Planetary boundaries 3 – chemical pollution

Week one: ozone
Last week: nitrogen
This week: chemical pollution

Having started this series with the neatly defined ozone boundary and then the nitrogen cycle, we come this week to something a little more scattered: the chemical pollution boundary, sometime referred to as the toxics boundary.

I say scattered because there isn’t one clear culprit here. Chemical pollution includes radioactive material, heavy metals, industrial chemicals and a whole variety of organic compounds in our water, air and soil. Since we’re not talking about a single pollutant, there’s no single limit or threshold either. So why is it considered a planetary boundary, one might ask.

The authors of the planetary boundaries report give two reasons. First, although pollution is often considered a local issue, many pollutants are now so ubiquitous that they can effect change on a global level. The second is that chemical pollution is a wild card factor in all the other boundaries, from freshwater and land use, to biodiversity loss or climate change. The fact that it is the cumulative effect of thousands of chemicals doesn’t make it less of a threat – if the ozone boundary is a bear and nitrogen overshoot is a tiger, think of chemical pollution as a swarm of killer bees.

It’s a big swarm too. There are an estimated 80,000 to 100,000 different chemicals in current use across industry, agriculture and science. The majority of those have never been studied in any detail to see how they might affect human or ecosystem health. Of those that have been studied, “1,000 are known to be neurotoxic in experiments, 200 are known to be neurotoxic in humans, and five are known to be toxic to human neurodevelopment.”

Neurodevelopmental disorders include autism, attention deficit disorder and cerebral palsy. We know that industrial chemical pollution might be a factor in these conditions, but to what degree? And how do we find out?

That’s just one category of chemicals. Others are better known, like radioactive waste or carcinogens. In that latter category are asbestos, benzene that is used in paints and solvents, or ethylene oxide, which is used to sterilise packaging and medical equipment. Then there are endocrine disrupting chemicals, which affect hormones and lead to reproductive disorders, infertility or early puberty. These are often found in plastics. If you’ve seen ‘BPA free’ on toys or baby products, it’s referring to Bisphenol A, an ingredient in clear plastic that has been banned in the EU and the US in the last couple of years. Phthalates are another, banned from use in toys in the EU, but still widely used in other products and packaging.

While it’s the risk to human health that usually gets these things banned, the dangers are usually identified through other species – bird die-off, transgender fish, bee colony collapse, or deformed antlers in deer.

The problem with all of these is that industrial chemistry moves faster than medical science and ecology, so we’re constantly playing catch-up with the innovations of industry. “Humans have now devised countless thousands of novel substances, never before seen on earth, and released them into the natural environment” says Mark Lynas. “Many appear perfectly benign at first pass – but turn out to be very different when they enter the food chain, whether on land or in the sea.”

When we do discover a problem, it often takes decades to confirm the risk, and then years of campaigning to deal with it. Even then, chemical pollution tends to be addressed one chemical or family of chemicals at a time. The recent EU moratorium on necotinoid pesticides is the latest example, aimed at protecting bees. Sometimes bans are negotiated globally. The ban on a variety of persistent organic pollutants through the UN’s Stockholm Convention began the phase out of dioxins and the agricultural use of DDT. PCBs used in industrial coolants were banned in 1977.

So where are the limits here? What’s the boundary? It’s almost impossible to say. Most industrial chemicals are little understood ‘in the wild’. As the boundaries report says, “some toxicity data exist for a few thousand of these chemicals, but there is virtually no knowledge of their combined effects.”

To give just one example, Polybrominated diphenyl ethers, or PBDEs, are used in fire retardants that are sprayed on electronics, mattresses, clothes, and lots of household goods. They’re a known endocrine disruptor, but they are so widely used that they are now ubiquitous. We carry around a certain amount of PBDEs in our bodies, where it accumulates in fat, blood and breast milk. Is that a problem? Sure, gender-altering chemicals in breast milk is not a good thing, but where’s the safe limit? And how do PBDEs interact with all the other endocrine disruptors out there?

PBDEs are banned in various US states, with different ones setting different thresholds from 0.1 to 7 micrograms per kilo of body weight. Other states haven’t banned them at all. The EU banned PBDEs in 2006, and concentrations of PBDEs in breast milk are now 40 times lower in Europe than in North America.

So, toxic substances are something we need to manage. We’re getting better at it, and some of the environmental movement’s biggest successes have been on this front, no doubt due to the emotive nature of the threat. EU regulation, usually dragging Britain along reluctantly, is often leading the way. These regional bans are useful, but it’s important that agreements are reached internationally, otherwise chemical pollution is simply outsourced along with production. As well as use in manufacturing, regulation dealing with waste is particularly important, as it is often when things are thrown away that they leach into the environment.

In short, chemical pollution is something that needs more work. “At present,” says the Stockholm Resilience Centre, “we are unable to quantify the chemical pollution boundary, although the risk of crossing Earth system thresholds is considered sufficiently well-defined for it to be included in the list as a priority for further research.”

Chemical pollution boundary

Safe limit: unknown

Status:  unknown


  1. The role of chemical pollutants of all kinds in the manifestation of everything from asthma to cot death has been conclusively proven time and time again. The basic problem is the amount of financial muscle that the big time producers of these pollutants have, and the degree to which this can be brought to bear by way of influencing government!

  2. Ethylene (which isn’t carcinogenic, and is in fact naturally given off by bananas, causing fruit left in an enclosed space with a bunch of bananas to ripen quicker) is the one used to ripen fruit, not ethylene oxide. Wikipedia does say that ethylene oxide is used “as an accelerator of maturation of tobacco leaves” but I would imagine the carcinogenic effects of ethylene oxide are the last of a smoker’s worries.

    In fact, that raises a wider point – while people still smoke, drink alcohol and burn noxious fuels, regulating other chemical pollutants will have a negligible effect on mortality.
    According to the WHO, alcohol kills 2.5 million people a year and tobacco kills 6 million (that’s the equivalent of a Bhopal disaster every day), while air pollution kills 3.3 million deaths (mostly from indoor air polution caused by domestic cooking and heating).


    1. Ethylene oxide has a bunch of uses, and I obviously picked the wrong one as an example. I’ve changed the post to reflect its main use, which is as a sterilant in hospitals. Thanks for the clarification.

      Of course, people are going to do things that are bad for their health, but that’s no excuse for not regulating poisons out of our atmosphere. Besides, we shouldn’t use mortality alone to decide whether something is dangerous or not. We want people to enjoy healthy lives, not just remain alive.

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