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Pollutants (POPs)

Persistent organic pollutants (POPs), their health and environmental effects

Alice Thompson

What are POPs?
The movement of POPs
Present global distribution
Health and environmental effects
Further reading


The twentieth century was a time of chemical revolution, and the production of synthetic compounds world-wide escalated from less than 150 000 tonnes in 1935 to more than 150 million tonnes in 1995. In fact, some reports have estimated that by the late 1980s the annual global production of synthetic chemicals was as high as 300 million tonnes, approximately one-third of which is believed to have eventually reached the environment. There are now over 75 000 different synthetic chemicals used in pesticides, pharmaceuticals, plastics, and other industrial and consumer products.

Among the chemical products of industrial society there is a group of substances which have come to be known as Persistent Organic Pollutants, or POPs, which share the characteristic that they are toxic to humans and other living organisms, and persist in the environment for long periods of time. Estimates of the number of substances with this property range from a dozen or so to several hundred.

There is a growing body of evidence that these chemicals are having adverse effects on the health of humans and other organisms, even in regions where they have not been produced or used, and their management is now recognised as a critically important issue for the international community.

What are POPs?

POPs are carbon-containing compounds and they share the following common characteristics: they persist in the environment; they can be transported long distances from their source; and they accumulate in the tissues of living organisms.

POPs compounds are mostly halogenated – that is, they contain chlorine, bromine or fluorine. There are two major sub-groups of POPs: (1) polycyclic aromatic hydrocarbons (2) other hydrocarbons.

An initial list of twelve chemicals has been identified by the United Nations Environment Program (UNEP) for special attention, because there is ample evidence of their serious effects on the environment. They are sometimes referred to as the ‘dirty dozen’(see Box 5.1). Nine of the twelve UNEP listed POPs are pesticides used on agricultural crops and for public health disease vector control. Some POPs are industrial chemicals, while others are unintentional by-products of manufacturing and combustion processes.

Box 1 UNEP’s "Dirty Dozen" POPs listed for international action

The initial twelve POPs






hexachlorobenzene (HCB)1,2,3


polychlorinated biphenyls (PCBs)2,3


polychlorinated dibenzo-p-dioxins (dioxins)3


polychlorinated dibenzofurans (furans)3

1Pesticide chemical 2Industrial chemical 3 By-product


POPs dissolve readily in lipids (fats and oils), but not in water. As a consequence, when they are taken in by living animals, they tend to accumulate in fatty tissues where they can reach concentrations up to 70 000 times higher than in the surrounding environment.

POPs are largely resistant to natural forms of degradation, including photolytic (by light), biological and chemical degradation. This is the reason why some of them persist in the environment for a very long time. They are also semi-volatile, and they have a high degree of mobility through the atmosphere, either as vapour or adsorbed onto atmospheric particles. This means that relatively large amounts of POPs enter the atmosphere, often to be transported over great distances before coming down to earth.

The movement of POPs

The physico-chemical properties of POPs have resulted in their transportation, by evaporation and atmospheric processes, to almost all parts of the world, often far from the area of initial use and release.

The movement and deposition of POPs is affected by local environmental conditions. While there are many uncertainties, it seems that temperature is an especially important factor. The natural processes of degradation (photolytic, biological and chemical), slow as they are, are even less effective in the cooler parts of the world, leading to the net accumulation and persistence of POPs in polar regions. Lower temperatures also encourage the transformation of POPs compounds from the vapour phase to adsorption onto particles in the atmosphere, increasing the likelihood of their deposition on the Earth’s surface in rain or snow.

In more temperate regions of the world the movement of POPs is primarily from the atmosphere to the oceans, while their movement in tropical regions is mainly from the oceans to the atmosphere. The residence time for POPs in tropical aquatic environments is relatively short

Movement of POPs between the northern and southern hemispheres depends partly on the specific properties of the different compounds. For example, because of the high molecular weight and low volatility of DDT and PCBs, movement of these compounds between hemispheres is relatively slow compared to some other POPs, such as HCB, which are more readily transported long distances away from the point source of use.

Present global distribution

While there are many difficulties in attempting to estimate the global distribution of POPs, sufficient information has been gathered to indicate that their levels in organic, aquatic and sediment samples are generally higher in the northern hemisphere than the southern hemisphere. This can be partly explained by the high rates of usage of POPs compounds in developed countries in Europe and North America and in Japan. There has, however, been a significant decline in recent years in POPs concentrations in mid-latitude oceans of the northern hemisphere and in Arctic lake sediments, reflecting changes in patterns of usage.

On the other hand, the contribution of tropical regions to global levels of POPs is currently increasing. Tropical Asia is now a major source, and in the northern hemisphere POPs levels in areas adjacent to tropical countries tend to be higher than in those close to developed countries. This pattern is the result of recent restrictions and bans on DDT and certain other POPs in developed countries, while their use is increasing in many tropical areas.

Although concentrations of POPs are generally higher in the northern hemisphere than in the southern hemisphere, recent measurements in Australia indicate that concentrations of some POPs exceed acceptable levels. For example, unacceptable levels of DDT and dieldrin have been recorded in fish and other aquatic organisms in Australian oceans, and elevated levels of PCB emissions have been reported in Sydney. However, recent work shows that, with these exceptions, current levels of most POPs compounds lie at acceptable levels in Australia. Most contamination in this country has originated from our own usage rather than from global transportation from other areas.


Animals can be exposed to POPs compounds in a number of ways. POPs can be taken up directly by organisms in contact with contaminated water. Chlordane, for example, is taken up directly from water, to be concentrated in the organs of aquatic species like minnows. POPs can also be taken up indirectly through the food chain, and they enter land animals mainly through the consumption of contaminated food.

Data from many countries have shown that POPs have become widely distributed in the food supply of humans. Around 90% of the total human intake of POPs is received through the consumption of foods of animal origin – mostly fatty foods, including meat, fish and dairy products including ice-cream. The use of organochlorine pesticides on agricultural crops is also an important route of exposure. While many toxic pesticides have been banned from use, a significant amount of POPs are still being released into the environment as a result of unauthorised use of certain chemicals on crops.

In Europe and the US, the levels of dioxins in breast milk are several times higher than that which is permitted in cows’ milk. Even higher concentrations of POPs are found in the breast milk of Canadian Inuit, who are among the most exposed people in the world. This is because they sit at the top of the food chain in the Arctic regions where they live, consuming a high fat diet of fish and marine mammals. The breast milk of Inuit women contains 2 to 10 times as much PCBs, and 10 times as much chlordane, as the breast milk of women in southern Canada, even though they are thousands of kilometres from the nearest agricultural area.

While most exposure to POPs is at low levels over long periods of time, humans can also be exposed to high concentrations of POPs compounds accidentally, or through their occupations. People working in agricultural industries are at greatest risk of such exposure, especially in developing countries in tropical areas where the use of pesticides containing POPs has resulted in a large number of deaths and injuries. Exposure to dioxins and furans often occurs in an occupational setting, for example during herbicide production. People can also be exposed to acute levels of these substances through industrial accidents and chemical fires.

Another source of high level, acute exposure to POPs is through the contamination of food. PCBs have been linked to several episodes of mass food contamination. In Taiwan in 1978 thousands of people consumed rice bran cooking oil that had been contaminated with a mixture of PCBs and furans. The intake of PCBs from this oil was estimated to be 1000 times higher than the average for Americans, and nearly 10 000 times higher than the average intake of furans. By 1983, over 2000 cases of ‘Yucheng’ or ‘oil disease’ had been recorded by health authorities, all showing signs of acute chemical toxicity.

Finally, people can become exposed to acute levels of POPs compounds under specific circumstances in warfare. One of the best known incidences occurred during the Vietnam war, with the spraying of the defoliant Agent Orang, which was contaminated with the dioxin TCDD.

Health and environmental effects

The impact of these chemical residues in the environment on living organisms and ecosystems is hard to assess, partly because it is difficult to be certain that a specific substance or group of POPs is a direct cause of disease. However, available evidence strongly suggests that POPs are causing disturbances to health in wildlife, in some domesticated species and in humankind. It has been shown experimentally that the feeding of minks with food containing PCB at a concentration of 0.64 parts per million (ppm) leads to a concentration of this chemical in their livers of 1.2 per cent, and to almost complete reproductive failure. Sea otters off the coast of central California have high concentrations of PCB in their livers, which also contain DDT related compounds and other POPs. It is believed that POPs are interfering with reproduction in these animals and in many other forms of wildlife.

There is much suggestive evidence that exposure to POPs as contaminants of human food may result in various forms of ill health, including diabetes, cancer and interference with the immune system and mental processes. As mentioned above, we all have POPs in our tissues. Epidemiological evidence suggests that there may be a relationship between breast cancer and the presence of organochlorine compounds in the body. The incidence of breast cancer has been steadily increasing over the past ten or twenty years in both developed and developing countries. In the USA it has risen 8 per cent in women under 50 years old, and just over 32 percent in women 50 years and older.


As in the case of other environmental problems that traverse geographical and political boundaries, approaches to the control of POPs and other hazardous chemicals over the past couple of decades have been piecemeal, and largely restricted to individual countries. The measures taken have mainly involved the banning of certain POPs and limits on imports. Many countries, but mostly developed nations, have now banned most of the POPs on UNEP’s initial list of 12 compounds. A complete international ban of some POPs, like DDT, would be particularly problematic for developing countries which rely upon this substance for the control of malaria-spreading mosquitos. The World Health Organisation (WHO) is currently taking steps designed to encourage other measures for controlling mosquitos, thereby reducing reliance on DDT.

Before 1992, international actions to control POPs largely involved the development of tools for the assessment of risk, and the identification of priority POPs for management by agencies such has WHO, UNEP and the Food and Agriculture Organisation (FAO). These three agencies formed a joint committee as early as 1963 to evaluate safe levels of pesticides in foods. More recently, in 1989, FAO established an International Code of Conduct for the Distribution and Use of Pesticides, which included certain POPs.

The first major regional agreement was the 1979 Convention on Long-Range Transboundary Air Pollution, but this was not actually completed until 1998. It contains a specific protocol aimed at the control, reduction and prevention of the release of POPs into the atmosphere, and it includes specific criteria for the further identification of POPs. Sixteen POPs are covered in this convention. Thirty six countries have now signed the agreement (no developing nations are included), and it entered into force in October 2003. While this treaty deals with POPs in the atmosphere, other treaties, have addressed the threat and control of toxic chemical substances, including POPs, in the marine environment.

Following initiatives by UNEP, a Diplomatic Conference was held in Stockholm in 2001, leading to .the Stockholm Convention that entered into force in May 2004. This Convention sets out control measures for the initial list of 12 chemicals, covering the production, import, export, disposal and use of POPs. The governments of individual countries are required to develop national legislation and action plans in order to fulfil their commitments. They must also promote the best available technologies and practices for replacing existing POPs, while prohibiting the development of new POPs. There is also the provision of a financial mechanism, the POPs Club, through which the international community help developing countries to meet their obligations to minimise and eliminate POPs.

There are now several international and national registers containing data on chemical stocks, and details on emissions, including POPs. These are known as Pollutant Release and Transfer Registers (PRTRs). This kind of database also helps communities identify the worst polluters, and bring public attention to the issues of hazardous chemicals, waste management and the threats these place on public health. Details of Australia’s National Pollutant Inventory can be found at Consumers play a vital role in reducing the use of POPs, as can be seen in the growth of the organic food industry in response to community concern over the health effects of the use of chemical pesticides and fertilisers in foods.

These efforts to reduce the production, use and emissions of POPs are currently hindered by the slow development of cost-effective alternatives to these chemicals. Although a variety of chemical and non-chemical alternatives to POPs exist, there are many barriers preventing the widespread adoption of such technologies, foremost their high cost. Financial mechanisms in the Stockholm Convention and other agreements attempt to address this problem, especially by providing assistance to developing countries.

There is still a shortage of data the amount of POPs still used, the reasons for use, available alternatives and impediments to the adoption of alternatives in different countries. However, several industries and communities have begun to implement new technologies, especially for waste management and pest control, and these play a crucial part in the broad shift away from harmful chemicals to alternative practices and cleaner production methods.

Further reading

For further information see


Alice Thompson was brought up with an appreciation of, and interest in the environment, leading her to study at the Australian National University, majoring in Geography/Human Ecology and Population Studies, and her involvement in the Nature and Society Forum (NSF). She now lives in Sydney where she currently pursues a career in Government working for the NSW Office of the Australian Bureau of Statistics (ABS). Before joining ABS Alice was employed by NSF as a Research Officer to prepare reports on important ecological issues in Australia.

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