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A planetary boundary for chemicals

Updated: Apr 6, 2023

I’ve been reading up on the concept of planetary boundaries. An idea first published in 2009, planetary boundaries are the limits of human impact on the environment beyond which there may be no return. There were nine boundaries described as part of the planetary boundaries concept. These were climate change, biosphere integrity, land-system change, freshwater use, biochemical flows, ocean acidification, stratospheric ozone depletion, atmospheric aerosol loading, and novel entities. Of these boundaries it is the last ― novel entities ― which is the focus of this blog.

Somewhat mysteriously named, the novel entities boundary is the limit of new substances, new forms of existing substances, and modified life forms beyond which unwanted geophysical and/or biological effects may occur. The boundary for novel entities has not yet been determined, despite the original 2009 planetary boundaries article being cited more than 5000 times and the updated planetary boundaries article published in 2015 more than 7000 times. In this blog, my focus is on only one component of the novel entities boundary ― chemical pollution.

Part of the problem with constraining a planetary boundary for chemical pollution is that the impacts of such pollution aren’t the result of one causal variable (e.g. CO2 emissions), one response variable (CO2 concentrations), and one medium (the atmosphere), as is the case for other boundaries. Instead, the chemical pollution planetary boundary must deal with multiple variables (different chemicals), multiple responses (harmful effects at cellular through to population levels), in a myriad of media (plants, animals, water, soil…). Diamond et al. describe the requirements for a chemical pollution planetary boundary in their 2015 paper:

“For a PBCP [a planetary boundary for chemical pollution] an ultimate effect or response variable subject to control is widespread adverse impact(s) to ecological and/or human health caused by exposure to (a) substance(s)…”

That is, the boundary must be a variable that aggregates the effects of all pollutants and it must be something that can be controlled. Diamond et al. (2015) considered that the constraint of a chemical pollution planetary boundary was not possible at the time of writing, stating:

“The combination of numerous substances with different use and emission patterns, affecting a multitude of different endpoints in a plethora of exposed species in the vastly different ecosystems of the world, plus consideration of human health, makes the derivation of a single quantitative PBCP or multiple PBCPs a daunting, if not impossible task… the extremely large number of commercial chemicals or mixtures of chemicals that cause myriad adverse effects to innumerable species and ecosystems, and the complex linkages between emissions, environmental concentrations, exposures and adverse effects. As well, the normative nature of a PBCP presents challenges of negotiating pollution limits amongst societal groups with differing viewpoints.”

So far, boundaries for chemical pollution have been drawn from the principle that the dose defines the poison. Using this concept, boundaries in the form of regulatory standards for individual chemicals have been created for key indicator species. Diamond et al. (2015) tried to extrapolate this idea of chemical doses being the limit for chemical pollutants by trying to conceptually identify a way to account for the effects of exposure to all chemicals across all species. However, I can’t help think that this concept doesn’t work for individual chemicals in the first place. While there’s evidence that individual limits to pollutants can reduce chemical loads (see my previous blog), chemical regulations and guidelines are too slow to produce, insufficient to protect the environment from chemical pollution, and are failing to keep the use of chemicals within safe limits — except in a very small proportion of cases.

Tools that focus on assessing the measurable impact of chemicals on organisms and even whole ecosystems do exist though and come in the form of environmental genomics. This term is used to describe the suite of molecular tools that sample, process, and analyse nucleic acids from an environmental sample. Some well-established environmental genomics tools are used to understand pollution impacts. Omics, for example, allow for a change in the metabolism of an organism due to chemical exposure to be assessed. While environmental genomics tools do not necessarily allow for the chemical that drives a biological response to be understood, the understanding of this response and the chemical driver(s) of this response, is not necessary for us to constrain whether a planetary boundary is being exceeded. Merely a measure of organisms’ response to chemical stressors is required. The logical follow on from this is, then, how humanity can withdraw from a chemical pollution planetary boundary if we do not know what is causing the boundary to be exceeded. This is where the precautionary principle comes in.

Defined differently in different jurisdictions, the precautionary principle is outlined in a highly recommended book focused on what the precautionary principle means to chemicals by Harramoës et al. (2001):

The main element of the precautionary principle they developed was a general rule of public policy action to be used in situations of potentially serious or irreversible threats to health or the environment, where there is a need to act to reduce potential hazards before there is strong proof of harm, taking into account the likely costs and benefits of action and inaction.

Harramoës et al. (2001) state that a precautionary approach to chemical regulation is warranted where there is enough evidence for concern, regardless of whether there is a paucity of substantial evidence or not. In the case of a chemical pollution planetary boundary, this to me would mean that the precautionary principle should be invoked if the chosen measure of the chemical pollution boundary has reached a point of no return for organisms.

And which chemical are we being precautionary about if the measure of the chemical pollution boundary does not allow us to understand what chemical is driving impacts? Well… why not all of them, except in the most pressing of situations (e.g. pesticide resistance)? The precautionary principle states that policy action should occur where there is indication of harm. In the event that environmental genomics indicates a planetary boundary is being surpassed, why not halt the production of new chemicals and even other chemicals for which there is insufficient evidence of the absence of harm, until the drivers of harm can be established?

The adoption of the precautionary principle into the environmental regulatory framework in a number of countries across the world means that governments have the ability to take action to prevent exposure to chemical pollution where potentially serious or irreversible threats to the environment might occur. This seems very much synonymous with the aim of planetary boundaries to me.

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