What are persistent organic pollutants?

However, humans have learnt to manufacture organic compounds that are extremely difficult to break down and, as a result, have become widely dispersed throughout the environment.

These chemicals are termed Persistent Organic Pollutants (POPs) and are extremely resistant to natural breakdown processes and therefore are stable and long-lived. Most do not occur in nature but are created through artificial processes.

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.

It is these three properties (persistence, toxicity and bioaccumulativity) that make POPs so dangerous. They are, arguably, the most problematic chemicals that natural systems can be exposed to.

The POPs that we have extensive information about, such as Polychlorinated Biphenyls (PCBs) and Dichloridiphenyl Trichloroethane (DDT), are known to cause serious negative impacts on wildlife.

There is also evidence suggesting that human health in different parts of the world, is influenced by exposure to these well-known POPs.

There is less information about other POPs, like brominated flame-retardants or tributyltin (TBT), and their effects on wildlife and humans.

However, it is known 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.

In addition, other chemicals exist where the possible effects are unknown. In essence our present 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 thought, but one that must, nevertheless, be highlighted.

Future research may reveal many more toxic effects caused by POPs contamination of the environment.

Due to their persistence and tendency to bioaccumulate, the potential for negative health affects are impossible to avoid, once the chemicals are released.

POPs in food webs

Many POPs 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. In the Arctic food chain this includes predators 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 food. It is estimated that 95 percent of 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 POPs 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 their health effects.

Research indicates that levels of dioxins and some organochlorine pesticides in humans have decreased or stabilised in some European countries over recent years.

However, other POPs may increasingly add to the existing levels of these POPs in breast milk. For instance, levels of brominated flame retardants present in the breast milk of Swedish women, have doubled every five years over the past 25 years. 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 not exposed to individual POPs, but to diverse mixtures of these chemicals.

The potential health effects of this multiple exposure remains poorly understood. Moreover, a substantial proportion of the chemicals humans may be exposed to cannot be identified, which further complicates the problem.

The diverse use of these chemicals, and their release into the environment, serve to illustrate the scale of the problem to be addressed.

Humans are all exposed to hazardous chemicals, including POPs, on a daily basis. Exposure most commonly occurs through the food we eat and consumer products.

The young, from wildlife and humans, are the most vulnerable to the toxic effects of POPs. Exposure occurs in the womb, as the chemicals are passed to the foetus from the mother's body via the placenta. Infants continue to be exposed through the mother's breast milk.

Effects on wildlife and humans

Exposure to POPs is associated with a wide range of impacts on the health of wildlife and humans.

Effects include carcinogenicity, toxicity to the reproductive, nervous and immune systems and adverse effects on development.

A substantial proportion of the POPs that have accumulated in a woman's body during her lifetime are passed to her child during its development in the womb and through breast-feeding.

Studies show that current levels of PCBs/dioxins in some women are associated with subtle, but significant 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 dioxins in their bodies.

Similarly in the Arctic, babies born to women who had higher levels of dioxins in their breast milk, had subtle undesirable effects on their immune system and on postnatal growth. These effects caused during foetal/infant development are likely to be irreversible.

Depending on the specific chemical in question, there are many mechanisms where POPs can exert biological effects on health.

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.

Global transport of POPs

In recent decades, vast numbers of POPs have been produced and used worldwide.

Many are still in production and used in everyday products. These chemicals have become widespread environmental pollutants.

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 via air currents.

These POPs migrate on air currents from warmer regions of the globe towards the 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. This is due to their long-lived nature and because they 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).