Dioxins and Dioxin-like Substances

Unique identifier / Notation

CH0013

Synonyms

Not applicable.

Dioxins and dioxin-like substances, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are persistent organic pollutants (POPs) and are unwanted by-products of combustion and various industrial processes, such as chlorine bleaching of paper pulp and smelting. They can travel long distances from the source of emission, and bioaccumulate in food chains These substances represent a major public health concern. They have been associated with a range of acute and long-term adverse health effects and diseases (WHO, 2019).

Primary reference(s)

WHO, 2019. Preventing disease through healthy environments: Exposure to dioxins and dioxin-like substances: A major public health concern. World Health Organization (WHO). Accessed 25 November 2019.

Additional scientific description

Dioxins and dioxin-like substances’ are three- or two-ring structures chlorinated to varying degrees. Polychlorinated biphenyls (PCBs) can have up to 10 chlorine atoms substituting for hydrogen atoms, and polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) can have up to eight. The compounds tend to have similar toxicity profiles and common mechanisms of action and are generally considered together as a group to set guidelines (WHO, 2019).

PCDDs and PCDFs are widely present in the environment, occurring naturally, but mainly as unwanted by-products of combustion and of various industrial processes. 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) was a contaminant of a herbicide (2,4,5-T) and chlorodibenzofurans (CDFs) were major contaminants of PCBs, but neither PCDDs nor PCDFs have ever been manufactured or used for commercial purposes other than for scientific research. PCBs were globally manufactured and used in the past (WHO, 2019).

Although PCB manufacture is now prohibited under the Stockholm Convention, release into the environment still occurs from the disposal of large-scale electrical equipment and waste, from metallurgical uses, and from some chemical manufacture and processing (WHO, 2019).

Human exposure to dioxins and dioxin-like substances has been associated with a range of toxic effects, including chloracne; reproductive, developmental and neurodevelopmental effects; immunotoxicity; and effects on thyroid hormones, liver and tooth development. Dioxins are also carcinogenic. Developmental effects are the most sensitive human health endpoint, making children – particularly breastfed infants – a population at elevated risk (WHO, 2019).

Metrics and numeric limits

Toxic equivalency factors (TEFs) have been derived to relate the toxicities of individual PCDDs, PCDFs and PCBs to the most toxic of these compounds: TCDD, which is used as a reference and given a TEF of 1. The common mechanism of action for these substances means that their effects are additive, and TEFs for individual compounds can be used to establish the summed toxicity of a mixture. This approach has proved robust as a method for establishing the relative toxicities of these compounds (WHO, 2019), and has resulted in the development of guideline values as follows:

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.

Air: 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 (WHO, 2019).

Key relevant UN convention / multilateral treaty

The Stockholm Convention on Persistent Organic Pollutants was adopted by the Conference of Plenipotentiaries on 22 May 2001 in Stockholm, Sweden (UNEP, 2001). The Convention entered into force on 17 May 2004. By September 2019, 183 UN member states and the European Union had adopted the Stockholm Convention.

Examples of drivers, outcomes and risk management

Dioxins and dioxin-like substances exposure occurs in the following ways (WHO, 2019):

Associated with natural hazards: Dixons can be generated by natural events, such as volcanic eruptions and forest fires.
Industrial processes: PCDDs and PCDFs are by-products of industrial processes, particularly waste incineration, cement kilns firing hazardous waste, chlorine bleaching of pulp, and thermal processes in the metallurgical industry, as well as the manufacture of chlorophenols and phenoxy herbicides.
Environmental media and food: Dioxin releases into air from inadequate incineration and releases into air, water or soil from industrial and waste sites contaminate soil and aquatic sediments, leading to bioaccumulation and bioconcentration through food chains. Most general population exposure is through ingestion of contaminated foods of animal origin.
Waste disposal: 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 health-care facilities) at low to moderate temperatures; guidance has been developed to identify and quantify releases from various incineration processes.

Actions to reduce emissions of these substances are required by the Stockholm Convention (UNEP, 2001). Interventions to reduce human exposure include: identifying and safely disposing of material containing or likely to generate dioxins and dioxinlike substances, such as electrical equipment; ensuring appropriate combustion practices to reduce emissions; implementing FAO/WHO strategies to reduce contamination in food and feed; and monitoring of food items, human breastmilk and air, as well as exposures in workers likely to be exposed to higher levels.