2. Gulf War Syndrome

3. Chemical weapons in history

4. The nerve of this gas

5. Agent Orange revisited

6. Dioxin on trial

7. Most poisonous substance?

Update: Gulf War Syndrome

Dioxin [more]
The dioxin problem is, in a sense, that we know too much -- or that the subject is just too complicated. One reason for that complexity is the fact that dioxin may tamper with hormones.

According to environmental health experts Thomas Webster and Barry Commoner (see p. 10 of "Dioxins and Health" in bibliography), dioxin's "fundamental molecular mechanism -- binding to a receptor that regulates gene expression -- has certain similarities to steroid hormones as well as differences. It alters cell growth and differentiation. It affects other hormones and growth factors, including altering the levels of their receptors. Finally, like hormones, TCDD and the other dioxins cause significant effects at very low doses."

Dioxin links to a receptor on cell membranes, causing a chemical signal to be sent to the cell nucleus that directs the genes to make chemicals that, in turn, affect the body in various ways. Similarly, hormones circulating in the blood direct specific cells to start dividing or take other action.

In animal research, TCDD (the most toxic dioxin) has affected such signaling mechanisms as growth factors and hormones. TCDD decreases testosterone in male rats exposed before birth and makes estrogen receptors more common in female rats. TCDD also affects immune cells, and in high doses kills by an ill-understood "wasting syndrome."

As a group, the animal effects of TCDD are disturbing, write Webster and Commoner. "TCDD is capable of disrupting a wide variety of biochemical processes which are likely to lead to an equally broad spectrum of microscopic toxic effects in animals. The latter include acute toxicity, 'wasting' and death, atrophy of the thymus (defined), liver damage, epidermal (defined) changes, immunotoxicity, birth defects, reduced fertility, and cancer."

The debate continues
However, the scientific community is hardly unanimous on dioxin. For example, John Constable, a Boston plastic surgeon who has had scientific contact with Vietnam since 1967, says most of the evidence for diseases other than soft-tissue sarcoma (blood cancers), is merely suggestive, and that many concerns about dioxin in Vietnam have not been proven in the long run. Vietnamese scientists have put in a lot of hard work, with not much money, he says, but it's difficult to show any papers that clearly meet Western standards of proof for an association between dioxin and other diseases.

One of the most convincing studies, Constable says, concerned partial hydatiform mole, a disorder of tissue connecting the placenta to the uterus. Women who reported more exposure to herbicides had more partial hydatiform mole. But when the dioxin level of their tissue was analyzed, Constable says, the relationship disappeared.

To plumb the scientific disagreement on dioxin, look at government standards for an "acceptable" daily intake. At the low end, the U.S. Environmental Protection Agency has a daily intake limit of 0.006 picograms/kilogram/day of TEQ (defined). The World Health Organization, however, considers 10 picogram/kilogram/day acceptable. (The daily dose in industrialized countries now ranges from 1 to 3 picograms TEQ/kilogram/day.)

Dioxin and the food chain
Crustaceans are low on the food web and prone to contamination.
Courtesy Agricultural Research Service, USDA

Since they are fat-soluble, dioxins and related compounds tend to bioaccumulate in animals. This means concentrations skyrocket as you go up the food web (defined). Tiny floating animals in lakes have more contamination than the floating plants they eat, and the fish that gobble those floating animals have higher still. The eagles that eat those fish have millions of times more dioxin and PCB than the water they fly above.

To some extent, people can protect themselves against contaminants that bioaccumulate by reducing or stopping their consumption of meat, dairy products and fish. As dioxin expert Arnold Schecter of the State University of New York, notes, these foods have much higher dioxin concentrations than fruits and vegetables.

Dioxin contamination can start before birth, since the chemical cross the placenta from the mother to the human fetus. In one study, Schecter found 5 parts per trillion of dioxin equivalent in fetal tissue. Since these levels could affect fetal development, Schecter finds them "rather disturbing, since the fetus is at the stage [three months] where organ development is most sensitive."

Need some protection
How to protect the next generation? Newborn babies face considerable exposure: "A newborn's [dioxin] intake through mother's milk is 50 times the amount the EPA considers safe," Schecter says. Even if a pregnant woman ate less meat, dairy products and fish, it would take years before dioxin was flushed from her system, he adds.

Fruits and veggies are less likely to contain high concentrations of dioxin than a salmon steak.
Photo by Keith Weller. Courtesy Agricultural Research Service, USDA

Schecter suggests that children can still benefit from breast milk, but get less dioxin, if mothers alternate breast milk with infant formula made from cow's milk or soybeans, which are less contaminated than mother's milk.

Unlike PCBs, which were made by the ton, dioxins are accidental byproducts of processes involving chlorine and high temperature. The largest current sources of dioxins are probably trash and toxic waste incinerators, particularly those burning plastic; and chemical, pesticide and paper factories.

Need some controls
This means dioxin can be controlled by regulatory and process changes, Schecter says. "In Europe, blood levels are going down rapidly" -- as much as 40 to 50 percent, due to the replacement of "sloppy" controls on incinerators with "pretty tough" controls.

We've gathered a dynamite bibliography on dioxin and chemical weapons for your reading ... pleasure?