Persistent Organic Pollutants | The Canadian Encyclopedia


Persistent Organic Pollutants

Persistent organic pollutants (POPs) are carbon-based chemical compounds or groups of chemical compounds of anthropogenic (resulting from human activities) origin that are biologically and chemically inert.

Persistent Organic Pollutants

Persistent organic pollutants (POPs) are carbon-based chemical compounds or groups of chemical compounds of anthropogenic (resulting from human activities) origin that are biologically and chemically inert. Their inertness enables them to become widely distributed around the globe, persist in the ENVIRONMENT, and bioaccumulate in fatty tissues up food webs (the food chain). Most POPs accumulate in the polar regions by atmospheric transport as a result of global temperature differences. Health effects due to excessive exposure can include nausea, anorexia, jaundice, vertigo, cancer and death in extreme cases.

There are many thousands of compounds that fit the above definition. A number of these are currently targeted for reduction or elimination by the Stockholm Agreement, an international treaty formed as a result of a meeting in Stockholm, Sweden in 1972. The current agreement lists 24 chemicals or groups. The original 12 are all chlorinated organic compounds and include pesticides, industrial chemicals and unwanted by-products. The second group, added in May 2009, includes additional chlorinated pesticides, brominated flame retardants (BFRs) and perfluorinated compounds (PFCs). A third group includes compounds currently under review: chlorinated and brominated flame retardants and a chlorinated pesticide. The Stockholm Agreement was ratified on 17 May 2004. Countries signing the agreement are committed to reducing and eliminating levels of POPs in humans and the environment.

A number of POPs are endocrine disrupting compounds (EDCs) - hormone-mimicking compounds that can effect or disrupt the body's normal functions. EDCs can affect the thyroid system, which can result in abnormal sexual development, learning disabilities and impairment of motor skill development. EDCs have also been connected with attention-deficit hyperactivity disorder (ADHD) and autism.


Organochlorine PESTICIDES have been used since the early 1940s when DDT was used to control mosquitoes for MALARIA and LICE for typhus. The use of pesticides increased exponentially in the 1940s and 50s in order to increase the quality of produce and crop yields. A number of other pesticides were also developed, including chlordane, aldrin, dieldrin, endrin and toxaphene, and by the late 1950s their usage became extensive. In the mid 1950s the reproduction of salmon in the MIRAMICHI RIVER was almost eliminated due to the spraying of nearby forests with DDT for the control of spruce budworm (seeINSECT PESTS). Pesticide use in the United States in the early 1960s resulted in startling effects on wildlife including great numbers of bird deaths resulting from the poisoning of their diet (insects and earthworms).

Silent Spring, published in 1962 by Rachel Carson, described the effects of pesticides. Carson is considered the originator of the environmental movement and the title of her book was a warning that birds could eventually disappear if the use of chlorinated pesticides continued to increase. Due to the efforts of Carson to bring awareness of the effects of pesticides on humans and the environment, all of the chlorinated pesticides except DDT, which could still be used for mosquito control in tropical areas, were banned by the mid 1970s.

Industrial Chemicals

Hexachlorobenzene (HCB) and polychlorinated biphenyls (PCBs) are 2 types of industrial chemicals. PCBs are a group of 209 compounds with 2 chlorinated 6-member carbon rings connected by a carbon-carbon bond. Their toxicity is dependent on the pattern of chlorine substitution on the carbon rings. A subgroup of 12 PCBs has dioxin character and toxicity.

PCB contamination in the environment was discovered by accident. During the analysis of environmental samples for pesticides, numerous additional unidentified compounds originally thought to be pesticide degradation products were observed in the pesticide scans of fish and sediments in the late 1950s. It took more than 5 years to identify and confirm that these unknown compounds were PCBs. PCBs were manufactured and used as heat transfer fluids, flame retardants, dielectrics, plasticizers in paints and plastics, and in carbonless copy paper from 1929 to the late 1970s, when they were banned.

Due to poor industrial practices and their use as synergistic agents for pesticides, many thousands of tonnes of PCBs have entered the environment. They are regularly detected in soils, sediments, wildlife and humans. PCBs can no longer be used in consumer products but are still present in older transformers, fluorescent light ballasts, paints, polymers, sealants and caulking materials. More environmentally conscious industrial practices and programs to destroy, replace or seal PCB contaminated products and materials have resulted in their reduction in the environment. Unfortunately, there are still numerous contaminated sites and the rate of decline in the environment is levelling off. HCB was used as a fungicide for seeds and a precursor for syntheses of other chemicals such as pentachlorophenol. It is considered a carcinogen and its use in Canada was discontinued in 1972.

Unwanted By-products

Unwanted by-products are POPs formed through a variety of different sources and include polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCBs and HCB were made for industrial purposes, but can also be formed through anthropogenic sources.

PCDDs and PCDFs are the most toxic of the POPs. Seventeen of them are highly toxic while the remaining 193 are not toxic at all. A compound is dioxinlike if it reacts (binds) with the aryl hydrocarbon receptor (AhR). This can result in a number of toxic effects including loss of body weight, immune impairment reproductive disorders, development toxicity and cancer.

Since dioxinlike compounds (DLCs) work through a common mechanism, the degree of their toxicological potency can be determined and compared through a relative toxicity scheme normalized to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-T4CDD), the most toxic dioxin. The toxic equivalent factor (TEF) is a value assigned to a specific congener representative of its relative potency compared to 2,3,7,8-T4CDD. The sum of the concentration of each individual dioxinlike compound multiplied by its TEF is the toxic equivalent quantity (TEQ), and this is the value of an equivalent dioxin (often referred to as the generic name for the group of PCDDs, PCDFs and other DLCs) concentration of the DLC compounds in the sample reported as 2,3,7,8-T4CDD equivalents.

PCDD and PCDFs are impurities in herbicides (eg, Agent Orange); are formed as products of incomplete combustion; and are by-products from pulp and paper bleaching processes. Agent Orange is now banned, paper production processes have been altered to use chlorine dioxide eliminating dioxin formation, and combustion and incineration processes have been modified to minimize dioxin formation.

Additional Compounds

Compounds added to the Stockholm Convention list in 2009 included several additional pesticides: chlordecone, a-hexachlorocyclohexane, ß-hexachlorocyclohexane, and lindane (gamma-hexachlorocyclohexane); industrial chemicals: pentachlorobenzene, perfluorooctanesulfonic acid (PFOS), its salts and perfluorooctanesulfonyl fluoride; and flame retardants: polybrominated biphenyls and tetra, penta and hexa brominated diphenylethers. Contrary to the legacy compounds discussed above, which are declining or levelling off in the environment, the environmental levels of many of these recently added compounds are increasing.

PFOS and other perflourinated compounds (PFCs) are used in a wide variety of applications including polymers like Teflon® and Viton®, surface treatment and surfactant formulations, fire-fighting foams (aqueous film forming foams, AFFF), paper coatings, stain repellants, cleaners and waxes, mining surfactants and insecticides. Many PFCs are highly stable and inert or degrade to highly stable and inert compounds. They were long considered to be nontoxic because of their highly inert properties and stability towards chemical and physical stress. However, over the last decade, studies indicating that many PFCs as well as their breakdown products can be toxic and persistent, and can bioaccumulate and undergo long-range global transport have been published.

Millions of tonnes of halogenated flame retardants have been produced since the early 1970s with the poly brominated diphenylethers (PBDEs) being one of the largest amounts. Flame retardants are an important chemical in our current society and the regulated use of them has significantly reduced fire-related deaths. In the late 1990s, brominated (39%) and chlorinated (23%) retardants accounted for the highest and second highest amounts of flame retardants produced. Halogenated flame retardants are the most effective, and therefore smaller amounts can be used than of non-halogenated flame retardants.

The major concern with PBDEs is that they can degrade to brominated dioxins, which exhibit similar toxicity to their chlorinated analogues. Many new flame retardants that do not form dioxins have been developed to replace PBDEs. Unfortunately, halogenated flame retardants are much more compatible and durable with plastics and polymers than classic flame retardants like asbestos and alum (aluminum sulphate), and there are millions of tonnes in use in consumer products, so that building fires are a significant source of halogenated dioxins and related compounds.


There are many thousands of chemicals used today. A significant number of them are POPs and in Canada, they are regulated under the Canadian Environmental Protection Act (CEPA). Other countries and the European Community have their own legislation. Globally, the Stockholm Agreement regulates POPs.