DDT is one of the most, if not the most, studied chemical known to man. DDT has been used in public health programs since the 1940s and for several decades was used very widely in agriculture. In all the years of use and after thousands of studies, no scientific study has been able to pinpoint actual human harm from DDT. The World Health Organisation conducts regular assessments of chemical risks in a program known as the International Programme on Chemical Safety (IPCS). The IPCS provides the most comprehensive review of the scientific literature on DDT. According to the IPCS:
"A 2000 Joint WHO/FAO international assessment is the latest WHO risk assessment of DDT and its metabolites. This assessment documents the wide spectrum of toxicological findings which have been observed in animal studies, including reproductive, developmental and neurotoxicological effects and recommends a provisional tolerable daily intake for human exposure. While biologically plausible, this wide spectrum of toxicological effects have so far not been confirmed by human epidemiological studies. An updated WHO report of the scientific literature relevant to an assessment of human risks will be available for peer review at the end of 2006. In conclusion, and in the meantime there is no reason to change the current WHO position regarding use of DDT for vector control."
This is why WHO renewed DDT's bill of health for malaria control in September, 2006.
Furthermore, according to A.G. Smith of the Lancet, "The early toxicological information on DDT was reassuring; it seemed that acute risks to health were small. If the huge amounts of DDT used are taken into account, the safety record for human beings is extremely good. In the 1940s many people were deliberately exposed to high concentrations of DDT through dusting programs or impregnation of clothes, without any apparent ill effect. In summary, DDT can cause toxicological effects but the effects on human beings at likely exposure are very slight."8
The Allies first used DDT in disease prevention. During World War II, they dusted it on troops and civilians to control the body lice that transmit typhus. They never recorded any adverse human health effects from this use of DDT; on the contrary, the use of DDT quickly controlled the typhus epidemics, and mortality and morbidity dropped dramatically. Indeed wherever public health programs have used DDT, disease incidence has dropped dramatically and people have enjoyed longer, healthier lives.
The International Agency for Research on Cancer classifies DDT as a possible carcinogen. It shares this classification with coffee, vegetables pickled in the Asian style and carageenan, a seaweed extract that is used as a thickener in ice cream, yogurt and puddings. Even during the years of widespread DDT use in agriculture, the carcinogenic risk to which you would have been exposed was 50 times lower than the risk from the known carcinogens in coffee. The average consumption of wine, beer, lettuce, apples, orange juice and potatoes all expose humans to higher carcinogenic risks than exposure to DDT9. During the 60 years of widespread DDT use, researchers have conducted hundreds of scientific studies on its human health effects. In all that time, not once has a researcher been able to affirmatively replicate a case-control study of DDT's human carcinogenicity10.
Some commentators and scientists express concern that DDT acts as an endocrine disruptor, affecting the reproductive capacity of humans and other mammals. While there is the potential for endocrine disruption, there is little evidence of actual human health harm as a result of this disruption. It is important, when discussing endocrine disruption, to link levels of potentially endocrine disrupting compounds with adverse human health effects because the human diet contains naturally occurring endocrine disruptors in fruits and vegetables. Indeed, the effect of naturally occurring endocrine disruptors in foodstuffs such as potatoes, carrots, peas, beans, apples, garlic and coffee is far stronger than the hormonal effect of synthetic chemicals. As Stephen Safe, Distinguished Professor of Toxicology at Texas A&M University and Director of the National Institute of Environmental Health Sciences, explains, "the amount of estrogenic compounds found in a single glass of cabernet wine is 1000 times greater than the estimated daily intake of estrogenic organochlorine pesticide residues."11
Claims also persist that chemicals such as DDT cause declining male reproductive capacity and breast cancer12,13. There is no evidence supporting a link between DDT and human cancers. Pressure against the use of organochlorines (among them DDT) persist because of the claim that they are linked to a fall in sperm quality. In 1992, Danish scientists of the Copenhagen University Hospital published a paper showing that the number of sperm cells in men's semen had fallen over the past 50 years14. Among the numerous reasons posited for the decline was the possible exposure to synthetic estrogens.
The media widely disseminated this study, and environmentalist groups such as Greenpeace used it very effectively in their campaign against synthetic chemicals. The study has sparked a great deal of debate in the scientific community and many investigations around the world into sperm quality. Subsequent studies conducted in Europe and the US found that in some cases sperm quality deteriorated and in some cases not; overall, scientists cannot find any meaningful decline in sperm quality. Part of the problem with these studies is that we do not have reliable pre-1970 data and so time series comparisons of sperm quality are inherently unreliable. According to Prof. Stephen Safe, "researchers have found no correlation between chemical exposures and measures of decreased male reproductive capacity. Demographic differences are more likely to account for the differences seen in the initial studies."15 Science simply does not support the assertion that DDT and other organochlorines are responsible for human health harms by disrupting the endocrine system.
Most recently, Eskenazi et al.16 published evidence linking DDT exposure to human harm. Like the innumerable studies before it, the study was roundly criticized by technical experts, and the authors' spurious recommendation to keep DDT out of malaria control was challenged by Africa Fighting Malaria.
The hypothetical risks to human health from the use of DDT in malaria control must be weighed against the very real benefits that it brings in malaria control. Even if a child survives a bout of malaria, the disease can severely damage his or her cognitive development, leaving him or her debilitated for life. The benefits of using DDT far outweigh any potential harm.
8 A. G. Smith, "How Toxic is DDT?" Lancet , Vol 356, No. 9226, July 22, 2000.
9 B. Ames & L. Gold, "Pollution, pesticides and cancer misconceptions," in What Risk? , ed. R. Bate, Butterworth Heineman, London (1997).
10 A. Attaran, D. Roberts, C. Curtis, W. Kilama, "Balancing risks on the backs of the poor", Nature Medicine, Vol 6 No 7 July 2000.
11 S. Safe, "Endocrine Disruptors. New Toxic Menace" in Earth Report 2000 , R. Bailey (ed), Washington DC: Competitive Enterprise Institute (2000) p. 190-191.
12 Liroff, R. "Reduction and elimination of DDT should proceed slowly" British Medical Journal , Vol 321. 2 December 2000, pp 1404-1405.
13 World Wildlife Fund, Resolving the DDT Dilemma , World Wildlife Fund (1998).
14 Carlsen, E, Giwercman A, Keiding, N, Skakkebaek, N "Evidence for decreasing quality of semen during the past 50 years" British Medical Journal Vol 305, pp 609-613.
15 Safe (2000) p 190. For instance, we know that the shorter the time since an ejaculation, the lower a man's sperm count is. During the time period chosen for a 1992 Danish study, statistics have shown that the frequency of masturbation has doubled for unmarried men (from 30 times a year to 60) and it also rose for married men (from 6 times a year to 24). At the same time the frequency of marital coitus also increased from around 1.9 times a week to 3 times a week (for married 30-year olds). So the sexual revolution of the 1960s could contribute greatly to the any observed decrease in sperm quality. Lomborg, B. The Skeptical Enironmentalist , Cambridge University Press (2001). p 240.
16 Brenda Eskenazi PhD et al. "In Utero Exposure to Dichlorodiphenyltrichloroethane (DDT) and Dichlorodiphenyldicholorethylene (DDE) and Neurodevelopment Among Young Mexican American Children" Pediatrics Vol 118, No. 1, July 2006.