No matter how improved or what they are called, incinerators present two problems that landfills do not. First, incinerators only transform garbage; they don’t provide a final resting place for it. There remains the question of where to put the ashes. Second, these cavernous furnaces create, out of the ordinary garbage they are stoked with, new species of toxic chemicals. In addition to producing electricity, they generate hazardous waste….
Moreover, the process of burning concentrates into the ash whatever hazardous materials are present in the original refuse. Heavy metal, such as mercury, lead, and cadmium, for example, are not destroyed by fire. Occurring as ingredients in household batteries, lightbulbs, paints, dyes, and thermometers, they are absolutely persistent. Air pollution control depends on the ability of an incinerator’s cooling chambers to condense these metals onto fine particles, which are then caught in special filters.
Once again, the irony of trade-offs becomes readily apparent: the less air pollution, the more toxic ash. An incinerator burning eighteen boxcars of trash per day, for example, produces about ten truckloads of ash per day. The trucks must then rumble out onto the highways, hauling their poisonous cargo through all kinds of weather. Once ensconced in special burying grounds, incinerator ash, of course, presents a hazard to groundwater.
The second problem is more an issue of chemistry than physics. Somewhere between the furnaces and the top of the stack, on the slippery surfaces of fly ash particles, in the crucible of heating and cooling, carbon and chlorine atoms rearrange themselves to create molecules of dioxins and their closely related organochlorine allies, the furans.
There are many dozens of dioxins and furans, but, as with snowflakes, their individual chemical configurations are all variations on a theme. Recall that benzene consists of a hexagonal ring of carbon atoms. This ring can then be studded with chlorine atoms. Two chlorinated benzene rings bonded directly together form a polychlorinated biphenyl, a PCB. By contrast, two chlorinated benzene rings held together by a single atom of oxygen and a double carbon bond are called a furan. A pair of chlorinated benzene rings linked by two oxygen atoms form a dioxin. There are 135 furans and 75 dioxins, each with a different number and arrangement of attached chlorines.
Dioxins and furans behave similarly in the human body, and they all to some degree elicit the set of biological effects described earlier. The most poisonous by far, however, is the dioxin known as TCDD. This particular molecule bears four chlorine atoms, each bonded to an outer corner. Because these points of attachment are located on the carbon atoms numbered 2,3,7, and 8, its full name is a mouthful. 2,3,7,8-tetrachlorodibenzo-p-dioxin. Imagine looking down from an airplane window at a pair of skydivers in a free fall, both hands joined together. Their geometry provides a reasonable impersonation of a TCDD molecule: the divers’ linked arms represent the double oxygen bridge, their bodies the benzene rings, and their splayed, outstretched legs the four chlorine atoms.
TCDD is scary because it is so stable. The symmetrical arrangement of its chlorine legs prevents enzymes–ours or any other living creature’s–from breaking TCDD apart. In human tissues, TCDD has a half-life of at least seven years. As we shall see, this particular geometry also allows TCDD admission into a cell’s nucleus and access to its DNA.
Ascertaining dioxin’s contribution to human cancers is one of the more frustrating challenges for public health researchers. Because dioxin is so potent at such vanishingly small levels, exposure is expensive to measure. Because it is so widely distributed, there remain no populations to serve as unexposed controls. Because dioxin so often rides the coattails of other carcinogens, confounding factors abound. U.S. military personnel exposed to Agent Orange in Vietnam, for example, were simultaneously exposed to 2,4-D and dioxin-contaminated 2,5,5-T, as noted in Chapter Three.
Animal studies provide a complex set of clues. In the laboratory, dioxin is an unequivocal carcinogen. As the dioxin researcher James Huff once noted, “In every species so far exposed to TCDD….. and by every route of exposure, clear carcinogenic responses have been found.” These include cancers of the lung, mouth, nose, thyroid gland, adrenal gland, lymphatic system, and skin. Dioxin also causes liver cancer in rats and mice, but it does so more often in females. Female rats whose ovaries have been removed, however, tend not to develop liver cancer when exposed to dioxin. On the other hand, they are far more likely to succumb to lung cancer. Clearly, an organism’s own internal hormones modulate dioxin’s carcinogenic powers, but through some unclear means.
(Portions from pages 215 – 223)
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