Persistent Organic Pollutants (POPs)  
What are POPs What are POPs Sources of POPs POPs Pestisides PCBs Dioxin an Furan Effects of POPs
The term "Persistent Organic Pollutants" or POPs is used to describe a class of toxic chemical substances that can harm human health and environment. POPs are long-lasting toxic substances that are produced and released into the environment by human activity. Some POPs are produced for use as pesticides; some are for use as industrial chemicals; and some are produced as unwanted byproducts of certain chemical and/or combustion processes.

What are the POPs

The Stockholm Convention identifies 12 substances as POPs, they are -Aldrin, Chlordane, DDT, Dieldrin, Dioxins, Furans, Endrin, Heptachlor, Hexachlorobenzene, Mirex, Toxaphane and Polychlorinated Biphenyls (PCBs). POPs are chemical substances having the properties:

4 Produced and mobilized into the environment as a result of human activity, i.e. they are anthropogenic
4 otential to cause harm to human health and/or to the environment, i.e. do adverse effects
4 Long life in the environment and not easily or quickly broken down when they are in air, in water, in soil, in sediments and/or in biota, i.e. they are persistent in environment
4 Become concentrated in the environment to levels of concern under circumstances where species accumulate POPs by eating smaller POPs contaminated organisms, i.e. undergo bioaccumulation
4 Travel long distances in the environment (through air, water or migratory species), and accumulate at locations that are distant from the sources of release

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Sources of POPs

Among these 12 POPs, some have been used as pesticides, some as industrial chemicals and some arise as unintentional by-products of chemical and/or combustion processes. To some extend these categories overlap. For example PCBs were produced as industrial chemicals in large quantities, are also generated as unintentional by-products, hexachlorobenzene fits into all three categories; pesticides, industrial chemicals and unintentional byproduct.

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POPs Pesticides

There are 9 pesticides namely Aldrin, Chlordane, Dieldrin, Endrin, Heptachlor, Mirex, Toxaphone, DDT and Hexachlorbenzene (HCB).
Specific uses of these pesticides are listed below:

  Name of pesticide

Specific uses

Used to control soil insects such as termites, corn rootworm, wireworms, rice water weevil, grass hoppers etc. and also to protect wooden structures from termites.
IARC - Group 3 (not classifiable as to be carcinogenic in humans)
Used as insecticides on agricultural crops including vegetables, small grains, maize, oilseeds, potatoes, sugarcane, sugar beets, fruits, nuts, cotton and jute, and also to control termites
IARC - Group 2B
(possibly carcinogenic to humans)
Used for the control of soil insects. IARC -Group 3
(not classifiable as to be carcinogenic in humans)
 Used on field crops such as cotton, jute and grains as insecticides. It has also been used as rodenticide to control mice.  IARC -Group 3
(not classifiable as to be carcinogenic in humans)
 Used against soil insects cotton insects, grasshoppers, crop pests and against termites. IARC -Group 2B
(possibly carcinogenic to humans)
Has been used against ants and to control leaf cutters, harvester termites, mealy bug etc. It has also been used as a fire retardant in plastics, rubber, paint paper and electrical goods.  IARC -Group 2B
(possibly carcinogenic to humans)

It is also an insecticide, intensively used for vector control in malaria eradication programme. During the second world war to protect the troops and civilians from disease. Continues production in many countries for public health purposes. IARC -Group 2B
(possibly carcinogenic to humans)

Hexachlorobenzene (HCB)
Used as pesticide. It has been widely used as fungicide for seed treatment. It is also produced as an unintentional by-product in chemical industries and is present as an impurity, and in combustion process too.  IARC -Group 2B
(possibly carcinogenic to humans)

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POPs - Polychlorinated biphenyl (PCBs)

Polychlorinated biphenyls (PCBs) are a class of chlorinated hydrocarbons that have been widely used as industrial chemicals since 1930. There are 209 varieties PCBs and most commercial PCB applications are in the form of mixtures of varieties. Large quantities of PCBs were produced for use as a cooling and dielectric fluid in electric transformers and in large capacitors. These have also been widely used as hydroulic fluids and as a heat exchange fluids. PCBs applications have included use as a sealant, as paint additive, additive in some plastics, as a component of carbonless copy papers etc. These can also be formed and released as unwanted byproducts in some chemical and combustion processes.

PCBs are linked to reproductive failure and suppression of the immune system in various wild animals, severe human intoxication occurred due to accidental consumption of PCB-containing oils. IARC classified PCB into Group 2B (possibly carcinogenic to human)
International production of PCBs were ended in most countries by 1980. The major exception to this was in the former soviet Union and some Central European countries. No country presently reports intentional PCB production.

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POPs - Dioxins and Furans

Dioxin and Furans are class of chlorinated hydrocarbons. They have never been produced commercially or intentionally except in small qualities for laboratory purposes and or as reference standards. There are 75 different dioxin congeners and 135 different furan congeners. IARC classified one congner of dioxin as group 1 carcinogen (human carcinogen); all others are carcinogenic in animals. Non carcinogenic effects on the immune, the reproductive, the developmental and the nervous systems are considered to be of great concern.

Dioxins and furans are generated as unwanted by-products in a variety of combustion and chemical process. The major sources include waste incinerators combusting municipal waste, hazardous waste, medical waste, sewage sludge etc. Incineration of medical wastes in small and poorly controlled incinerators was found to be a major source of dioxin and furans. Kilns firing of cement/tiles industries, open burning of wastes etc. may also generate dioxins and furans. Other dioxin and furan sources are: pulp and paper mills using chlorine bleach processes, certain thermal processes in metallurgic industry and chemical production process.

Though dioxins and furans are formed as byproducts in a wide range of processes, they are directly dispersed to the environment may also be present in manufacturing processes as raw materials or products.

Dioxins and furans are persistent in the environment and transfers can occur between media, e.g. from air to water through rain water and by run off from soil to water reservoir. This type of transfer may also make an important contribution to human exposure to these organic compounds.

Direct release or transfer of dioxins and furans follow in to the following five components mainly:
4 Air
4 Water (fresh, marine, estuarine; then subsequently into sediments)
4 Land (soil)
4 Waste (including liquid, sludge, and solid residues which are handled and disposed of as waste or recycled)
4 Products (such as chemical formulations ar consumer goods such as paper, textiles, ceramics etc.).

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Toxicity of dioxin

It is said that dioxins are “more toxic than cyanide and the most toxic of the manmade chemicals” . Toxicity of dioxin overall is expressed as Toxic Equivalents (TEQs). Internationally, FAO and WHO Expert Committee on Food Additives decided to express the tolerable intake of dioxins as a monthly value, the Provisional Tolerable Monthly Intake (PTMI). Recommended PTMI is 70 pg dioxin toxic equivalent (TEQ)/kg body weight. The Tolerable Daily Intake of dioxins in Japan was set at 4 pg in 1999.


Effects of POPs on Human Health and Environment

The harmful effects of POPs on human health and the environment was unknown until the 1960s. As a result the use of these compounds were encouraged until the scientists have developed a large body of evidence associating POPs to do injuries to human health and to the environment. The first evidence pointed by the researcher is the population declines in wildlife due to POPs. The scientists observed a range of health injuries in birds, fishes and mammals. Wildlife species exposed to POPs in the environment may exhibit any of a number of different kinds of injury. These includes :

4 Reproduction failure and population declines;
4 Abnormal functioning of thyroids and other hormone system;
4 Feminization of males and masculization of females;
4 Immune system becomes weaker;
4 Abnormalities in behavior;
4 Tumors and cancer;
4 Birth defects;

Human exposure to POPs are either directly from the environment a occupational and through food chain. Some groups of people like farmers and workers are exposed to POPs in their occupations. POPs can be found in the blood, muscles and other tissues particularly in fatty tissues of the general human population in all over the world. Research on the ways POPs impact on human health is continuing.

Good evidences are there that human exposure to specific POPs or classes of POPs may cause following injuries:

4 Cancers and tumors at multiple sites;
4 Neurobehavioral impairment including learning disorders, reduced performance on standard tests and changes in temperament;
4 Immune system weaker;
4 Reproductive deficits and sex-linked disorders;
4 Shortened period of location in nursing mothers;
4 Diseases such as endometriosis (a painful, chronic gynecological disorder in which uterine tissues are affected), increased incidence of diabetes and others.

The evidence suggests that women, infants, and children are especially vulnerable to certain effects of POPs. POPs are transferred from mother to the child both prenatal and postnatal, ultimately causing harm to the developing infant.
For more than 2 decades awareness has grown about the toxic effects to human health and the environment caused by certain chemicals that persist for decades in the environment, travel great distances through various media from their point of origin, bio-accumulate in humans and other organisms, and are bio-concentrated through food chains. The evidence of detrimental effects on living organisms at the population level demonstrates that POPs are a thread to biodiversity and even have potential for disruption at the ecosystem level.

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