[Gladwin]

Chlorinated Hydrocarbons

DDT was the first of a long line of insecticides based on hydrocarbons with chlorine atoms replacing some of the hydrogen atoms. Its chemical name is dichloro, diphenyl, trichloroethane (see figure). Some others:

• methoxychlor
• dieldrin (see figure)
• dicofol (Kelthane®)
• endosulfan (Thiodan®, Phaser®)

DDT was introduced during World War II and, along with penicillin and the sulfa drugs, was responsible for the fact that this was the first war in history where trauma killed more people - combatants and noncombatants alike - than infectious disease. DDT is effective against

• vectors of human diseases such as
  • malaria and yellow fever (both transmitted by mosquitoes)
  • plague (transmitted by fleas)
• many crop pests

Prior to the introduction of DDT, the number of cases of malaria in Ceylon (now Sri Lanka) was more than a million a year. By 1963 the disease had been practically eliminated from the island. However, growing concern about the hazards of DDT led to its abandonment there in the mid-1960s, and soon thereafter malaria became common once again.

DDT was especially effective against malarial mosquitoes because of its persistence - its resistance to breakdown in the environment. One or two sprays a year on the walls of homes kept them free of mosquitoes. But DDT has several serious drawbacks.

Insecticide resistance

As early as 1946, Swedish workers discovered populations of houseflies resistant to DDT. This was quickly followed by many other reports of developing resistance. Other chlorinated hydrocarbons (like dieldrin and methoxychlor) were developed as substitutes, but in time insects developed resistance to these as well.

Persistence

DDT is

• stable and
• fat soluble.

These properties cause it to accumulate in fat tissue. People who were heavily exposed to DDT (during its manufacture or application) often showed concentrations of DDT in their fat 1000 times higher than that in their blood.

Even these high levels were probably of little harm to the workers. In the early stages of exposure, the blood levels of DDT (and its metabolite DDE) rise rapidly at first and then reach a steady level. From that point on, the body excretes it as fast as it acquires it.

Biomagnification

Although no harmful effects from average exposures to DDT have been seen in humans, DDT and other chlorinated hydrocarbons have been shown to harm other species, such as fishes, earthworms, and robins. The hazard of DDT to nontarget animals is particularly acute for those species living at the top of food chains.

Link to discussion of how DDT becomes concentrated as it moves up a food chain.

Carnivores at the ends of long food chains (e.g., ospreys, pelicans, falcons, and eagles) once suffered serious declines in fecundity and hence in population size because of this. High levels of chlorinated hydrocarbons interfere with forming eggshells of normal thickness.

Correlation between DDE concentrations in the eggs of Alaskan falcons and hawks and reduction in the thickness of their eggshells (compared with shells collected prior to 1947). DDE is a metabolite of DDT. Data from T. J. Cade, et. al., Science 172:955, 1971.
Species	Location	Average Concentration of DDE in Eggs (ppm)	Reduction in Shell Thickness
Peregrine falcon	Alaskan tundra (north slope)	889	-21.7%
Peregrine falcon	Central Alaska	673	-16.8%
Peregrine falcon	Aleutian Islands	167	-7.5%
Rough-legged hawk	Alaskan tundra (north slope)	22.5	-3.3%
Gyrfalcon	Seward Peninsular, Alaska	3.88	0

Another group of nontarget victims of DDT (and other pesticides) are insects that prey upon insect pests; that is, the natural enemies of the pests. Killing these has serious ecological - and economic! - effects.

Once apple growers began controlling pests with DDT, they quickly found their orchards being attacked by scale insects and mites. The reason: DDT had killed off their natural enemies.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Insecticides.html

[ijcr]

By Kenneth Smith
The Washington Times
September 2, 1999

The National Academy of Sciences called it the greatest chemical ever discovered, a lifesaver for 500 million people whose deaths were otherwise inevitable. And environmentalists want to make sure the world can never use it again.

The chemical is DDT. Though it is banned here in the United States as a possible threat to man and animals, public health authorities around the world have been using it for years to control the mosquito that carries the dreaded malaria parasite. The disease already kills some 2.7 million people and afflicts half a billion annually - about 90 percent of them children and pregnant women - and it could get much worse. Next week United Nations diplomats are scheduled to gather in Geneva to vote on a treaty that would ban alleged pollutants, among them DDT, by 2007.

Pushing hard for the ban are the World Wildlife Fund (WWF) and other environmental activists who argue that even trace exposures to the chemical can cause cancer and disrupt human hormones. On the other side are the Malaria Foundation and some 370 medical researchers, including three Nobel Prize winners from 57 countries who consider the proposed ban an exercise in Third World population control rather than healthy public policy.

In an open letter to U.N. negotiators, they accuse the WWF of "surprisingly blatant" distortion of the debate by its selective use of scientific findings. For example, the WWF cites studies linking breast cancer to exposure to DDT and other chemicals. But when the medical researchers went back to look at the studies, they found that one actually concluded just the opposite, that is, the "data do not support the hypothesis that exposure . . . increases risk of breast cancer." Environmentalists are misleading the world about the real risk here. "It would be ironic indeed if in running from the bogeyman of these speculative health risks," the open letter says, "we banned DDT and ran directly into the familiar and deadly hands of malaria."

Just how familiar and how deadly the developed world has largely forgotten thanks to a combination of eradication efforts and general migration away from those mosquito incubators known as wetlands. But for peoples who can't afford a mosquito net much less a townhouse, malaria is a genocidal tyrant on a scale beyond anything, say, a Hitler or even Stalin could comprehend. Partly that's because of the size of its army. Four different parasites carried by 35 species of mosquitoes - flying syringes, some call them - can infect humans. Once injected inside, the parasites slip past the body's unwitting defenders and regroup in the liver, gathering their strength and multiplying until finally they launch back into the blood system, gorging themselves on red blood cells and debilitating, even killing their human host within a matter of hours. Wrote Ellen Ruppel Shell in the Atlantic magazine two years ago: "I have seen greatly enlarged photographs of malaria parasites pouring from the ghostly white hulks of dead blood cells, like soldiers fleeing a scorched-earth spree, and the sight is frightening."

There is no vaccine against malaria. It evolves and mutates, hiding like a sci-fi villain, making it hard for a vaccine to find, much less attack. It's true that the disease is treatable in most cases, but after a while the malaria parasite develops a resistance to drugs. The result is that in Sub-Saharan Africa alone, the malaria researchers say, the disease destroys 70 percent more years of life than do all cancers in all developed countries combined.

Faced with so formidable a foe, researchers continue their hunt for the vaccine. But in the meantime, public health officials decided that if they couldn't beat the parasite, they would take on the mosquito. Armed with DDT, they discovered that by spraying the interior walls of huts twice a year, they could kill or at least deter the female mosquito carrying the parasite before she could plant her deadly kiss. And for a while that approach worked. The number of malaria cases around the world fell sharply. Researchers wondered if the disease might go the way of smallpox.

Then came a woman named Rachel Carson. In an apocalyptic book titled "Silent Spring," she predicted that man would destroy the Earth, chiefly through the use of sinister, profit-making pesticides like DDT, which would essentially poison the food chain. At its heart, the book was a religious, rather than scientific, tract whose premise was a creationist myth: Man had eaten of the forbidden tree of technology, and for that he would lose his access card to Eden, a gated community; he gets the card back when he gets rid of DDT and other pesticides. The book helped bring about the U.S. ban on the chemical.

Well, the Africans live (in a manner of speaking) in this version of paradise, and they're dying to get out. It has been a very silent spring for the tens of millions of people who have died of malaria.

"Malaria keeps Africa down, and down is where the rest of the world wants us to be," a medical editor in Senegal told the Atlantic. "If this was a disease of the West, it would be gone." Several Western scientists even told the magazine that population control, not disease control, is the central mission of the U.S. Agency for International Development in Africa. Said one scientist, "I'd rather die of malaria than of starvation."

There's no need for such a choice. There was no evidence at the time DDT was banned in the early '70s, and there is no evidence now that when used as directed the chemical posed a serious human health threat. Nor is it clear that its use had anything to do with declines in certain bird species, many of which had been having problems long before the advent of DDT. Why then deny the Third World access to a cheap, effective pesticide for which there is no substitute?

DDT, editorialized the British Medical Journal in 1969, "has incontrovertibly been shown to prevent human illness on a scale hitherto achieved by no other public health measure entailing the use of a chemical." Who now wants to save malaria from it?