Peter Montague
After three years of study, the US Environmental Protection Agency (EPA) is about to publish a nine-volume draft "scientific reassessment" of dioxin and other dioxin-like chemicals, including dibenzofurans and some PCBs (polychlorinated biphenyls). PCBs are industrial poisons now banned in the US because of widespread environmental damage. Dioxins and furans are highly toxic by-products of certain industrial operations, created when chlorine combines with other chemicals at high temperatures.
A copy of EPA's summary volume (Chapter 9) was leaked to the press in mid-May.
The vast majority of dioxin is created by human economic activities. Since about 1920, industrial emissions, and inattention to the potent toxic effects of dioxin-like chemicals, have allowed the environment to become widely contaminated with significant quantities of dioxins, furans and PCBs. As a result, all North Americans eat and breathe small but important amounts of dioxin every day.
Exposure to dioxin begins early in life. Unlike many other poisons, dioxin crosses the placenta and begins affecting the foetus. The human body retains dioxin, so a "body burden" begins accumulating in each of us during our early months in the womb. (The "half-life" of dioxins in humans, the time it takes for half of today's dioxin intake to be excreted, is somewhere between 5.8 years and 7 years.)
In humans and other species, the growing embryo is most sensitive to the toxic effects of dioxin-like chemicals. EPA: "A general finding in fish, bird, and mammalian species is that the embryo or fetus is more sensitive to TCDD-induced mortality than the adult". (TCDD is a shorthand name for dioxin.)
Growth occurs in two ways: cells multiply, and cells of one type turn into cells of another type (a process called differentiation). Thus some cells become eyes and other cells become fingers by differentiation. Dioxin-like chemicals can disrupt both cell multiplication and cell differentiation. Damage that occurs in the womb can last a lifetime.
After a baby (or animal) is born, rapid growth continues, so sensitivity to the toxic effects of dioxin continues as well. Human infants who breast feed get a particularly high dose of dioxins. EPA's report calculates that an infant who breast feeds for a year will receive 4% to 12% of his or her full lifetime dose of dioxin during that one year. (Despite the presence of dioxin-like chemicals in human milk, breast feeding is still the best way to nourish an infant; all of the alternatives are worse.)
Although dioxin can presumably interfere with every bodily system in the growing infant, there is evidence that the developing immune system is one of the most sensitive to disruption by low-level exposure to dioxin-like chemicals.
EPA points out: "Concern over the potential toxic effects of chemicals on the immune system arises from the critical role that the immune system plays in maintaining health. It is well recognized that suppressed immunological function can result in increased incidence and severity of infectious diseases as well as some types of cancer. Conversely, the inappropriate enhancement of immune function or the generation of misdirected immune responses can precipitate or exacerbate the development of allergic and autoimmune diseases."
EPA clearly considers these immune system hazards important; the report spends considerable time discussing them: "Animal host resistance models that mimic human disease are available and have been used to assess the effect of TCDD on altered host resistance [to disease]. Results from host resistance studies provide evidence that exposure to TCDD results in increased susceptibility to bacterial, viral, parasitic, and neoplastic [cancer] disease. These effects are observed at relatively low doses and likely result from TCDD-induced suppression of immunological function."
The immune system is as complex as the brain and central nervous system. Scientists speak of two basic parts of the immune system: those that work via cells ("cell-mediated") and those that work in the bloodstream without entering cells ("humoral").
EPA: "Both cell-mediated and humoral immune responses are suppressed following TCDD exposure, suggesting that there are multiple cellular targets within the immune system that are altered by TCDD. Evidence also suggests that the immune system is indirectly targeted by TCDD-induced changes in nonlymphoid tissues."
EPA goes on: "One potentially important indirect mechanism is via effects on the endocrine system. Several endocrine hormones have been shown to regulate immune responses, including glucocorticoids, sex steroids, thyroxine, growth hormone, and prolactin. Importantly, TCDD and other related compounds have been shown to alter the activity of all of these hormones."
The EPA's draft report speaks of "a window of sensitivity of biological processes". In other words, there are certain times during the life of an animal (or human) when it is more sensitive to dioxin's effects than at other times. The perinatal period is one such "window of sensitivity." But there are evidently other "windows." EPA suggests that any time the immune system begins to respond to a challenge, disruption by dioxin can have far-reaching effects. Thus even a short-term exposure to dioxin at the wrong time might cause disease in a person by suppressing the immune system, even though the person's average lifetime body burden of dioxin may not be greatly increased.
Dioxin may also cause inheritable genetic changes: "While dioxin and related compounds are not generally considered to be 'genotoxic' in traditional terms, both empirical data and the results of modeling efforts suggest that they may be functioning indirectly to produce irreversible genetic changes in exposed cells."
EPA's draft report emphasises that most people get their daily dose of dioxin from their food (about 90% from meat, fish and dairy products). However, people who live near sources of dioxin emissions should consider that inhalation may be an important hazard for them.
EPA says, "The use of incineration as a means of solid and hazardous waste management results in the emission of contaminated particles that may contain TCDD and related compounds into the environment. Thus, exposure to TCDD and related compounds may result from inhalation of contaminated fly ash, dust and soil."
How much dioxin is "safe"? EPA's answers: For cancer hazards, 300 to 600 times less than we all now take in every day. For non-cancer hazards, 10 to 100 times less than we all now take in every day.
EPA's "dioxin reassessment" raises one key public policy question: How much additional dioxin is acceptable in the environment? The answer seems clear: zero. To protect public health, no new sources can be allowed, and present sources must be sharply reduced.
[From Rachel's Hazardous Waste News (US). Second of two articles.]