Preliminary Assessments of Sites Contaminated with Obsolete Pesticides in Khatlon region, Tajikistan in 2023

DDT – General Information

The term DDT refers to p,p’-dichlorodiphenyltrichloroethane.  DDT, prepared by the reaction of chloral with chlorobenzene in the presence of sulfuric acid, was first made in 1874; and its insecticidal properties were discovered in 1939 by a Swiss chemist, Paul Hermann Müller.  DDT (dichloro-diphenyl-trichloroethane) is considered one of the first of the modern synthetic insecticides.  

DDT is applied as a dust or by spraying its aqueous suspension. The term DDT is also applied to commercial products consisting predominantly of p,p‘-DDT, but also containing smaller amounts of other compounds. All DDT isomers are tasteless, almost odorless solids.  

Major uses of DDT

DDT is a non-systemic contact insecticide with a broad spectrum of activity.  Growers used DDT on a variety of food crops in the United States and worldwide.  Some of the crops were beans, cotton, soybeans, sweet potatoes, peanuts, cabbage, tomatoes, cauliflower, brussel sprouts, corn, and other crops.  DDT was also used in buildings for pest control.  

It was banned in several countries in the early 1970s because of ecological considerations, and many other countries have more recently restricted or banned its use except when it is needed for the protection of human health.  Despite a ban on sales, organochlorines may still be found in storage in many countries such as Tajikistan; thus, exposure is still possible.  DDT is still used in some countries for the control of vectors that transmit yellow fever, sleeping sickness, typhus, malaria and other insect-transmitted diseases. 

DDT Persistence

DDT was designated as a persistent organic pollutant (POP) in 1997 by the Governing Council of the United Nations Environment Programme. Pesticide applicators are exposed primarily to p,p‘-DDT, whereas it is the p,p‘-DDE metabolite to which the general population is exposed in the diet or drinking-water.

DDT and its metabolites are persistent in the environment and resistant to complete degradation by microorganisms, although photochemical degradation does occur.  The persistence of DDT is substantially lower in tropical climates than in temperate ones (a few months compared with years). DDT and its metabolites are readily adsorbed onto sediments and soils, which can act both as sinks and as long-term sources of exposure.  Because of its strong tendency to be adsorbed onto surfaces, most DDT that enters water is and remains firmly attached to soil particles.  If it does find its way into water, it is gradually lost by adsorption onto surfaces.

In soils, DDT is immobile under aerobic conditions with a mean half-life ranging from 2 to 15 years.  DDT is metabolized by microbial systems in soils and is broken down into DDE and DDD.  Significant degradation has been demonstrated in soils under anaerobic conditions, while little or no degradation was observed under aerobic conditions. Biodegradation, however, is highly variable and influenced by the populations of required microorganisms.  Various amendments to soils such as energy and carbon sources, were shown to increase degradation under anaerobic but not aerobic conditions. DDT has been shown to readily degrade in certain flooded soils. DDT is apparently co-metabolized by microorganisms and is not used as a sole carbon source.  Products of biodegradation include DDD and DDE and occasionally DBP (4,4′-dichlorobenzophenone).

The physical and chemical properties of DDT and its metabolites enable these compounds to be taken up readily by organisms from the surrounding medium and from food.  In aquatic organisms, uptake from water is generally more important, whereas food is the major source for terrestrial fauna.  High lipid solubility and low water solubility lead to the retention of DDT and its stable metabolites in fatty tissue.  In general, organisms at higher trophic levels tend to contain more DDT-type compounds than those at lower ones.  These compounds can be transported around the world in the bodies of animals, as well as in ocean and air currents.

In the United States, populations of bald eagles and other raptors crashed when DDT thinned their eggs, killing their embryos.  The pesticide, known for accumulating in food webs and persisting in soil and river sediment, was banned in the United States in 1972.  Studies in animals have also shown that oral exposure to DDT can cause liver cancer.  

DDT is classified as “moderately toxic” by the US National Toxicology Program (NTP) and “moderately hazardous” by WHO, based on the rat oral LD 50 of 113 mg/kg.  Indirect exposure is considered relatively non-toxic for humans.  The International Agency for Research on Cancer (IARC) classified DDT as Group 2A “possibly carcinogenic to humans”.  EPA has determined that DDT, DDE, and DDD are probable human carcinogens as of January, 2015.

Current concerns surrounding DDT are that it is an endocrine disruptor.  Endocrine disruptors are chemicals that can interfere with endocrine (or hormone) systems at certain doses.  These disruptions can cause cancerous tumors, birth defects, and other developmental disorders.  Any system in the body controlled by hormones can be derailed by hormone disruptors.  A wide and varied range of substances are thought to cause endocrine disruption.  Chemicals that are known endocrine disruptors include diethylstilbestrol (the synthetic estrogen DES), dioxin and dioxin-like compounds, polychlorinated biphenyls (PCBs), DDT, and some other pesticides.

Pesticides in Tajikistan

Before the collapse of the Soviet Union, pesticides and other chemicals were actively used in Tajikistan’s agriculture. The application of different pesticides was often done without proper adherence to the existing rules and standards and without consideration of the climatic conditions in different areas. 

The range of pesticides used in Tajikistan included the following POPs: aldrin, dieldrin, heptachlorine, endrin, hexachlorinebenzene, toxaphene, chlordane, DDT, endosulphane, and lindane. On average, about 14 thousand tons of pesticides were brought to Tajikistan every year. The share of DDT constituted from 33 to 80% of the total volume of pesticides. About 90 thousand tons of DDT were brought to Tajikistan during the period of active use of this insecticide. 

In 1970, by the decree of the Minister of Health of the Soviet Union, application of DDT was banned in agriculture and, in 1987. DDT was banned for use to control vector-borne diseases. In the 1980s and through the beginning of 1990s, the use of other pesticides-POPs was also banned. Despite the ban, the remaining pesticides were still used, though in much smaller volumes. 

Currently, the threat of obsolete pesticides to the health of people and the environment in Tajikistan still exists. During the Soviet time, the amount of pesticides brought to Tajikistan exceeded actual application needs by 1.5-2 times. As a result, excessive volumes of pesticides were accumulating in multiple storages and distribution centers of “Tajikselkhozkhimiya” Republican Service. Because there were so large volumes of unused pesticides, various violations occurred: pesticides were distributed to individuals for uncontrolled use in their gardens, some pesticides were dumped in municipal landfills or secretly buried. In the 2000s and because of small supplies of pesticides to Tajikistan, the existing volumes of obsolete pesticides were used again in agriculture. People particularly preferred using DDT and excavated the waste chemical from the Vakhsh polygon, where more than 9 thousand tons of DDT were buried. The customs of Tajikistan registered some cases of illegal import of DDT, including the case of bringing 7 tonnes of DDT from Uzbekistan. 

Today, agriculture is an important part of Tajikistan economy, generating about 24.2% of GDP (2015). The total area of land used for growing various crops exceeds 900 thousand hectares. Much attention was also paid to the agricultural sector while Tajikistan was part of the USSR. In Tajikistan there were significant areas of agricultural land for growing crops, especially cotton, and millions of rubles were spent on building infrastructure, as well as providing the industry with agrochemicals and pesticides. The development of agriculture, and, above all, cotton growing, in Tajikistan was closely linked to the widespread use of pesticides to control agricultural pests, plant diseases, and weeds.