DDT, Human Health, and the Environment

At the 2005 G8 Summit at Gleneagles, Scotland, two of the major policy issues addressed were Africa and climate change. Obviously, the topic of climate change is of interest to many environmental engineers and scientists, but I was also interested to learn of another topic of concern to those working on human and environmental health issues that was raised at the Summit in the context of Africa. Specifically, the Roll Back Malaria Partnership of the World Health Organization reports that the G8 pledged to contribute an additional $1.5 billion (United States) annually “… to help ensure access to antimalaria insecticide-treated mosquito nets, adequate and sustainable supplies of Combination Therapies including Artemisin, presumptive treatment for pregnant women and babies, household residual spraying and the capacity in African health services to effectively use them …” (Roll Back Malaria Partnership 2005a). The topic of malaria control is important, particularly in Africa. According to the Roll Back Malaria Partnership, malaria annually causes more than 300 million acute illnesses and at least 1 million deaths, 90% of which occur in Africa south of the Sahara, primarily amoung young children (Roll Back Malaria Partnership 2005b). Significant direct and indirect economic effects are also associated with malaria. The Roll Back Malaria Partnership acknowledges that no single solution exists for malaria and recommends a combination of effective low-cost strategies, like those described in the preceding. Of these interventions, one that has caused much debate is household residual spraying, because the pesticide DDT (dichloro-diphenyl-trichloroethane) has been the mainstay of household spraying. Why the controversy over using DDT for malaria vector control? I am not an expert on this topic, but I think that it is a fascinating example of the complex interactions between human health and environmental protection, as well as between science and policy, that face environmental engineers and scientists. Thus, the purpose of this editorial is to highlight some key aspects of DDT use in the past and present—in the United States and abroad—that make it so controversial.

If you grew up in the United States—like I did—after the problem of malaria had been largely eradicated and if you had been educated as an civil or environmental engineer, you may be most familiar with the negative ecological aspects that are associated with the use of DDT. Open just about any introductory environmental engineering or science textbook and you are likely to find DDT presented as a classic example of a persistent organic chemical that accumulates in fatty tissues and undergoes biomagnification in the environment (e.g., Woodwell et al. 1967). The ill effects of DDT on carnivores at the top of the food chain—especially fish-eating and predatory birds—are well documented and are attributed to interference with calcium metabolism caused by DDT and its metabolite DDE (dichloro-diphenyl-dichloroethene), which leads to production of thin-walled eggshells that are easily broken (Ratcliffe 1970). Rachel Carson dramatized the potential negative impacts of DDT on wildlife in her well-known book Silent Spring, which contributed to the debate that ultimately led to the United States ban on DDT in 1972 (USEPA 1975). The publication of Silent Spring also helped set off a controversy that has been raging ever since between opponents and advocates of DDT use (Miller 1988). However, as Berenbaum (2005) contends in a recent editorial, the issue of DDT is more complex than the simplistic superhero versus supervillain argument, as it is sometimes portrayed.

Many of the pros and cons of DDT use that have been considered over the years are highlighted in a story that involves my father’s family, who were farmers during the post–World War II era. In the years immediately following World War II, agricultural and commercial use of DDT in the United States became widespread, with approximately 1,350,000,000 pounds of DDT used domestically before the ban (USEPA 1975). This popularity was largely attributable to DDT’s low cost, effectiveness, versatility, and persistence. Its persistence, of course, contributed to the ecological concerns related to DDT, which prompted singer Joni Mitchell to pen lyrics with her plea to farmers—“Hey, farmer, farmer, put away your DDT now.” I doubt that my Grandpa Ted was aware of singer Joni Mitchell, nor would he have agreed with her on much, but the subject of DDT is one topic on which they may have seen eye to eye. During those years, he refused to use DDT in the dairy barn to keep down the flies, even though others told him how well it worked. Grandpa Ted based his position on two interesting and insightful concerns. First, he said that anything that killed insects as well as DDT did should not be used around food produced for human consumption. Second, he said that even though DDT was working well at the time, he thought that those insects would eventually get used to the pesticide, making it less effective over time.

Although my Grandpa Ted was concerned about the effects of DDT on humans, DDT was actually first used—and continues to be used—effectively to improve human health through control of vector-borne diseases. After its insecticidal properties were discovered in 1939, DDT was first successfully used for combating insect-borne diseases (e.g., typhus carried by body lice) during World War II. (USEPA 1975). The World Health Organization (WHO) subsequently used DDT and related pesticides to try to control the spread of such insect-transmitted diseases as malaria, which is caused by one of four species of protozoa of the genus Plasmodium and is transmitted by a bite from the female of about 60 of the 400 different kinds of the Anopheles mosquito (Miller 1988). The use of DDT was very successful, and is credited with freeing more than 1 billion people from the risk of malaria, as well as with saving millions of lives.

So, was my Grandpa Ted right to be concerned about human exposure to DDT? The jury still seems to be out on this issue. DDT and other organochlorine insecticides have relatively low acute human toxicity (Smith 2000). However, their long-term effects are less clear (Turusov et al. 2002). Because of its biomagnification, DDT is found in human tissues, and Turusov et al. (2002) states that “there is now not a single living organism on the planet that does not contain DDT.” For example, in the United States, storage of total DDT in body fat increased from $5\mathrm{ppm}$ in 1950 to $15.6\mathrm{ppm}$ in 1956, although by 1980 it had decreased to $3\mathrm{ppm}$ . Turusov et al. (2002) also notes the following about DDT and its metabolites: (1) they have been reported to be associated with premature births and to affect neurobehaviorial functions; (2) they may act as endrocrine disrupters; and (3) there is convincing experimental evidence of their carcinogenicity. However, Turusov et al. also concludes that “… epidemiologic studies have provided contrasting or inconclusive, although prevailing negative, results” regarding possible carcinogenic effects in humans. Nonetheless, the USEPA has determined that DDT, DDE, and DDD are probable carcinogens; the International Agency for Research on Cancer has determined that DDT may possibly cause cancer in humans; and the Department of Health and Human Services has also determined that it is reasonable to believe that DDT may be a human carcinogen (Agency for Toxic Substances and Disease Registry 2002).

The accuracy of my Grandpa Ted's prediction that the insects would get “used to” the DDT is more clear, and in fact, increased insect resistance was one of the factors that contributed to the decline in DDT use. When a poison like DDT kills a large proportion of the insect population, the individuals that survive are those that have genes that make them resistant or immune to the pesticide (Brown 1986). Through a process of selection, the proportion of resistant genotypes increases as, generation after generation, the susceptible organisms are killed off after the organisms are repeatedly sprayed with the same chemical. In fact, my grandfather’s prediction came true relatively quickly, even though widespread use of DDT did not begin until World War II. Berenbaum (2005) notes that by 1947, DDT-resistant houseflies were in Europe; and by 1949, DDT-resistant mosquitoes had been documented on two continents. Furthermore, by 1972, 19 species of mosquitoes capable of transmitting malaria were resistant to DDT, including some in Africa, and pockets of DDT-resistant mosquito species are well-documented in Africa today. Unfortunately, the mosquito’s increased genetic resistance to DDT and other insecticides, along with other factors, resulted in malaria’s making dramatic comebacks in many areas of the world after 1970 (Miller 1988). Berenbaum (2005) concluded that “…it’s essential to determine whether target populations are resistant; if they are, then no amount of DDT will be effective” and noted that new means are available for determining whether populations are genetically prone to developing resistance.

Where are we today with respect to DDT? The United Nations Environment Program has identified DDT as one of 12 persistent organic pollutants (POPs), the so-called dirty dozen, which are scheduled for worldwide reduction in use or elimination (Berenbaum 2005). This decision to designate DDT as a POP caused heated debate and sharp, criticism, in part because of concerns about the effect of the potential ban on DDT for malaria control (e.g., Attaran and Maharaj 2000; Liroff 2000). The dilemma of how to move away from DDT was illustrated by a 1996 ban on the use of DDT for indoor spraying in South Africa, which resulted in a sudden increase in malaria cases. Therefore, the Stockholm Convention, an international treaty designed to eliminate POPs, included an exemption that allowed DDT use in malaria control, as a result of calls by the WHO “that DDT, although an environmental hazard, was a necessary public-health weapon in poor tropical countries” (Kapp 2000). Pending the development of safer solutions, 25 countries with endemic malaria, including South Africa, are allowed to use DDT against malarial mosquitoes. Importantly, the WHO notes that only small amounts of DDT are needed for malaria control compared with what was used in the past as a pesticide (WHO 2005). Furthermore, WHO also recommends that DDT be used only for indoor residual spraying. In addition, WHO’s Roll Back Malaria campaign is working to mobilize funds for finding alternatives to DDT.

DDT and its use clearly remain controversial. WHO notes that opponents of the use of DDT for vector control point to DDT’s persistence and biomagnification in the environment, which has been linked to negative ecological effects, with potential negative long-term impacts on human health (WHO 2005). However, advocates of DDT use for disease vector control point to the unacceptable human and economic burden caused by malaria, the proved effectiveness of DDT for reducing malaria transmission, the relatively low cost of DDT, and the lack of sustainable alternatives in endemic countries.

Where do environmental engineers contribute to this debate? Relatively recent papers on DDT published in the Journal of Environmental Engineering indicate a focus on the fate and transport (e.g., Ackerman and Schiff 2003), as well cleanup of DDT in the environment (e.g., Jafvert et al. 1997). However, the broader debate over DDT highlights two of the mandates that drive environmental engineering—to protect and improve environmental quality and to protect and improve human health. In striving to strike the appropriate balance in meeting these goals, it is interesting and instructive to understand the broader historical and contemporary context and concerns surrounding DDT use and other complex issues.