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Wednesday 12 February 2020


What are POPs?

There’s increasing concern about the growing mass of our discarded plastics – yet not because of their direct effects on wildlife (e.g. entanglement), but because plastics could be a crucial vector for the transport of key environmental contaminants: persistent organic pollutants, or POPs.

There are many thousands of POP chemicals, originating from agriculture, combustion processes, industrial syntheses, and products such as flame retardants, plasticisers and antimicrobials. In fact, our understanding and classification of them is ever evolving. They’re named not because of their chemical groups, but because of their behaviour. As the name “persistent organic pollutants” suggests, they’re long-lived and harmful.

Plastic debris on a beach. By (Flickr).

What chemicals are we talking about?

Mosquito – By Alvesgaspar via Wikipedia Commons.
Typical culprits include DDT, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), halogenated flame retardants, and polybrominated diphenyl ethers (PBDEs). Although DDT is now banned in most countries, it’s still permitted in some places with a high incidence of malaria to fight off mosquitoes[1].

POPs are polluting and persistent because of certain properties, such as their solubility, reactivity and volatility.

What they do[2]

...In the soil?
POPs are typically hydrophobic and weakly bound to water, they either fix or fly… binding to organic matter (with a long half-life of years or decades before they even think of breaking down) or vaporising and entering the atmosphere, which happens readily at environmental temperatures.

…In the air?
POPs last for days in the atmosphere, which is considered long-lived in this phase (in fact, there is no universal agreement on how long-lived chemicals have to be, and where, to count as persistent). In the atmosphere, POPs can be particles and aerosols, depending on temperature, but don’t react or break down easily. As such, POPs can be transported long distances, spread out, and get everywhere.

...In the water?
POPs are not only hydrophobic, but also lipophilic. Lipids make us think of body fats – and body fats are lipids – but the group also contains hydrocarbons such as waxes and plastics. This means POPs are only sparingly soluble in water, but readily dissolve in plastics, concentrating in trace waste in the ocean.

...In wildlife?
Plastic particles inevitably get eaten by wildlife, especially by indiscriminate eaters like the lugworm, which lies at the bottom of a long food chain. Inside the animal’s gut, POPs in the plastic can move between the fat tissues and plastic because they dissolve well in both. If there is a concentration gradient, i.e. the POPs are more concentrated in the plastic than the animal, they will preferentially move from the plastic to the animal, until concentrations are equal. As the POPs travel up the food chain and enter larger and larger animals, they bioaccumulate.
Lugworms are most often seen by the trails they leave in the sand. By Nveitch via Wikipedia Commons.


Consequences to wildlife

1. Carcinogens
Many POPs are known or suspected carcinogens, such as polynuclear aromatic hydrocarbons (PAHs).

Egg shell thickness. By Tuomas 'tuos' Räsänen via Wikipedia Commons.
2. Reproductive health
POPs such as polychlorinated biphenyls (PCBs) are suspected endocrine disruptors, and may cause fertility problems. Evidence has been seen in fish-eating birds, seals and whales.

3. Egg shell thickness
Egg shell thickness in ducks and birds of prey may be reduced by chemicals including DDT metabolites.

4. Predator populations
Scientists are concerned that POPs will accumulate in and harm predators. Notably, as use has declined in some areas, predator populations of seals, eagles and other fish-eating birds have gone up[3].

How much is there?

As analytical techniques have improved, scientists have been able to detect concentrations of POPs so small that they were previously "unseen", and their host samples considered clean.

Samples are typically collected as wildlife blood specimens or collection pellets placed in seawater round the world, where they concentrate pollutants to millions of times higher than surrounding water[4].

Techniques include gas chromatography and mass spectrometry. These techniques use the distinct isotope patterns of chlorine and bromine to pinpoint compounds that contain them and identify structurally related compounds that can only be grouped using very high resolution measurements[5].


POPs, whilst long-lived, don’t hang around forever. They can be removed from the environment naturally or through human interventions through biological, chemical, or physical means. In soils, sediments, water, or the bodies of living things, biological reactions can take place that break POPs down into simpler chemicals (or metabolites). Chemical reactions can take place in the atmosphere, such as photolysis, when hydroxyl radicals facilitate their decomposition. They can also be occluded, bound, or simply buried in soils, peat bogs or sediments. In the atmosphere, if POPs can’t be removed they can at least be diluted by spreading them out into uncontaminated areas, with low POP output.

Atmospheric pollution can travel long distances “unseen”. By PiccoloNamek via Wikipedia Commons.

Things we don’t know about POPs

How much there is
Poor or non-existent records about past and present POPs emissions (especially in some countries), let alone the fact that new chemicals are being added to the group every year, makes identifying how much there is in the environment really hard. This is true even for agrochemicals – chemicals such as fertilisers which are deliberately (and supposedly measurably) added to soil. We still know relatively little about how fast POPs biodegrade or break down in the atmosphere, only that rates can vary enormously depending on variable factors such as wind speed, pH and temperature. Measurements aren’t very helpful either: as the resolution of detection techniques increases, we keep finding more trace evidence of POPs.

Where it is
We still don’t understand the “POP cycle” well – how they are transported in the environment and end up where they do. Mathematical modelling suggests that the plastic pathway may only be a small contributor, but remains so inaccurate that only broad deductions can be made[6].

Which ones are bad
There are so many different POPs that working out which one is causing harm to wildlife is almost impossible. It’s also possible that their metabolites or several chemicals acting synergistically are responsible for toxic consequences. As new chemicals are being made or discovered and added to the POP category, older conclusions about which chemicals are likely to be responsible for adverse effects need to be revisited. Following legislation, several harmful POPs have been phased out – but these are often replaced by similar chemicals that probably do the same harm[5].

why don't all references have links?

[1] M. Cone, Should DDT Be Used to Combat Malaria?, Sci. Amer. 2009.
[2] K. Jones, Persistent Organic Pollutants (POPs), ECG bulletin July 2001.
[3] R. Fletcher-Wood, S. Ball, Plastic debris in the ocean — a global environmental problem, ECG Bulletin, July 2014.
[4] Mato, Y., Isobe, T., Takada, H., Kanehiro, H., Ohtake, C., Kaminuma, T. (2001) Plastic resin pellets as a transport medium for toxic chemicals in the marine environment. Environmental Science and Technology 35, 318–324.
[5] D. Megson, M. Pena-Abaurrea, X. Ortiz, Non-targeted methods for identifying new and emerging persistent organic pollutants of environmental concern, ECG Bulletin July 2015.
[6] R. Thompson, Plastic debris in the ocean – a global environmental problem, ECG Bulletin July 2014.

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