Environmental Toxicology: How chemicals interact with environmental species and human physiology

environmental toxicology

The European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) give a detailed perspective on environmental toxicology, a life science discipline that seeks to understand how chemicals, both natural and man-made, can interact with environmental species and human physiology to cause harm

Toxicology, a life science discipline, seeks to understand how chemicals, both natural and man-made can interact with environmental species and human physiology to cause harm. The art of toxicology has been known since the time of Socrates who was executed in 399BC using hemlock; a poison that acts slowly.

Toxicology as a science matured from this time with notable figures, such as Paracelsus and came of age after the thalidomide catastrophe of the early 1960s (human) and the discovery of the adverse effects of chemicals such as the insecticide DDT (environment). These adverse events directly led to the methods and supporting international legislation that now underpins the risk assessment of chemicals (in the widest context including drugs) on both human and environmental health. In counterbalance to chemical risk, it must be recognised that chemical use has brought substantial benefit to society.

Thus, in deciding whether to use a chemical, any identified risk has to be balanced against benefit in the context against which it is going to be used. This is called the risk/benefit ratio, with the aim being to make the ratio as small as possible. The least controversial risk/benefit ratios are achieved with chemicals used in therapy where the benefit of having a positive effect on the disease process usually outweighs toxicological risk by a substantial margin. For the use of chemicals in the natural environment this ratio can be more difficult and controversial (usually due to uncertainty discussed below) to compute. It is best addressed by ensuring that the risk part of the ratio is a small as possible, which leads to a need to have the best testing regimes that can be devised using current scientific knowledge.

To achieve this, the chemical industry, the EU and national governments dedicate significant resources to assess the potential environmental toxicity of chemicals. 1 EU legislative regimes relating to the manufacture and use of chemicals require comprehensive environmental toxicology assessments to be conducted for chemicals placed on the market.2 This allows regulators (those responsible for assessing the risk), the chemical industry (responsible for providing the data from toxicology testing) and society (who derive and largely assess benefit) to achieve the maximum benefits from chemical use, at minimum risk. Ultimately, the most important part of these three is the end user, who makes the ultimate call on whether to use a chemical, relying on the expertise of the industry and the regulator in providing and assessing toxicological risk, respectively and of course, their own analysis of the benefit.

At this point, three common and important toxicological terms must be introduced, these are hazard, exposure and risk. Consider again for a moment, Socrates. At the time of his execution, he held in his hand a vial containing Hemlock. Hemlock affects the nervous system; thus, it is a hazardous poison, but it is contained within a vial and as such, it does not pose a risk. It becomes a risk when Socrates drinks it because at this point, exposure occurs. When there is both hazard and exposure, then there is risk. The risk increases with increased hazard and/or increased exposure with the greatest risk occurring for a substance that has both a high hazard and high exposure. Those chemicals with the greatest hazard potential, usually interact very specifically with a critical part of a biological system. Those having the greatest exposure, are often those released without control into the environment. The absence of either hazard or exposure removes the risk.

Toxicology concerns itself with assessing and reducing the hazard, so risk management concentrates on reducing or eliminating exposure. Both are effective in reducing risk. A chemical in the environment is not hazardous if it has no interaction with any biological component of that environment and, thus, does not perturb the homeostasis (normal equilibrium) of the environment. Similarly, if a chemical is used but not released into the environment, it does not pose a risk (though risk management would include in this case, an assessment of accidental release). An environmental risk occurs only when a chemical has an adverse effect on the environment (hazard) and is released into that environment (exposure). Therefore, we can define risk as the product of exposure and hazard, which is the essence of toxicology.

We cannot quite stop there though. There are two further factors that need to be taken into account when assessing risk and these can be amongst the most challenging facing the environment and indeed human health, as well as the toxicologist. These factors are both time and uncertainty.

Let’s deal with time first. It is fairly apparent that the longer a chemical is in an environment, the greater the probability and likely magnitude of exposure is. Once released into an environment, a chemical may start to degrade. This can be a slow or rapid process, or any time in between. Additionally, alterations of the chemical may take place in the environment from the actions of species in that environment, such as microorganisms. These processes may remove a chemical from an environment but can produce new forms that also have their own hazard properties that may be greater or less than the parent chemical. Therefore, part of the work of the environmental toxicologist is to assess what happens to a chemical in the environment, over what time frame and how this changes the risk. The time factor is, therefore, found in the majority of today’s toxicological assessments.

Secondly, let’s look at uncertainty. Arguably, this is the greatest challenge for the environmental toxicologist. No testing is perfect, no assessment of exposure is perfect. Both have uncertainties associated with them that can be due to measurement or biological uncertainty, often arising from an incomplete understanding of the exposed biological system. For this reason, testing methods and existing risk assessments need to be continually re-visited as scientific understanding advances to determine how new measurement technology or a greater understanding affects the assessment of risk.

For example, a notable current challenge in the field of environmental toxicology is the question as to whether current standardised test methods have greater uncertainties associated with them when used for new materials, such as chemicals in nano form (nanomaterials), microplastics and polymers. If the uncertainty of the testing method is too great for these new materials, then the data obtained is not fit for purpose. Similarly, there are questions about the appropriateness of current approaches in the field to assess the degradation of chemicals in the environment and, therefore, the time factor of the exposure assessment.

Risk is simple, it is the product of hazard and exposure. But, it is a big ‘but’, indeed, determining hazard and exposure can be challenging, as the above examples illustrate. Therefore, to ensure that the field of environmental toxicology continues to advance in practice, there is a need for a new generation of scientists who are well-trained in fundamentals and familiar with the most recent techniques and tools (in silico, in chemico, in vitro, in vivo). With enough people holding such fundamental training and experience, we can continue to improve our understanding of the impacts of chemicals on the environment and ensure that no unacceptable harm to the environment or to ourselves is caused by their use. Thus, we can continue to reap the benefit of chemicals with minimal risk.


1 E.g. http://cefic-lri.org/ http://www.ecetoc.org/ https://ec.europa.eu/programmes/horizon2020/

2 E.g. REACH Regulation (EC) No 1907/2006; CLP Regulation (EC) No 1272/2008; Plant Protection Products Regulation (EU) No 1107/2009; Biocides Regulation (EU) No 528/2012; Veterinary medicinal products Directive 2001/82/EC; Medicinal products for human use Directive 2001/83/EC; Cosmetic Products Regulation (EC) No 1223/2009.

Professor Timothy Gant

Member of the ECETOC Scientific Committee

(European Centre for Ecotoxicology and Toxicology of Chemicals)

University of Surrey

Tel: +32 2 675 3600





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