What are POPS?

The building blocks of living organisms are organic compounds, which are chemical compounds that contain carbon and hydrogen (and in some cases other elements as well). These compounds are never indestructible and many break down relatively easily.

But man has learnt to manufacture organic compounds that are extremely difficult to break down and, as a result, have become widely dispersed through the environment. These chemicals are termed Persistent Organic Pollutants (POPs).

POPs are a group of chemicals, which are very resistant to natural breakdown processes and are therefore extremely stable and long-lived. Most do not occur in nature but are man-made.

Once released into the environment, many POPs persist for years, even decades. Therefore, even if production of all POPs ceased today, they would continue to pollute the environment for many years to come.

Many POPs are also highly toxic and build up (bioaccumulate) in the fatty tissues of animals and humans. These three properties - persistent, toxic and bioaccumulative, make them, arguably, the most problematic chemicals to which natural systems can be exposed.

The POPs that we have extensive information on, like PCBs and DDT, are known to cause serious negative impacts on wildlife. There is also evidence to suggest that human health in different parts of the world is *influenced by exposure to these well-known POPs.

About other POPs, like brominated flame-retardants or tributyltin (TBT), we have less information when it comes to the effects on wildlife and humans. However, we do know that these chemicals have similar properties when it comes to their persistence in the environment, their potential to accumulate in organisms and their inherent toxicity.

On top of that, there are even more poorly described chemicals of which nobody knows the possible effects. In essence therefore, our present state of knowledge on exposure to POPs and their health effects in wildlife and humans may only be the tip of the iceberg. This is an alarming conclusion, but one that must, nevertheless, be drawn. Future research may reveal many more toxic effects caused by POPs contamination of the environment.

Because these chemicals are resistant to break down and can accumulate in the environment and organisms, the potential for negative health effects is impossible to avoid once the chemicals are released. In effect, the release of POPs to the environment is akin to playing a large scale laboratory experiment with the very livelihood of humankind and the environment.

POPs in Food Webs

Many POPs, which pollute the environment, become incorporated into food webs. They accumulate and persist in the fatty tissues of animals and humans because they are soluble in fats and are not easily broken down in the body. Even low environmental levels of POPs can lead to high levels in the body tissues of animals and humans.

For many POPs, the levels in fat increase as one animal eats another, so that the highest levels are found in predator animals at the top of food webs, such as polar bears, seals, toothed whales, birds of prey and humans.

Marine mammals accumulate particularly high levels of POPs because of their large quantities of fatty blubber and, compared to other species, a reduced capacity to break down some POPs.

Exposure to POPs

The main exposure to POPs is through our food; e.g. it is estimated that 95% of the dioxin exposure in the Netherlands comes from food.

The ubiquity of POPs in the global environment is reflected by their presence in human tissues. Research from numerous countries throughout the world has demonstrated that measurable quantities of persistent organochlorines are present in human adipose tissue, blood and breast milk. Some other POPs such as brominated flame retardants and nitro musks are also detectable. Measuring levels of POPs in tissues is one of the most direct ways of studying human exposure to these environmental pollutants. This information is also useful for investigating relationships between human exposure to POPs and health effects.

Research indicates that levels of dioxins and some organochlorine pesticides in humans have decreased or stabilised in some European countries in recent years. However, other POPs may now add increasingly to the already existing levels of these POPs in breast milk. For instance, levels of brominated flame retardants have doubled every 5 years over the past 25 years in breast milk from Swedish women. Nitro musks are also detectable in breast milk in European countries.

Man-made chemicals occur in the environment and in our bodies not as single entities but as complex mixtures. We are exposed not to individual POPs, but to diverse mixtures. The significance of such multiple exposure remains poorly understood. Moreover, a substantial proportion of the chemicals which occur in the environment and to which we may be exposed simply cannot be identified. This further complicates the problem.

The diversity of uses of these chemicals, and their releases to the environment, serve to illustrate the scale of the problem to be addressed. We are all exposed to hazardous chemicals, including POPs, during daily life, both because of their pervasive distribution throughout the environment, including the food we eat, and because of their use in consumer products.

The developing young of both wildlife and humans are the most vulnerable to toxic effects of POPs. They are exposed to these chemicals because POPs are passed to the foetus in the womb from a mother's body via the placenta and through breast milk to the infant.

Effects on Wildlife and Humans

Exposure to POPs has been associated with a wide range of impacts on health in wildlife and in humans. Effects include carcinogenicity, toxicity to the reproductive, nervous and immune systems and adverse effects on development.

A substantial proportion of the POPs which have accumulated in a woman's body during her whole lifetime are passed to her child during development in the womb and through breast-feeding. Studies show that current levels of PCBs/dioxins in some women of the general population are associated with subtle, but significant undesirable effects on the nervous system, immune system and post-natal growth of their babies.

These effects were evident in healthy infants from the Netherlands whose mothers' had slightly higher levels of PCBs/dioxins in their bodies. Similarly, in the Arctic, babies born to women who had higher levels of PCBs in their breast milk had subtle undesirable effects on their immune system and on postnatal growth. This is obviously of great concern, particularly as effects are occurring at current background levels and effects caused during foetal/infant development are likely to be irreversible.

There are many mechanisms by which POPs can exert biological effects on health; these depend on the specific chemical in question. One mechanism which appears to be common to a wide range of POPs is disruption of the hormone (endocrine) system. Chemicals that have the capacity to disrupt hormones are known as endocrine disrupters.

Other mechanisms by which POPs could cause effects on health include those mediated via enzyme systems. Cytochrome p450 enzymes located in the liver are involved in both the regulation of steroid hormones and in the detoxification of chemicals. This enzyme system is common to all animals with backbones. Some POPs, notably persistent organochlorines, are known to stimulate the production of certain cytochrome p450 enzymes. Various biological effects, including reproductive and immunological effects, may result from induction of these enzymes by POPs (Reijnders 1994, Tanabe et al. 1994).

Global transport of POPs

In recent decades, vast numbers of POPs have been produced, used worldwide and are still in production and use in everyday products. These chemicals have become widespread pollutants in the environment.

POPs contaminate local areas close to sites where they are released into the environment from industry and agriculture. However, volatile and semi-volatile POPs also contaminate regions remote from their source because they can be transported for thousands of kilometres on air currents.

These POPs migrate on air currents from warmer regions of the globe towards colder polar regions. Once they reach colder temperatures they condense, precipitate out, and are deposited again on the Earth's surface. POPs may also be transported for long distances by rivers, ocean currents and as contaminants in wildlife.

Only a few POPs have been banned in most countries. However, even these banned POPs continue to contaminate the global environment because of their long-lived nature and because some are still manufactured and used in some countries.

Scientists have proposed a hypothesis that explains how POPs could be transported for long distances on air currents from warmer regions of the globe towards cooler polar regions. The proposed mechanism is known as global distillation (or global fractionation).

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