Persistent Organic Pollutants
Persistent organic pollutants (POPs) are chemicals of global concern due to their potential for long-range transport, persistence in the environment, ability to bio-magnify and bio-accumulate in ecosystems, as well as their significant negative effects on human health and the environment.
Primary reference(s)
WHO, 2020. Food safety: Persistent organic pollutants (POPs) World Health Organization (WHO). Food safety: Persistent organic pollutants (POPs) (who.int) Accessed 12 June 2024
Annotations
Additional scientific description
The most commonly encountered POPs are organochlorine pesticides, such as DDT, industrial chemicals, polychlorinated biphenyls (PCB) as well as unintentional by-products of many industrial processes, especially polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF), commonly known as dioxins.
Humans are exposed to these chemicals in a variety of ways: mainly through the food we eat (HIP CH0601), but also through the air we breathe, and through water, in the outdoors, indoors and at the workplace. Many products used in our daily lives may contain POPs, which have been added to improve product characteristics, such as flame retardants or surfactants. As a result, POPs can be found virtually everywhere on our planet in measurable concentrations.
POPs bio-magnify throughout the food chain and bio-accumulate in organisms. High concentrations of POPs are thus found in organisms at the top of the food chain. Consequently, trace levels of POPs can be found in the human body.
Human exposure - for some compounds and scenarios, even to low levels of POPs - can lead to many health effects including increased cancer risk, reproductive disorders, alteration of the immune system, neurobehavioural impairment, endocrine disruption, genotoxicity and increased birth defects.
Dioxins (HIP CH0502) are one of the most common classes of POPs . Dioxins and dioxin-like substances' are three- or two-ring structures chlorinated to varying degrees. Dioxin is also used as the colloquial (short) name of the toxic by-product (and sometimes contaminant) of herbicides: 2,3,7,8-tetrachlorodibenzo[b,e][1,4]-dioxin (IUPAC 2014). 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-tetrachlorodibenzo[b,e][1,4]-dioxin (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 environmentally circulating POPs, 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).
Another common example of POPs are organochlorine pesticides such as DDT (HIPs CH0501, CH0502),. Half a million tonnes of obsolete pesticides are scattered throughout the developing world. These toxic chemicals, often stored outdoors in leaking containers, are seeping into the soil and water. Eliminating these dangerous stocks is a development priority. Rural communities cannot hope to develop if the soil and water are contaminated with pesticides. People cannot hope to prosper if they are suffering from severe illnesses caused by pesticide poisoning. The Food and Agriculture Organization of the United Nations (FAO) Programme on the Prevention and Disposal of Obsolete Pesticides is working to inform the world about the dangers of obsolete pesticide stocks. It collaborates with developing countries to prevent more obsolete pesticides from accumulating and assists them in disposing of their existing stockpiles (FAO, 1999).
POPs are found in many obsolete pesticides, including insecticides, fungicides, herbicides, larvicides, acaricides, rodenticides, molluscides, nematocides, and aphicides. Around a thousand active ingredients are used to manufacture the wide array of pesticides in countries all over the world. Pesticide ingredients come in many thousands of different formulations. All these formulations degrade over time although some are more persistent in the environment than others. The chemical by-products that form as the pesticide deteriorates can be more toxic than the original product.
Once pesticides enter soil, their fate depends on the physico-chemical characteristics of the soil, such as moisture, texture, and soil organic matter content, as well as the pesticide properties. A relatively small amount of spilled pesticides can therefore create a much larger volume of polluted soil. For example, approximately 30 tonnes of pesticides buried at a site in Yemen in the 1980s contaminated over 1500 tonnes of soil. This can pose a serious health and environmental threat to nearby communities (FAO, 2000)
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).
The TEFs were updated in 2022 (WHO, 2023; DeVito, 2024).
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.
United Nations (2015) Sustainable Development Goals. Preventing soil pollution could reduce soil degradation, increase food security, contribute substantially to the adaptation and mitigation of climate change, and contribute to the avoidance of conflict and migration. Therefore, taking immediate actions to combat soil pollution contributes to the achievement of almost all Sustainable Development Goals (SDGs) (FAO, 2018).
The Food and Agriculture Organization of the United Nations (FAO) Programme on the Prevention and Disposal of Obsolete Pesticides is working to inform the world about the dangers of obsolete pesticide stocks. It collaborates with developing countries to prevent more obsolete pesticides from accumulating and assists them in disposing of their existing stockpiles (FAO, 2025)
Drivers
Agricultural Sector: The agricultural sector is a major source of POPs. Pesticides and herbicides used in farming contribute significantly to POPs contamination.
Manufacturing Industries: Secondary sources include manufacturing industries. Chemical production, especially older processes, can release POPs into the environment1.
Recycling and Thermal Processes: Recycling and thermal processes (such as waste incineration) can release POPs when handling materials containing these chemicals.
Improper Waste Disposal: Ill-planned waste disposal, including landfill sites, can lead to POPs contamination of soil and water.
Reservoirs: Reservoirs, such as large bodies of water, can accumulate POPs over time.
Impacts
Impacts of POPs cause direct or indirect impacts on the environment due to interactions among biotic and abiotic elements. Their contamination disrupts ecological systems and threatens the environmental integrity and health of living organisms. The global production of these pesticides started to decline in the 1980s in response to increasing concerns about negative impacts on environmental and human health, mainly because POPs are resistant to degradation, bioaccumulate in food chains, and have adverse effects on ecosystems and human health.
The wide use of PCBs in electrical equipment such as transformers means that any events (weather, geological, technological) that damage electrical facilities may release such molecules. Industrial sites where PCDDs and PCDFs are produced (for example as by-products) may be contaminated and need remediation. Accidents at chemical plants or during the transport of hazardous materials can result in the sudden release of POPs. Use of certain pesticides can introduce POPs into the environment, which can be further spread by natural and anthropogenic processes. Poorly managed landfills and improper disposal of electronic waste can lead to the leaching of POPs into soil and groundwater, leading to contamination of food and feed.
Multi-hazard context
The figure below summarises common interactions between persistent organic pollutants and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.
Multi-hazard diagram
Risk Management
To address the risks associated with these substances, several steps have been taken to reduce or eliminate emissions of dioxins, dioxin-like PCBs and other related persistent organic pollutants. Many countries have now implemented the Stockholm Convention on POPs (2001), which suggests ending commercial use of 12 POPs and reduction or elimination of their emission into the environment. To monitor reduction in human exposure WHO and UNEP run a monitoring program for POPs in breastmilk. A steady decline in levels has been observed since 1980 for most countries.
In regard to food contamination, the FAO/WHO Codex Alimentarius Commission, which is a risk management body comprised of 169 member countries, is developing a draft code of practice for dioxins and dioxin-like PCBs in food, which identifies source-directed measures to reduce their presence in food, including fish, as well as a position paper, which provides an evaluation of the need for possible regulatory measures, such as limits in food and feed.
FAO and WHO consider fish to be an important component of a nutritious diet, and that the risk of consuming contaminated fish must be weighted in view of the beneficial nutritive effects of fish. FAO and WHO plan to develop general guidance for such risk-benefit considerations, with the contamination of fish as case studies. Dioxins and dioxin-like substances exposure occurs in the following ways (WHO, 2019):
- Associated with natural hazards: Dioxins 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 dioxin- like 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.
Monitoring
The section and the table below offer an overview of monitoring persistent organic pollutants. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
Which institution(s) produce(s) Disaster Risk Data/Information? | Global Monitoring Plan (GMP) under the Stockholm Convention Aims to provide a harmonized framework for monitoring the presence of POPs in the environment and human populations globally. European Monitoring and Evaluation Programme (EMEP) Focuses on monitoring air pollution, including POPs, across Europe to assess transboundary air pollution and support policymaking. Integrated Atmospheric Deposition Network (IADN) A U.S.-Canada initiative to monitor the deposition of toxic substances, including POPs, in the Great Lakes region. |
| How is the Hazard Observed/Monitored/Forecast? | The hazard posed by persistent organic pollutants (POPs) is monitored and forecasted through a combination of environmental sampling, predictive models, and real-time monitoring systems. Monitoring efforts are globally coordinated through organizations like UNEP, WHO, and national agencies, with data being shared internationally through frameworks such as the Stockholm Convention. These systems help assess current risks, forecast future trends, and issue early warnings in response to contamination events. The integration of meteorological, environmental, and health data plays a critical role in understanding and mitigating the long-term impact of POPs on both ecosystems and human health. |
References
DeVito, M., Bokkers, B., van Duursen, M. B. M., van Ede, K., Feeley, M., Fernandes Gaspar, E. A. Haws, L., Kennedy, S., Peterson, R. E., Hoogenboom, R., Nohara, K., Petersen, K., Rider, C., Rose, M., Safe, S., Schrenk, D., Wheeler, M. W., Wikoff, D. S., Zhao, B., van den Berg, M., 2024. The 2022 world health organization reevaluation of human and mammalian toxic equivalency factors for polychlorinated dioxins, dibenzofurans and biphenyls, Regulatory Toxicology and Pharmacology, 146 (2024) 105525. Accessed 12 May 2024.
FAO, 2000. Assessing Soil Contamination: A reference manual. FAO Pesticide Disposal Series 8. Food and Agriculture Organization of the United Nations (FAO). Accessed 12 June 2024.
FAO. 2025. Terminal evaluation of the project “Strengthening the Environmentally Sound Management of Pesticides, Including Persistent Organic Pollutants” – Project code: GCP/URU/031/GFF, GEF ID: 5144. Project Evaluation Series, No. 31/2024. Rome. Accessed 30 April 2025
Global Monitoring Plan for Persistent Organic Pollutants Under the Stockholm Convention Article 16 on Effectiveness Evaluation. Third Regional Monitoring Report of the Central, Eastern European and Central Asian Region. Stockholm Convention Regional Centre, RECETOX, Masaryk University, Brno, Czech Republic, March 2021
IUPAC, 2014. 'dioxin' in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. Accessed 12 June 2024.
UNECE, 2023. Globally Harmonised System (GHS) of Classification and Labelling of Chemicals (2023). United Nations Economic Commission for Europe (UNECE). Accessed 11 May 2024.
UNEP, 2001. Stockholm Convention on Persistent Organic Pollutants. UN Environment Programme (UNEP). Accessed 12 June 2024.
WHO, 2019. Preventing disease through healthy environments: Exposure to dioxins and dioxin-like substances: A major public health concern. World Health Organization (WHO). Accessed 12 June 2024.
WHO, 2023, WHO Fact Sheets Dioxins. Dioxins (who.int). Accessed 12 May 2024