite bans and restrictions that may be in effect. For instance, despite a longstanding ban, the United States exported more than 96 tons of DDT in 1991.[6] A study by the Foundation for Advancement in Science and Education found that banned chemicals are often exported to countries that also have banned the chemical.[6]

Bans and restrictions do not immediately yield decreased detection of DDT in breast milk, because of the chemical's persistence. However, it is clear that banning DDT use can eventually lead to declines in exposure and drops in detectable residue levels. Since exposure can occur through food, and since global food security relies on the import and export of food crops, a worldwide ban is the only real solution to exposure.

The table below is a partial list of countries that have taken steps to reduce or eliminate the use of DDT.

Countries where the use of DDT is banned or restricted[7]
Argentina, Australia, Austria, Bulgaria, Burkina Faso, Canada, Colombia, Costa Rica, Cuba, Cyprus, Denmark, Dominican Republic, Egypt, El Salvador, Ethiopia, Fiji, Finland, Hong Kong, China, Indonesia, Ivory Coast, Japan, Korea, Lebanon, Liechtenstein, Mozambique, New Zealand, Nicaragua, Paraguay, Poland, St. Lucia, Singapore, Sweden, Switzerland, United States, Yemen, Zimbabwe.

Assessing the Extent of DDT Exposure: Limits and Benchmarks

Most chemicals that are either in widespread use or that have caused widespread contamination are subject to national and international benchmark levels, established to protect public health. In the case of DDT, conclusive evidence demonstrating that any one of these benchmarks is protective or superior to the others does not exist. DDT has also been associated with shortened duration of lactation and difficulty producing breast milk, but the benchmark levels may not protect against adverse effects on lactation.[8] Conversely, there are no data linking exposure to DDT via breast milk with specific health outcomes. However, many scientists consider any level of contamination unacceptable and believe that benchmark levels are not protective.

In 1984, the World Health Organization (WHO) established an acceptable daily intake (ADI) level for children's consumption of DDT in milk. This value of 20 micrograms per kilogram per day (µg/kg/day) can be converted into an "acceptable contamination level" of between 5,000-6,000 µg/kg DDT in milk fat.[9]

Most countries' current (1990) average DDT residue level in breast milk is now below this standard. However, a number of countries and particular regions have average levels well above the WHO standard. An examination of data from periods of heavy DDT use in the 1960s and 1970s shows that many countries' levels surpassed the WHO benchmark at one point or another.

Breast-milk Monitoring Studies and DDT

Researchers around the world have published more than 200 values for detectable levels of DDT in human breast milk. Studies have been conducted in the following countries:

AustriaUnited KingdomMexicoSweden
BrazilGuatemalaNew ZealandTajikistan
Canada Hong Kong, ChinaNigeriaThailand
ColombiaIndiaPapua New GuineaTurkmenistan
Costa RicaIranPolandUnited States
Czech RepublicIrelandPortugalUganda
JapanRwandaVietnamEl Salvador
Jordan Saudi ArabiaWalesFinland
KazakhstanSouth AfricaYugoslaviaFrance

Although studies have been conducted in all of these countries, data are not always complete. The following section discusses issues that make it difficult to interpret the data. In addition, countries not on this list may also have detectable levels of DDT residues in breast milk, since the list reflects only those areas where studies have been conducted.

Limitations of Studies Measuring DDT in Breast Milk

It is often difficult to draw conclusions about national and international trends in DDT contamination, because of the many factors affecting measured levels, and because of limitations in the way data are reported. Some of these challenges include:

  • Absence of standardized methodology. No accepted and standardized method for conducting breast-milk monitoring studies has been established. Thus, differences may arise in the sampling time - when the breast milk is collected, or in the birth histories of the mothers - women who have breast-fed multiple children might be mixed in with women who are breastfeeding for the first time.

  • Distinct regional differences in DDT use. Many countries have a clear demarcation between agricultural and urban zones. In such situations, general averages are inadequate because they do not reveal peak levels in areas with heavy agricultural use. The same problem arises in countries where DDT has been used for vector control only in specific regions. Again, average values will not adequately represent the unique exposure situations in these areas.

  • Few studies. Many countries do not have multiple studies over a range of time. Instead, the information on DDT residues may be nothing more than a snapshot of a particular time. Where that is the case, it is difficult to generalize about how the conditions in a country have changed over time.

  • Small study populations. Because of the cost and time involved, many studies measuring DDT levels in breast milk test only a few people. When the only data for a country are from studies with very small sample sizes, it is difficult to draw conclusions about the entire population.

  • Differences in measurement methodology and data reporting. Although many studies look at and report the total concentration of DDT (in all its metabolite forms) in breast milk, some only look at the levels of the metabolite DDE. This complicates data comparisons and may underestimate the effects of recent exposures.

  • Bias. The selection of study participants often presents challenges. In many studies, women may have been chosen to participate based on potentially high exposure to the chemical of interest, thereby driving average values from the study higher.

Some Important Examples of DDT in Breast Milk

Several useful studies on DDT are available.

In general, studies looking at DDT levels in breast milk have shown that as DDT use has declined and changed, detectable levels in breast milk have also declined. Levels are usually much lower in developed nations than in developing countries - not surprising, since many developed nations restricted or banned DDT in the 1970s, while restrictions in the developing world were not common until the 1980s.

Despite the overall trend of decreasing DDT levels in breast milk, ongoing challenges remain in some areas of the world. In some regions, DDT use for malaria vector control has made exposure common, and has led to high levels in breast milk. In other regions, continued use or disregard of bans and restrictions on agricultural use has led to widespread and high-level exposure.

Although measurements of DDT residues in breast milk have been taken in more than 60 countries, only a few nations have comprehensive trend data (multiple studies over time, large study populations, consistent analysis methods). Among those countries with such comprehensive data are some important environmental success stories.[10] After the restriction and ban of DDT in some nations, average breast-milk levels have decreased substantially. One study involving a statistical analysis of trend data from around the world found that the average national levels of DDT found in breast milk directly correlate with the time since DDT restriction.[9] In other words, the longer DDT was restricted, the lower the average levels of DDT in breast milk.

The examples presented here are divided into three types:

  • time trend examples - studies that have looked at average levels of DDT in breast milk in a location over a number of years;
  • differences in average levels among different countries; and
  • comparisons and differences within countries depending on different regional use patterns.

Time Trend Examples

Sweden has excellent data from breast-milk monitoring studies spanning more than 30 years. DDT levels in breast milk continuously declined from 1967 through 1997. The use of DDT was severely restricted in Sweden in 1970 and completely banned in 1975.[3] Figure 5 shows the marked decrease in the average concentrations of DDT found in Swedish women's breast milk.

Figure 5

Germany has also witnessed a rapid decline in average concentrations of DDT in breast milk. Between 1969 and 1995, detectable residue levels decreased 81 percent. DDT was banned in Germany in 1972. However, trend data in Germany is difficult to assess on a national basis because East and West Germany had different use patterns before reunification. [11-16] Figure 6 shows the declining trend of DDT residues in the former West Germany. The decline has been similar in the former eastern state but the data are far less complete. In addition, the average concentrations in Eastern Germany were much higher during the 1970s, with the highest detected residue levels (~11,500 µg/kg DDT in milk fat) recorded in Greifswald, East Germany in 1971.[11]

Figure 6

Other countries where studies have revealed a downward trend include Canada, Denmark, Norway, Switzerland, Turkey, Yugoslavia, Czech Republic, United Kingdom, Hong Kong, China, Israel, India, and Japan.

International Comparisons

It is difficult to make comparisons between countries because relatively few studies were done at the same point in time. But in order to illustrate the vast differences in exposure from country to country, Figure 7 shows average national levels between the period 1974-76 for 21 different countries. It was during this time period that many industrialized nations banned or began to restrict the use of DDT. At about the same time, however, use of DDT in developing nations was peaking. The red line in Figure 7's graph represents the breast-milk concentration that corresponds to the World Health Organization Acceptable daily intake (ADI) for DDT in breast milk.[3, 11, 17-27] This figure illustrates the wide variability between countries, although it has limitations in terms of estimating the true average concentrations of DDT in any single country. For some of the countries included in the table, multiple studies may have been conducted in the same time period. Where multiple studies were conducted, the study with the largest study population was included in the figure. Some of the included studies had very limited study populations but were included because no other data for that country existed. In addition, variations in the type of measurement methods used among studies must be considered.

Figure 7

National Variations In DDT Levels

In the United States, a pooling of all studies done in the last 50 years shows an overall continuous decrease in DDT levels in breast milk. However, distinct differences emerge between different states and regions. Regions of the United States with more intensive agriculture have commonly shown higher levels.

Figure 8 shows the average levels of DDT found in women's breast milk in different regions of the United States in 1975.[28] These regional differences in detectable breast-milk levels probably reflect differences in use patterns. The Southeastern United States includes areas where intensive agriculture led to much greater overall DDT use. In addition, crops with higher intensity of DDT use dominated this region - cotton, for example.

Figure 8

Further variations have been noted in DDT levels measured in breast milk in the United States based on more specific differences in agricultural pesticide use. Figure 9 presents data gathered in Mississippi in the mid-1970s in areas with no known DDT use, as compared to areas with recorded pesticide use.[29] The data show sharply higher levels of DDT in breast milk in areas with ongoing pesticide application.

Figure 9

Data from Australia also has shown variation in the levels of DDT found in breast milk based on use patterns. Figure 10 shows a distinct difference in DDT levels found in women living in rural and urban areas of Australia in 1971.[30]

Figure 10

Levels of DDT found in breast milk are not always higher in rural areas than in urban areas. Figure 11 presents 1989 data from Brazil. This study showed that urban areas had approximately twice the level of contamination as rural populations, likely the result of DDT applications for mosquito abatement in Brazil, particularly in urban areas.[31]

Figure 11

The difference between areas that currently apply DDT and those that have only the residue of past exposures is particularly evident in the data from Zimbabwe presented in Figure 12. DDT was banned for agricultural use in Zimbabwe in 1982,[32] but national averages for DDT in breast milk still show moderately high levels (~6,000 µg/kg DDT in milk fat). Exposure is generally in the form of DDE, indicating that exposure to DDT was not recent. However, the Kariba region of Zimbabwe, the only region that still actively employs DDT for malaria control, shows much higher levels of total DDT residues.[32] The pesticide's use in the Kariba region is mainly in the rainy season via aerial and ground spraying. DDT likely makes its way into rivers and the lake that is the region's main water source and supplier of fish, and humans are exposed when they eat DDT-contaminated fish. [32]

Figure 12

Data from Mexico also reveal regional differences in DDT levels found in breast milk. DDT was first restricted in Mexico in 1972. However, the restrictions were mainly put into place in Northern Mexico, where pressure from the United States was strongest. DDT use continued in the southern part of the country for many years. DDT was finally restricted to vector control for the entire country beginning in 1990. Through this string of regulations, an overall gradual downward trend was seen in Mexico for DDT in breast milk.[27, 33-35]

Despite this downward trend, regional data give cause for concern in those parts of Mexico where DDT continues to be used in malaria control. Figure 13 shows data from a 1998 study[36] that showed distinct differences in breast-milk levels related to DDT use for vector control. The suburban area of Veracruz City employs frequent malaria control, built as it is on swampland. DDT is sprayed at least every six months on indoor surfaces and dwellings. Women from suburban areas of Veracruz have higher levels of DDT in their breast milk than do urban or rural residents of the same area. In addition, the ratio of DDE/DDT is lower in suburban women, suggesting that these women's residue levels stem from recent, direct exposure. In contrast, the women from the urban and rural areas have high DDE/DDT ratios suggesting that their breast-milk levels have risen from historical exposures or from exposures through food.[36]

Figure 13

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32. Chikuni, O., et al. An Evaluation of DDT and DDT Residues in Human Breast Milk in the Kariba Valley of Zimbabwe, Bulletin of Environmental Contamination and Toxicology 1997; 58: p. 776-778.

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34. Waliszewski, S.M., et al. Organochlorine Pesticide Residues in Human Breast Milk from Tropical Areas in Mexico, Bulletin of Environmental Contamination and Toxicology 1996; 57: p. 22-28.

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last revised 5.22.01

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