BLOG: Erring on the side of caution: the case of pesticides

Published 23 Mar 2017

Are pesticides hazardous to your health? If you were to drink a bucket-full of any pesticide, then probably yes.

But what about the relatively tiny amounts that you encounter through residues in your food? Or the particles that you breathe in after pesticides are sprayed on fields, in parks or perhaps in your own garden?

You might be hoping that for each pesticide there is a simple answer, either it is hazardous to your health or it is not. Unfortunately, it’s not that simple. Gathering the scientific evidence and then using that evidence to agree on regulations can be a complex and controversial process.

The science

There are different ways of gathering evidence on the risks posed by a chemical used in a pesticide:

  • In vitro experiments, where chemicals are tested on cell cultures.
  • In vivo experiments (experiments using living organisms*), where (predominantly) rats and mice are fed or exposed to a specific chemical
  • Epidemiological studies, which look for links between exposure to a chemical and illness

There can be disagreement between scientists on how much weight to give to which kinds of studies, whether studies were done properly (e.g. whether the sample size is large enough, and all relevant factors have been considered), and whether there is any bias in the research undertaken by pesticide companies or environmentalists.

It takes time to gather this evidence, especially when diseases take time to develop, and for scientists to come to a consensus on how hazardous a chemical is, and at what dose, and how much of a risk it poses to the environment and human health.

Regulations – the precautionary principle

When there is disagreement between scientists over the dangers posed by a pesticide, how should it be regulated? This is where the precautionary principle comes in.

If it is plausible that a pesticide could cause harm to the environment or human health, then the precautionary principle states that action should be taken to avoid or diminish that harm. The worse the possible harm, the stronger the action should be.

In other words, if there is evidence that a pesticide is hazardous and poses a risk to your health, then we should act to restrict or ban its use. If a consensus develops that it does not pose a risk, then we can lift those restrictions. Put simply, we should err on the side of caution.

Lessons from the past

There are some striking examples of chemicals that have been widely used, which later were found to be harming our health. In pesticides, an infamous example is DDT (or to give its full title, dichloro-diphenyl-trichloroethane).

DDT was developed in the 1940s and was widely used as one of the first modern synthetic insecticides to combat insect-borne diseases, especially malaria carried by mosquitoes. After being heavily promoted, evidence began to emerge about the damage that DDT could do to the environment and human health, including cancer. The environmentalist Rachel Carson highlighted these issues in her 1962 book ‘Silent Spring’. Thanks to the work of Carson, and others, the use of DDT has been banned or heavily restricted.

However, in the case of DDT, much damage had already been done, and because it is a persistent organic pollutant (POP), DDT continues to cause problems in the environment and to human health, to this day.


Currently, pesticide regulations are developed in the EU, where the precautionary principle is enshrined in law. While current regulations are from perfect, we do have some of the most effective regulations in the world. Brexit means that in the future, the UK might have its own pesticide regulations that differ from those in the EU. If this happens, it is essential that these regulations are based on the precautionary principle.

We must continue to learn lessons from the past, and when it comes to the environment and human health, err on the side of caution.



Pesticide Action Week runs from 20 - 30 March 2017.  To find out more and how you can get involved visit:

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