Dioxin: Some Questions and Answers

A great deal has been written and said about dioxins by scientists, journalists, government officials and others. Chances are the public debate surrounding these chemical by-products has raised questions in your own mind about the possible effects of dioxins on human health and the environment, and posssibly in relationship to chlorine-treated swimming pools.

This document was written and produced by the Chlorine Chemistry Council (CCC) to answer questions commonly asked about dioxins. It is intended to present complex information about the nature, sources, exposure and human and environmental effects of dioxins as plainly and succinctly as possible. Since dioxins will continue to engage the interest of the scientific and public policy communities, the CCC will update this document as new information becomes available.

It is generally accepted that the use of residential swimming pool chlorine will not contribute to the production of dioxins as the factor of high temperature is not present, as it would be in (for example) the production of paper from wood pulp.

There is no risk of dioxin contamination by bathers in swiming pools that use chlorine as the sterilisation agent - either by direct dosing of chlorine gas, powder, or liquid, or the production of chlorine by a saline generator (salt chlorinator) - even of the pool water is accidentally or deliberatey ingested. Backwashing small amounts of pool water into a soak trench will not materially affect dixoin levels in the ground.


What are dioxins?

The term "dioxin" often refers to a family of chemicals comprising 75 dioxins and 135 related compounds called furans. These compounds differ from one another primarily by the location and number of chlorine atoms on the molecule, and their degree of toxicity varies greatly. The more chlorine atoms that are on the molecule, the lower the toxicity is. The term "dioxin" is also used to refer specifically to 2,3,7,8-tetrachlorodibenzo-p-dioxin (commonly abbreviated as 2,3,7,8-TCDD, or just TCDD), the most thoroughly researched and widely publicized form of dioxins and furans.

Throughout this document, the word "dioxins" is used as a shorthand reference to the family of dioxins and furans, unless otherwise noted.

What are some of the physical and chemical properties of dioxins?

Dioxins do not dissolve readily in water, but are highly lipophilic or "fat loving"-that is, they are soluble in fatty substances and in organic matter with fat-like properties. Dioxins do not react readily with other chemicals. In their pure form, dioxins are colorless, odorless solids with high melting and boiling points and low vapor pressure-that is, they are slow to evaporate. These characteristics explain why dioxins usually are found adhered to or dissolved in fat tissue, where they can accumulate. In lakes and rivers, scientists have found dioxins largely bound to sediment and other organic substances.


Where do dioxins come from?

Dioxins are not intentionally manufactured, except in small quantities for research purposes. They are formed by the presence of carbon, oxygen, hydrogen, chlorine and heat, and therefore can be unwanted contaminants in a variety of manufacturing processes. They can be formed as a consequence of waste, wood, coal and other fuel combustion, for example, in municipal, hospital and hazardous waste incinerators, power plants, wood stoves and motor vehicles. Dioxins can also be formed in some chemical reactions and can occur to some extent in nature-in forest fires, volcanoes and compost piles. With so many sources, it is not surprising that scientists have detected dioxins virtually everywhere they have looked. There may still be unidentified sources of dioxins, and the amount they contribute to the environment is not known.

The U.S. Environmental Protection Agency (EPA), in a public review draft of an extensive dioxin reassessment document released in 1994, grouped the sources of environmental release of dioxins into four major types: combustion and incineration sources; chemical manufacturing/processing sources; industrial/municipal processes; and reservoir sources, or those dioxins that were already present and recirculating in the environment. As awareness of these possible sources has grown in recent years, EPA noted, releases of dioxin-like compounds have been reduced due to technological advancements and regulations such as: the switch from leaded to unleaded gasoline; process changes in pulp and paper mills and chlorine manufacturing; and new emissions standards and controls for waste incinerators. Due to the amount of natural and reservoir sources, total elimination of dioxin in the environment is not possible.

Dioxin Deposition Trends in the Environment
(measured in three lake sediment cores)

Source: Hagenmaier, 1996; Hites, 1990.
*1 Picogram (pg) = 0.000000000001 of a gram

How long have dioxins been in the environment?

There is evidence that some dioxins were present in the environment long before they were first observed. It is clear that from as far back as the 1800s there have been some dioxins in the environment. The amount of dioxins observed rose between 1930 and 1960, peaked in the 1960s and 70s, then started declining. The level of dioxin in the environment has continued to decrease since then. Sediment samples taken from isolated lakes around the world indicate this decline could be as high as 80 percent since 1970 (see graph below).

Are dioxins found globally?

Yes. Dioxins have been identified as contaminants worldwide.


How am I exposed to dioxins?

Most human exposure to dioxins comes from what are called "secondary" exposure pathways, that is, through the ingestion of meat, fish or dairy products containing small amounts of dioxins. According to EPA's draft dioxin reassessment, because dioxin-like chemicals are persistent and accumulate in biological tissues, particularly in animal tissues, the major route of human exposure is through ingestion of foods containing minute quantities of dioxin-like compounds. Indeed, scientists in Japan, North America and Europe report that food-mainly animal products such as fish, meat and dairy products-is the source of 90 percent of human levels of dioxins.

Obviously, the exact source of the dioxins in the human body is influenced by individual dietary habits. In commenting on the agency's draft dioxin report in May, 1994, Lynn Goldman, M.D., Assistant Administrator of EPA's Office of Prevention, Pesticides and Toxic Substances, said, "It should be stressed that the benefits of a balanced, nutritional diet far outweigh any risk from dioxin to the general population."

Human exposures to dioxins resulting from "primary" exposure pathways-that is, by direct skin contact, inhalation and water and soil ingestion-are very small.

Have I been exposed to dioxins?

Based on numerous published articles about human exposure to dioxins, virtually everyone in the world has some small amount of dioxins in his or her body. Human fat concentrations of 2,3,7,8-TCDD throughout the world have been reported in the range of 5 to 15 parts in 1,000,000,000,000 parts of body fat-that is, between 5 and 15 parts per trillion.

What happens to dioxins once they get into my body?

Once dioxins enter the human body, a small amount is metabolized and eliminated, while the rest is stored in body fat. As fat is burned during normal metabolic processes, some of the stored dioxins will be mobilized, metabolized and excreted in feces. If this elimination is faster than intake through food, then total dioxin levels in the body will decrease. Just as dioxin levels in the environment are on the decline, it appears that the dioxin levels in humans are also decreasing.


What do we know about the health effects of dioxins in the general population?

Only a few studies have been conducted to evaluate the health effects of dioxins on the general population. Based on existing information, many scientists do not expect dioxins at the levels derived from current sources to cause adverse health effects. In 1994, Dr. Genevieve Matanoski, a physician and epidemiologist at Johns Hopkins School of Hygiene and Public Health, noted when analyzing EPA's report on dioxin, "The estimates of risk state that there may be one cancer caused by dioxin in 10,000 people exposed to the chemical. This number is based on mathematical models, which deliberately overestimate the risk. At the other end of the scale, the risk actually could be zero. Again, some people may be more at risk than others." Matanoski is also chair of an independent Science Advisory Board (SAB) to EPA that reviewed the agency's draft dioxin reassessment.

As with any chemical, the potential health effects associated with dioxins are related directly to the level of exposure: the lower the exposure, the less likelihood of adverse health effects. Most research into dioxins' human health effects has focused on people who have received dioxin doses-through occupational or accidental exposures-as much as thousands of times higher than the levels that the general population is estimated to be exposed to from current sources.

What have studies of occupationally and accidentally exposed populations shown about the health effects of dioxins?

Residents of Seveso, Italy, who were exposed to high levels of dioxins following an industrial accident in 1976, showed evidence of a skin disorder known as chloracne and other skin effects, as well as reversible effects on liver enzyme levels. In fact, in an article published in American Family Physician, researchers convened by the Agency for Toxic Substances and Disease Registry (ATSDR) stated that "Chloracne is the only overt clinical sign of dioxin exposure in humans." ATSDR is a branch of the U.S. Department of Health and Human Services' Public Health Service.

Some industrial workers manufacturing chlorophenol or chlorophenol-based pesticides showed evidence of chloracne and other skin effects. A few studies of individuals exposed to high levels of dioxins years ago also have suggested a slight increase in cancer, but these studies are inconclusive. It is unclear whether that increase was due to exposure to dioxins, or to other chemicals that were present in the workplace, or to non-occupational or lifestyle factors, such as smoking.

In addition, other studies of highly exposed populations suggest that there are other non-cancer health effects from dioxin. Evidence of these effects, however, is even more uncertain and debated more widely than the cancer issue.

"Allowable Daily Intake" of Dioxin

The term "allowable daily intake" (or ADI) as used in this figure refers to the amount of dioxin (expressed in picogram per kilogram of body weight per day) an individual can be exposed to which is believed to pose little, if any, risk. This figure presents some examples of an "ADI" for 2,3,7,8-TCDD as calculated by different scientific organizations and regulatory agencies around the world. Note that the "ADI" varies dramatically from country to country and from agency to agency depending on the differences in the method of calculation. For example, the World Health Organization's "ADI" value is 1,000 times greater than EPA's risk-specific dose (RSD) of .01 pg/kg/day. (This RSD level corresponds to a calculated cancer risk of one in a million.)

Sources: Ahlborg, 1992; Appel, 1986; Dutch State Institute of National Health, 1982; Kimbrough, 1984; Ontario Ministry of the Environment, 1985; United Kingdom Department of the Environment, 1989; U.S. Environmental

Are dioxins present in soil, and if so, do they accumulate in plants?

Unless there is an obvious source of contamination, concentrations of dioxins in most soils are quite low.

When dioxins are present in soil, they attach strongly to particles rich in organic matter. Consequently, dioxins are not easily removed from soil particles by water percolating through soil, nor are they likely to move freely in soil. Most studies show that crops or natural vegetation grown in soils with a known dioxins concentration do not accumulate dioxins because dioxins bind so tightly to the soil.

Are there dioxins in the air, and if so, can they deposit on plant surfaces?

Scientists have detected very low levels of dioxins in ambient air. Although dioxins can break down in the atmosphere, some dioxins will attach themselves to particulates and eventually settle either in sediments or soils.

Some evidence suggests that dioxins in the air can collect on the leaves of plants. Animals that eat these leaves can accumulate dioxin in their fat. However, scientists consider the direct ingestion of dioxins which may be present on vegetables and fruits a relatively insignificant source of current human exposure.

What is being done to reduce dioxins in the environment?

Dioxins continue to receive a great deal of attention from regulatory authorities around the world. For instance, the U.S. Environmental Protection Agency is reassessing the risk to human health and the environment due to exposure to dioxins. After review by its external Science Advisory Board and extensive public comment, EPA is revising its initial reassessment document and is expected to release a final draft of the document in Spring, 1997. In addition, industry and government research is still being conducted to better understand the way dioxins act in living systems.

Specific regulations on the operation of combustion facilities will have a significant impact on the generation of dioxin in the United States. EPA has promulgated regulations for municipal waste combustion units and is currently working on promulgating regulations for medical waste and hazardous waste combustors. These proposed emissions limits are expected to reduce dioxin emissions from these sources significantly. The pulp and paper industry, through regulatory compliance and individual company initiative, has reduced its output of dioxins by more than 90 percent. The chlorine industry, through its stewardship efforts, is also working on characterizing and reducing its emissions of dioxins.

To meet new regulations and help achieve industry stewardship goals, technologies are being developed to help identify and further reduce the amount of dioxins released into the environment, as well as to treat dioxin-containing materials and wastes more effectively. Further reductions will occur as new technologies are introduced and implemented.

  Abridged for clarity & relevance