Dichlorodiphenyltrichloroethane (DDT) was first synthesized in 1874 and its insecticidal properties were discovered around 1939. It belongs to a group of chemicals referred to as organochlorine or chlorinated hydrocarbon. The chemical has since been used as an insecticide for a long time. It is crystalline, colorless, has a weak odor and, like other organochlorines, it’s hydrophobic with good solubility in most organic solvents, fats, and oils (lipophilic). The bio-magnification of DDT is attributed to its lipophilic property. It was widely used successfully used in the control typhus and malaria during the second half of the Second World War. The chemical was widely used as an agricultural insecticide after the war. However the environmental demerits of DDT was brought to light in 1962 with concerns that some pesticides, including DDT, could cause cancer and be injurious to non-target organisms such as birds. Subsequently the use of DDT in the US was banned in 1972 and later the international community banned its use in agriculture after the Stockholm convention. Albeit the ban; DDT continues to be in malaria-endemic countries for indoor residual house spraying with the approval of the World Health Organization (WHO). WHO approval is based on the fact that DDT has a longer residual efficacy (6-12 months depending on dosage and nature of substrate) once sprayed on walls and ceilings. Since DDT has a high chemical stability, it accumulates in the environment through food chains and in tissues of exposed organisms, including people living in treated houses. DDT showed a biomagnification factor of more than 200,000 times. DDT is not biodegradable but is gradually broken down to DDE and DDD. This paper evaluates the position statement of WHO on the use of DDT for indoor residual house spraying.
2.0 Toxicology Evaluation of DDT
In order to determine whether the use of DDT for malaria vector control should be continued several factors and toxicological data needs to be considered. The major consideration would be negative health impact (both short term and long term) of its use. Data from this could be collected from controlled animal model studies. The interaction of DDT with other chemicals that would exacerbate its toxicity should also be considered as well other factors that aggravate the toxicity such as sex, diet, lifestyle, state of health etc. Data on the biodegradability, bio-magnification and bioaccumulation rates and levels of DDT could also be important in determining if the chemical should continue to be used.
3.0 Evaluating the toxicity of DDT based on the four phases of Disposition
There are several factors that verify the potential of DDT usage is noxious to humans and the environment (non-target organism) based on the four disposition phases as outlined below.
Absorption: this refers to the ability of a chemical agent to enter the body (orally, the skin or other routes). DDT enters the body mainly through eating contaminated foods but small amounts enter through inhalation. DDT is not easily absorbed through the skin
Distribution; refers to the process through which a chemical compound moves from the point of entry to the entire system. Once in the body, DDT, is absorbed through the GIT into the circulatory system through which it is distributed. The little DDT that enters the body through inhalation is attached to large particles and travels up the air passage through mucus and is swallowed. Being lipophilic DDT and its metabolites are mostly stored in the fatty tissues in which they remain at the same level or increase gradually with increased exposure.
Metabolism: refers to the fate of a chemical once inside the body. As earlier stated DDT metabolized to DDD or DDE which are further broken down to other metabolites.
Excretion; refers to the elimination of toxicant from the body. DDT metabolites are eliminated from the body mainly through urine or through breast milk of lactating mothers.
3.0 Exposure and responses
As earlier stated DDT can get into the body through swallowing contaminated food or through inhalation. There two major types of exposure: long term and short term exposure. The response to DDT exposure depends on the concentration of active compound at the target site over time. DDT and its metabolites mainly affect the nervous system leading to nervousness and hyperactivity. The effect of DDT on the nervous system is associated with seizures, tremors, sweating, vomiting, sweating, nausea, dizziness and headache. It also affects the endocrine system particularly interfering with calcium metabolizing hormones. In pregnant women exposure to DDT can result in Pre mature births and low weight births. DDT is also presumed to be a possible human carcinogen and a risk factor of breast cancer and there is data from animal studies showing that it causes liver cancer.
Based on the possible negative health implication of the use of DDT I would recommend the use of DDT be stopped. The proponents of the use of DDT may argue that when inhaled in small quantities it has no or little harm but the chemical bio-accumulate resulting in long term injury. In addition there are safer and equally effective insecticides such as the pyrethrum-based insecticides. More efforts should be directed towards development of safer vector control methods and effective antimalarial drugs and vaccines.There is also the ecological measures of draining stagnant waters and clearing of bushes.
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WHO. (2007). The use of DDT in Malaria vactor control:WHO Position statement. Gineva: WHO press.
Wright, D. A., & Welbourn, P. (2002). Environmental Toxicology (Cambridge Environmental Chemistry Series) . cambridge: Cambridge University Press.