Pesticide is a generic term for a substance used for preventing and controlling insects or other organisms harmful to cultivated plants or animals¹. Within agriculture they can be split into three broad categories: herbicides, fungicides and insecticides.

They can exist in many forms, such as solid granules, powders or liquids, and consist of one or more active substances co-formulated with other materials.

Today there are approximately 1,300 approved pesticides within the UK¹.  The majority of pesticides used on arable farmland are fungicides (such as tebuconazole) and herbicides (such as glyphosate)².

Regulatory Regime

Pesticides have been around for a long time, for instance during the mid-18^th Century inorganic substances such as arsenic and copper sulphate were used to prevent fungal diseases in wheat.  However, traditional methods of pest control such as crop rotation and tillage were prevalent until the development and commercialisation of synthetic insecticides (organochlorines and organophosphates) from the 1930s to 1950s³.

The environmental impacts of these substances were not known by the general public, but a notable decline of wildlife during the 1960s, and the publication of the infamous book ‘Silent Springs’ by Rachel Carson⁴, brought the potential detrimental impact of pesticides (namely Dichlorodiphenyltrichloroethane, commonly known as DDT) into the public eye and instigated the establishment of the Countryside Act 1981.  This act was not specifically related to pesticides, but prohibited methods of killing wild animals e.g. poisoning.

As the human and environmental risks became more apparent, further legalisation was passed.  The ‘Control of Pesticides Regulations’ (1986)⁵ introduced a registration system for pesticides as well as requiring appropriate labelling and training on their application.  In the UK, pesticides such as DDT were withdrawn from registration in 1984 by the Health and Safety Executive, followed by the  ‘drins’ ( Dieldrin, Aldrin, and Endrin) during the 1990s and therefore were no longer authorised for use in the UK.

The Stockholm Convention was adopted by the UK in 2001⁶ (enacted in 2004), which sought to protect human health and the environment from persistent organic pollutants (POPs), which are resistant to environmental degradation, by reducing or eliminating harmful use of POPs.  This included aldrin, chlordane, DDT, dieldrin, heptachlor, mirex, toxaphene and PFAS (per- and polyfluoroalkyl substances).

Further regulation was introduced in 2008¹ (Pesticide Maximum Residual Levels Regulations, England and Wales) which was enacted to ensure compliance with the European Union pesticide residual limits on food, animal feed and animal origin to protect human health and environment.

In 2025, the UK government introduced the Pesticide National Action Plan⁷, which aims to reduce pesticide use by at least 10% by 2030.  The action plan proposes to do this by encouraging uptake of integrated pest management, setting targets for the reduction of the use of pesticides, and strengthening compliance.

Human health risk assessment.

Greater awareness of the environmental and health risks and increased regulatory oversight have increasingly prevented or limited the use of pesticides that are known to pose an environmental risk, although there is still some way to go.  For instance, there are currently over 30 pesticide products in the UK that contain PFAS, which are known to be persistent in the environment, and increasingly are being recognised as a risk to health, land and aquatic ecology.

Pesticides that are spread over wide tracts of agricultural land represent a diffuse source of contamination, and following breakdown in the environment, or removal by surface run-off  / leaching, the risk to human health reduces.  Organic pesticides naturally break down in the environment through volatilisation, photodegradation, and microbial degradation, with the latter most prominent.  The break-down process depends on a variety of factors such as the chemical composition of the pesticide and soil type.  The longevity of pesticide in the environment is measured by its half-life (the time it takes to reduce by half) and may be taken into consideration as part of a risk assessment.  Examples of pesticide half-lives in agricultural soils are presented in Table 1, below. However, in oxygen-depleted or waterlogged environments, the half-life can be significantly extended.

Table 1 Pesticide half-lives

Pesticide Group	Topsoil half life (aerobic & photolysis)10
Organochlorines	2 to 15 years
Organophosphates	1 to 3 months
Carbamates	10 days to 2 months
Pyrethroids	1 to 4 months
Triazines & triazoles	2 months to 4 months
Neonicotinoids	2 months to 5 months (longer in dry conditions)
Glyphosate	Up to 1 week

 

The Pesticides Properties Database (PPDB) is a useful source of information developed by the Agriculture & Environment Research Unit (AERU)⁹ at the University of Hertfordshire. The database contains information on pesticide approvals, physio-chemical properties, environmental fate, human health and ecotoxicological data.   However, the database does not contain any generic assessment criteria.

An increased risk and therefore focus of any human health risk assessment should be on pesticides which can remain in the environment, such as POPs  (including PFAS), and where their usage could have been concentrated through intensive use, cleaning of equipment, and disposal, or where they can be trapped in anaerobic conditions (e.g. sediments and burial of wastes). Examples include sheep dips, greenhouses, orchards, former slurry pits and vehicle wash down areas. These features can often be identified on historical maps, however without knowledge from the previous landowners it can be very difficult to determine exactly which pesticides have been used. It may therefore be prudent to screen against a range of POPs and other historical pesticides that could have been used within a particular agricultural setting. Table 2 below sets out a list of widely used pesticides in agriculture (not exhaustive) and their uses.

Table 2 Widely used pesticides and their uses

Type	Use
Organochlorines such as DDT, the ‘drins’ heptachlor, chlordane, endosulfan, dicofol and lindane.	Used as insecticides since the 1940s – banned/withdrawn from use in the UK. Classified as POPs under the Stockholm Convention
Organophosphates such as Chlorpyrifos, Malathion, Diazinon, Parathion	Used as insecticides since the 1940s – banned/withdrawn from use, except for Malathion.
Pyrethroids	Insecticides used since the 1950s – not banned, but used as a last resort.
2,4-Dichlorophenoxyacetic acid	Herbicide – used since the 1940s, still approved for use within the UK.
Triazine pesticides such as atrazine, simazine, and propazine	Herbicide used since the 1950s.  Atrazine was withdrawn from use in 2004.
Glyphosate	Herbicide used since the 1970s – still commonly used today. Classified as a possible carcinogen with reproductive/developmental effects on human health.
Tebuconazole	Fungicide used since the 1970s – still commonly used today.  Classified as moderately persistent with highly hazardous properties*
Note:  Pesticides highlighted in bold are classified as POPs by the Stokholm Convention. * Agriculture & Environment Research Unit

 

To quantify risk where pesticides are encountered, generic human health assessment criteria (Suitable for Use Levels (S4ULs)), has been published in the UK by Land Quality Management and the Chartered Institute of Environmental Health for nine pesticides⁸.  These are, Aldrin, Dieldrin, Atrazine, α-Endosulfan, β-Endosulfan, α-Hexachlorocyclohexane, β-Hexachlorocyclohexane and Lindane (y-Hexachlorocyclohexane).   Assessment criteria for other pesticides can be found on the US Environmental Protection Agency website, but should be used with caution given the differences in the UK and US approaches to risk assessment.

Conclusions

There has been a long history of pesticide use within the UK, which really intensified from the 1940s onwards along with agricultural practices.  The risk to human health and the environment was overlooked or poorly understood until decades later when legislation and registration were enacted, and some of the harmful pesticides such as DDT were banned.  Legislation and research on toxicology and environmental fate still struggle to keep up with chemical use in agriculture, and there remain numerous pesticides in use that will potentially lead to an ongoing legacy of contamination, such as the continued use of PFAS.  However, awareness of these issues is increasing, and efforts are being made to limit pesticide use.

In relation to the risks to human health, the greatest risks are from the more persistent pesticides that can remain in the environment for long periods of time, and where their use was concentrated.  These should be the focus of any risk assessment.

Only a limited number of generic assessment criteria for human health risk assessment have been derived to date for pesticides. The UK contaminated land industry would certainly benefit from the publication of some additional criteria as well as specific guidance on the assessment of pesticides if someone would be prepared to take on the challenge.

References

¹ Health and Safety Executive, 2026, ‘Pesticides’, (www.hse.gov.uk/pesticides/).

² Fera, December 2024, Pesticide Usage Survey Report (https://pusstats.fera.co.uk/published-reports).

³ Pollenzie, May 2022, ‘The History of Pesticide Use in the UK: A Pollenize Perspective’, (https://www.pollenize.org.uk/the-buzz-blog/the-history-of-pesticide-use-in-the-uk-a-pollenize-perspective).

⁴ Carson, R, 2002, ‘Silent Spring, Penguin Classics.

⁵ Control of pesticide Regulations (1986), (https://www.legislation.gov.uk/uksi/1986/1510/made).

⁶ Secretariat of the Stockholm Convention, 2026, ‘Stockholm Convention on Persistent Organic Pollutants (POPs)’, (https://chm.pops.int/TheConvention/Overview/tabid/3351/Default.aspx)

⁷ Gov UK, 2025,’ UK Pesticides National Action Plan’ ( https://www.gov.uk/government/publications/uk-pesticides-national-action-plan-2025)

⁸ LQM/CIEH, 2015, ‘Suitable 4 Use Levels’.

⁹ University of Hertfordshire, 2026, ‘Pesticides Database’ ,https://sitem.herts.ac.uk/aeru/bpdb

¹⁰ Dhakal, G., Thapa Magar, S., & Fujino, T. (2025). Pesticide Degradation by Soil Bacteria: Mechanisms, Bioremediation Strategies, and Implications for Sustainable Agriculture. Environments, 12(12), 492. https://doi.org/10.3390/environments12120492

Article by Andrew Tranter, Senior Principal Geoenvironmental Consultant at Stantec UK