Dioxin has been billed as "the most toxic man made chemical" and "the most potent carcinogen." It is especially notorious because of its presence in Agent Orange, which is often cited as the cause of diseases among Vietnam veterans and birth defects among their children.
Moreover, nearly all of us have been exposed to dioxin. For some of us, exposure has occurred through past contact with some now-banned herbicide. Almost certainly we have been exposed through inhalation or ingestion of dioxin produced by various combustion processes and through contact with bleached paper, which contains traces of the chemical.
Results from recent studies suggest that North Americans slowly accumulate dioxin in their tissues over their lifetimes. Among the few hundred people to have been examined so far—each test to measure dioxin in human tissue costs about $1,000—concentrations are low or undetectable in children, increase with age, and are highest in older people. With the decay of residual dioxin from herbicide use and the control of emissions from combustion sources, and in the absence of unforeseen new sources, levels of dioxin in human tissues will probably decrease in the future.
Risks remain
Despite this likely decrease, some risks will remain. Although numerous investigations of possible human health effects of dioxin and intensive laboratory studies of dioxin toxicity have been carried out, many questions have not been answered. The most fundamental is, have environmental exposures to dioxin made people sick?
Our inability to answer that question to everyone's satisfaction keeps alive the controversy about how harmful dioxin may be to human health. Strongly held opinions on both sides of the question can be and are being expressed in regulatory proceedings, legal actions, and Vietnam veterans' claims for compensation.
Dioxin is used here to refer to the chemical 2,3,7,8-tetrachlorodibenzo-para-dioxin, which is also referred to as 2,3,7,8-TCDD or TCDD. It is the most toxic of 75 chlorinated dioxins and is chemically closely related to 135 chlorinated furans, which are also toxic. Both dioxins and furans occur in combustion products and in bleached paper goods.
The controversy is also fueled by such contradictory statements and actions as those that occurred after several thousand Boy Scouts camped on an army base during a national Boy Scout jamboree. When the scouts had departed, dioxin was discovered around a shed that had formerly been used to store herbicides. Although there was no evidence that any scouts had come into contact with the contaminated soil, parents and others were naturally concerned about possible exposures.
Various federal agencies studied the information about the levels of dioxin in the soil and announced that no scout's health had been endangered. Then, thirteen days later, a photograph appeared in the New York Times showing Environmental Protection Agency employees in moonsuits investigating the site (see photo). If Boy Scouts running around in shorts, tee-shirts, and sneakers had not been at risk, why were the workers moon-suited? Among the reasons are EPA work rules and liability concerns that mandate use of protective gear. Nevertheless, the government's assessment of no risk for the boys appeared to be contradicted by the moonsuits.
Sources and health effects
Although no one has ever intentionally manufactured dioxin, it was present as an unavoidable contaminant in a precursor chemical in the synthesis of the herbicide 2,4,5-1 and some closely related herbicides, and it was always carried along into the herbicides. By the early 1980s, those herbicides had been banned, but other sources of dioxin had been found.
Dioxin is produced by many kinds of fires, such as those in municipal and industrial waste incinerators, as well as by automobile engines that burn leaded gasoline. Significant sources of airborne dioxin in developed countries include industrial activities, refuse incineration, and automobiles.
Dioxin is also present in many paper products, as was announced in an EPA study released in the fall of 1987. Formed by chemical reactions between chlorine used to bleach paper pulp and organic matter in the pulp, dioxin is present in all products that include bleached paper. Some human exposure is possible from food wrapping, baby diapers, toilet tissue, tampons, paper napkins, and other paper products.
Past and present exposures to dioxin can be divided into four general types. Historically, the highest exposures have been to workers in some chemical plants that manufactured the herbicide 2,4,5-T and its precursor from the 1940s through the early 1980s in this country. Many workers developed the serious skin disease chloracne—an indicator of high exposures to dioxin—from routine leaks or spills or from more spectacular occupational accidents that sprayed dioxin-containing materials across work areas.
The next highest exposures have been to applicators who sprayed dioxin-containing herbicides from airplanes, various vehicles, and backpacks and hand-held apparatuses during roughly the same time period. The most famous group of applicators was the Ranch Hands, the U.S. Air Force personnel who sprayed Agent Orange in Vietnam.
Most exposures to dioxin have been lower than those of military and civilian applicators and have involved either incidental exposures in areas that were herbicide-sprayed in the past, or else past or current exposures to combustion products and paper products. Probably everyone has experienced these "environmental" exposures, which are, for practical purposes, unavoidable in our modern society.
The fourth kind of exposures are characterized by special conditions that have existed for only short periods of time in specific locations. The best known involve towns in Missouri and in Italy.
The dirt streets of Times Beach, Missouri were sprayed with oily chemical wastes in the 1970s to suppress dust. Several years later, the residents learned that the wastes contained dioxin, and, because of the risks, the federal government purchased the town in 1983. Many of the town's former residents have sued chemical companies that produced the dioxin-containing wastes, and million-dollar settlements have been reached. Only one of these cases has gone to a jury. On June 8, 1988, the jury decided that there was no medical evidence that the former residents had been harmed.
In a separate incident, an explosion occurred in 1976 at a chemical plant in Seveso, Italy. Following the explosion, which harmed no one in the plant, a cloud of chemicals drifted over the town, leaving behind a fallout that contained dioxin. Although exposures among Seveso residents were high enough to cause chloracne in some children, no other adverse health effects have been confirmed in that population.
In view of dioxin's notoriety, it is surprising that the consequences of human exposures to it remain an enigma. Without doubt, dioxin is the most potent carcinogen yet tested in animals, and, equally clearly, many people have been exposed to it. Despite its demonstrated toxicity and verified exposures to it, its effects on humans are much less dramatic than might be expected.
Some exposed chemical workers and some Ranch Hands have dioxin concentrations of a few hundred parts per trillion (ppt) in their body fat. This level can be contrasted to the 2 to 20 ppt of dioxin in people who have had no known occupational exposures, including the vast majority of veterans of ground warfare in Vietnam.
Nevertheless, there is no consistent, convincing evidence of long-term adverse health effects, even in the most highly exposed people. It can be argued that too few people with high exposures have been studied to know for certain that those levels are not harmful. But there is no measurable increase in adverse health effects in these highly exposed people, which contrasts sharply with the widely held impression that dioxin has caused many human diseases.
Regulatory activities
Three major ongoing regulatory activities focus on dioxin. The first originated with a petition from the Environmental Defense Fund (EDF) filed in 1985. EDF contended that dioxin can pose an "unreasonable risk" that requires regulation under the Toxic Substances Control Act of 1976 (TSCA). EDF's petition requested that the Environmental Protection Agency (EPA) regulate dioxin in air, water, and some industrial and consumer products under the provisions of that act. EPA denied the petition, stating that other statutes provide adequate authority to regulate dioxin levels. In response, EDF filed a legal suit to force EPA to regulate dioxin in air, water, and some industrial and consumer products. A hearing has been scheduled for the summer of 1988.
The second major regulatory activity involves EPA and many state agencies. Everyone knows that modern society produces heaps of garbage and that disposal is a growing problem. Incineration is an efficient disposal method, and heat from burning garbage can be used to generate electricity. Those positive features have to be balanced against the fact that incineration emits small amounts of dioxins and related toxic substances called furans. E
PA claims to be 95 percent certain that emissions from all 321 municipal incinerators now in operation and planned for the near future will, at worst, cause no more than 10 to 120 cancer deaths annually (there are about 485,000 cancer deaths annually in the United States). The lower limit is zero. Those calculations, like all quantitative estimates of risks from dioxin, depend on estimates of the carcinogenic potency of dioxin that are derived from animal tests.
There are many reasons for opposition to incinerators: they are large structures that can change skylines and may change property values, and heavy trucks, carrying garbage that may stink, will rumble to and from the incinerators. But much opposition to them focuses on emissions of dioxins and furans and some carcinogenic metals. EPA administrator Lee H. Thomas said in April 1988 that a combination of guidance documents and regulations expected in November 1989 cause him to believe that air emissions can be safely controlled. However, opponents claim the cancer risks make incinerators unacceptable.
Disputes about incinerator siting and construction are going on in many states, and some states may set their own standards for emissions, as Connecticut did in May 1988. It is altogether possible that some state standards will be written to allow incinerator construction and operation and that others will be so stringent as to prevent it.
The third ongoing regulatory issue strikes at the heart of the controversy about possible human health effects from dioxin. In December 1987, EPA circulated a draft document that proposed a change in its quantitative estimate of dioxins carcinogenic potency. The change would be a 17-fold reduction. If adopted, it would have an immediate effect on estimates of cancer risk. For instance, EPA's calculation of an upper limit of 10 to 120 annual cancer deaths from incinerator emissions would drop to a range of from less than 1 to 17 (again, with a lower limit of zero). The proposed change in potency has been attacked, but as of this writing EPA appears determined to adopt it.
Progress to date
The federal government so far has spent about $1 billion on assessing the effects of dioxin and Agent Orange. Those expenditures include the nearly $36 million to purchase Times Beach, Mo. The bulk of that amount, perhaps $500 million, has been spent by the Veterans Administration to examine Vietnam veterans who believe themselves to have been exposed to Agent Orange. Lesser amounts have been spent on environmental monitoring, testing, and basic research.
We know that most human exposure is to dioxin that has been transported through the air rather than the water or soil. Although we have no direct measurements of past airborne dioxin levels, scientists can estimate those levels from measured concentrations of dioxin in lake sediments. To obtain that information, researchers bore into the sediment of lake bottoms, and then slice up the sample for chemical analysis. The information they gain can provide evidence about whether the general public's exposure to dioxin is increasing, decreasing, or remaining constant.
Concentrations of dioxin in borings from a lake bottom on a small island in Lake Superior are shown in the figure below. The island has no industry, agriculture, or automobiles, and the only source for dioxin in the sediments is from deposition of airborne particles. The annual deposition rate began increasing about 1940, increased steadily until 1972, and then began a decline that continues.
Several conclusions can be drawn from those data. Dioxin was relatively uncommon until about 1940, when significant production of chlorinated hydrocarbons began, and there is an apparent association between those chemicals and the amounts of airborne dioxins. No decrease in production accompanied the decrease in deposition rates that began in the 1970s. However, that was the period during which inefficient incinerators were shut down, leading many to conclude that reduced emissions of combustion products, including dioxin, are the reason for decreased dioxin deposition rates. Since the deposition rate is related to the airborne concentration of dioxin, it can be concluded that the airborne concentrations have been decreasing since that time also. Similar observations to those shown in figure 2 have been made from borings in Lakes Huron and Erie; they support the conclusion that airborne concentrations of dioxin have decreased in the last decade or so.
It is impossible to measure directly any changes in health that might have resulted from reductions in dioxin exposure, with one important exception. The outbreaks of chloracne and perhaps other diseases among 2,4,5-T production workers have been eliminated by banning the herbicide. By contrast, the disease burdens that are estimated for environmental exposures are so low as to be undetectable, making it impossible to measure any changes that may have accompanied the phase-out of leaded gasoline and shutting down of inefficient incinerators.
Exposure risks
At how much risk from dioxin is the average person, when all is said and done? EPA and a number of researchers around the world estimate that the average person in an industrialized nation is exposed to about 70 picograms (a picogram is 10^-12 grams) of dioxin daily. That value and estimates of carcinogenic potencies can be combined to calculate an upper limit on the number of cancer cases expected among people exposed to average levels of dioxin.
Use of EPA's former potency estimate predicts the upper limit of cancer risk to be 528 cases annually in the United States if everyone is exposed to 70 picograms of dioxin per day for his or her lifetime. Use of the new potency factor predicts an upper limit of 31. It is important to remember that risks are upper limits on the probabilities of harm. They are not the same as actual cancer cases, and EPA states it is possible that average exposures will cause fewer or no cancers. There is also some chance that the risks are underestimated. Either of these estimates, 31 or 528, is large compared to target levels for U.S. regulation of carcinogens, which often sets exposure levels so as to limit risks to one extra cancer case in one million people exposed to that level throughout their lives. A one-in-a-million lifetime risk for the country is equivalent to the risk of an additional 3 cancer cases annually. This is a rigorous goal.
It is impossible for any epidemiological study to provide information about cancer risks as low as 528 annual cases in the U.S. population. As a rule of thumb, an agent usually has to cause a doubling of cancer rates to be detectable as a carcinogen, and exposures associated with such rates have only been found in workplaces and some applications of chemotherapy. No convincing evidence for cancer in populations occupationally exposed to dioxin has been found, and studies of environmentally exposed people can reveal neither whether those exposures have caused cancer nor whether decreased exposures are reducing cancer rates.
If dioxin is such a potent carcinogen in animals, why has it not been shown to cause cancer in people? There are at least three possible answers. First, it may be that humans are more resistant. Second, methods for extrapolating from animal studies to predict human risks may be inappropriate. Third, epidemiology is an imperfect tool, and some cases of cancer caused by dioxin may have been missed.
Dioxin in context
In some ways, dioxin is typical of many environmental carcinogens. Animal studies convincingly show it to be carcinogenic, and there is no doubt that humans were and are exposed. Yet there is no convincing evidence that dioxin has caused human cancer.
But dioxin differs from all other environmental carcinogens. First, it is the most potent carcinogen yet tested in animals. Second, because of its presence in Agent Orange, it is intertwined with our country's feelings and confusion about the Vietnam War, which keeps it in the public's eye. Third, many—perhaps hundreds—of legal actions around the country involve claims that dioxin has caused disease and death. Those cases will continue to be filed and heard. Those reasons, plus the media's interest in the "most toxic chemical" and "most potent carcinogen," set dioxin apart from other environmental carcinogens and make it more visible and more important.
Finally, the stakes are high in the current EPA reevaluation of the carcinogenic potency of dioxin. The "old" evaluation was made according to the guidelines that EPA had heretofore used to evaluate the potencies of all carcinogens. Many scientists have objected to use of a single method of evaluation because different carcinogens play different roles in the carcinogenic process.
Until now, EPA has resisted these arguments, adhering to use of a single method to estimate potency. The EPA draft reevaluation, on the other hand, concedes that the standard method for risk estimation may not be appropriate to all cases. It seems especially ironic that EPA first departed from its standard method in the case of dioxin. If the potency of the most potent carcinogen can be reevaluated, surely the same can be done for other chemicals.
Objections to the reevaluation of the potency estimate for dioxin and any subsequent evaluations can be expected from environmental organizations, citizens, and members of Congress who believe that adequate protection from carcinogens requires and depends on EPA's standard approach. Equally, many scientists, from industry, academe, and other countries, will applaud a more case-by-case approach to carcinogen evaluation and regulation that considers the mechanism by which a chemical acts in the cancer process.
Disagreements about dioxin will someday be clarified by science. I fully expect that what I wrote two years ago will come true: "The consensus among most scientists—that harm [from dioxin] has been limited to highly exposed industrial populations and that none has been shown from environmental exposures—may often be ignored, and the old claims about harm will be brought up again and again. But l am confident that the information gathered by science will eventually prevail."
Yet I don't want to bet on when that will happen.
Michael Gough is senior fellow in the Center for Risk Management at RFF. He formerly was director of special projects at the Congressional Office of Technology Assessment.
A version of this article appeared in print in the June 1988 issue of Resources magazine.
