The term “dioxins”, commonly covering polychlorinated dibenzo-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs), refers to a group of chlorinated organic chemicals with similar chemical structures.
Chlorine atoms can be attached to 8 different places on the molecule, numbered from 1 to 8. Dioxins can have varying harmful health effects depending on the number and position of the chlorine atoms. 2,3,7,8-TCDD or simply TCDD, a molecule with 4 chlorine atoms, is one of the two most toxic dioxins. Only dioxins having more chlorine atoms added to the 2,3,7,8-TCDD structure are also toxic, but to a lesser extent. Other dioxins do not show this dioxin-type toxicity.
Chlorinated chemicals with comparable structural and biochemical properties, such as certain polychlorinated biphenyls (PCB), are called “dioxin-like compounds” and can act similarly in terms of dioxin-type toxicity.
Dioxins are almost insoluble in water but have a very high affinity for lipids (fat). They also tend to associate with organic matter such as ash, soil and plant leaves.
Dioxin is not produced intentionally, other than for research purposes, such as to prepare standard material for analysis. Dioxins are by-products generated from processes when heat is applied to substances containing carbon, oxygen, hydrogen and chlorine.
The main source of dioxins at present is waste incineration, with most being generated in combustion processes and released to the ambient air without being fully captured by waste-gas treatment equipment. Other sources exist, such as emissions from electric steel-making furnaces, cigarette smoke, and automobile exhaust. Some reports indicate that dioxins may have accumulated in sediment in the environment due to the past use of PCBs and some types of agricultural chemicals, which contained dioxins as impurities.
The behavior of dioxins in the environment is not fully known. Taking the atmospheric pathway, for example, dioxins in the air are associated with particulate matter and fall to the ground, contaminating soil and water. It is thought that over long periods of time these dioxins, together with those released into the environment via other pathways, ultimately accumulate in aquatic sediments and enter the food chain when ingested by plankton and fish, thereby starting to concentrate in organisms.
Sources of exposure to dioxins and dioxin-like substances
Industrial processes and natural events
PCDDs and PCDFs are by-products of industrial processes, including the manufacture of chlorophenols and phenoxy herbicides, chlorine bleaching of paper pulp and smelting. They can also be generated by natural events, such as volcanic eruptions and forest fires. PCBs were previously manufactured for use as dielectric fluids (with low electrical conductivity) in larger-scale electrical products such as transformers and capacitors, in heat transfer and hydraulic systems and in industrial oils and lubricants. PCDFs were common contaminants of commercial PCB mixtures.
Food, water and air
Generally, levels of PCBs, PCDDs and PCDFs in air are very low, except in the vicinity of inefficient incinerators. Concentrations of these compounds in drinking-water and surface water are also very low, because they are poorly soluble in water. Releases to air from inadequate incineration and releases from waste sites contaminate soil and aquatic sediments, leading to bio-accumulation and bio-concentration through food-chains. The higher chlorinated components and components with specific positions of chlorination persist longer in the environment and show greater bio-accumulation. The substances have high fat solubility, which may lead to higher concentrations in fatty foods, such as dairy products, some fish, meat and shellfish. Most human exposure is through ingestion of contaminated food. These compounds persist in fatty tissue, with typical half-lives in humans in excess of 7 years.
Any source of organic materials in the presence of chlorine or other halogens will generate dioxins and furans during combustion. PCDDs and PCDFs are generated through the incineration of waste (domestic, industrial and hospital) at low to moderate temperatures; guidance has been developed to identify and quantify releases from various incineration processes. The use of modern incineration technology destroys dioxins and furans, whereas inadequate incineration creates them. Disposal of electrical equipment may release PCBs (and PCDF contaminants); guidance is available on equipment likely to contain PCBs. Stockpiles of old industrial lubricants containing PCBs are also a potential source of emissions.
Provisional tolerable monthly intake
In 2002, the Joint Food and Agriculture Organization of the United Nations (FAO)/WHO Expert Committee on Food Additives (JECFA) established a provisional tolerable intake of 70 pg/kg body weight per month for PCDDs, PCDFs and coplanar PCBs expressed as TEFs, based on reproductive end-points. The value is expressed “per month” to reflect that exposure is cumulative and chronic rather than acute.
No water quality guidelines have been set for these substances because of their low water solubility.
An air quality guideline for PCBs was not established, because direct inhalation exposures constitute only a small proportion of the total exposure, in the order of 1–2% of the daily intake from food. Although this air concentration is only a minor contributor to direct human exposure, it is a major contributor to contamination of the food-chain
Short-term exposure to high levels of dioxins and dioxin-like substances in occupational settings or following industrial accidents may cause skin lesions known as chloracne, which is persistent.
Longer-term environmental exposure causes a range of toxicity, including immunotoxicity, developmental and neuro-developmental effects, and effects on thyroid and steroid hormones and reproductive function. The most sensitive life stage is considered to be the fetus or neonate. Guidance values have been based on reproductive and developmental effects.
Experimental animal studies indicate carcinogenicity in a range of species with multiple sites of tumours. Epidemiological studies in occupational settings also indicate human carcinogenicity at multiple sites. The International Agency for Research on Cancer (IARC) classified TCDD in Group 1 (carcinogenic to humans) and some other dioxins in Group 3 (not classifiable as to their carcinogenicity to humans). PCBs as a group are classified in Group 2A (probably carcinogenic to humans). In addition, IARC recently classified 2,3,4,7,8-pentachlorodibenzofuran and 3,3′,4,4′,5-pentachlorobiphenyl in Group 1.13
These substances are not genotoxic carcinogens. It is considered that the mechanism of carcinogenesis, involving the aryl hydrocarbon receptor, means that there is a threshold for carcinogenicity. Tolerable intake guidance based on non-cancer endpoints is considered protective for carcinogenicity.
How do dioxins act on living organisms?
The toxic dioxins can alter key biochemical and cellular functions by interacting with a cellular receptor called Ah, affecting the hormonal system and the way cells grow and develop. The mechanism of action of dioxins appears to be the same in both humans and animals.
Being highly lipophilic, dioxins dissolve in fat. They need to be transformed in the liver to become water soluble before they can be excreted. However, dioxins are metabolized slowly and therefore tend to bio-accumulate, especially in fat and in the liver.
The speed of elimination of dioxins can vary with dose, quantity of body fat, age and sex. The process of elimination of dioxins and PCBs is similar in animals and man, but it is faster in most other mammals. Rodents only reach the same body burdens, or tissue concentration, at much higher exposure compared to humans. This is why body burden must be used as a reference when comparing risks for humans and animals.
The biological half-life, which technically characterizes the speed of elimination, varies largely for the various dioxins and dioxin-like compounds. However the average half-life of 2,3,7,8-TCDD is being used for practical purposes.