Pesticides

Additional scientific description

Pesticides can also be grouped according to the types of pests which they kill: insecticides (insects); herbicides (plants); rodenticides (rodents); bactericides (bacteria); fungicides (fungi); and larvicides (larvae) (WHO and FAO, 2016). Pesticides are used in many different sectors (e.g., agriculture, forestry, food industry, domestic etc.).

Insecticides are chemicals used to control insects by killing them (CDC, 2019). Insecticides are classified based on their structure and function. They include:

1. Carbamates/Organophosphates: Humans may be exposed via inhalation, ingestion or through skin contamination; and acute and chronic exposure can produce varying levels of toxicity. Similar to the insects exposed, these pesticides disrupt the function of the human nervous system, mainly the brain. They have also been linked to cancer risk. Their toxicity to humans has led to bans and use restrictions in many countries (CDC, 2019);
2. Organochlorines: These are a group of chlorinated compounds that typically affect the central nervous system. They are known for their high toxicity, slow degradation and bioaccumulation. Examples include dichlorodiphenyltrichlorethane (DDT), methoxychlor, chlordane, and lindane. Human exposure may occur via inhalation, ingestion or through skin contamination. Many organochlorines have been banned in many countries, although a few are still registered for use (CDC, 2019).
3. Boric acid: This type of pesticide comes in many forms (most commonly in pellets or tablets or a finely ground powder) and has many uses. It is a combination of boron and other elements. Toxicity depends on the amount of boron in the product. In addition to controlling insects, boric acid can be used to control growth of moulds, fungi and weeds. Boric acid is low in toxicity but can still cause irritation to the skin or eyes, and nausea, vomiting, stomach aches and diarrhoea if ingested (CDC, 2019).
4. Pyrethroids/Pyrethrins: Pyrethrum is a naturally occurring mixture of chemicals found in certain chrysanthemum flowers. Pyrethrum was first recognised as having insecticidal properties around 1800 in Asia and was used to kill ticks and various insects such as fleas and mosquitos. From pyrethrum extract, individual chemicals called pyrethrins which have active insecticidal properties were developed. Pyrethroids are manufactured chemicals that are very similar in structure to the pyrethrins, but are often more toxic to insects, as well as to mammals, and last longer in the environment than pyrethrins (ATSDR, 2003). These insecticides are used widely in households, agriculture, on pets, and in mosquito control. While they are generally less toxic to mammals than some other insecticides, they can still have harmful health effects (CDC, 2019).

Fungicides are pesticides that kill or slow the growth of fungi and their spores. They can be used to control fungi that damage plants, including rusts, mildews and blights. They might also be used to control mould and mildew in other settings. Fungicides work in a variety of ways, but most of them damage fungal cell membranes or interfere with energy production within fungal cells (NPIC, 2019).

Pesticides are inherently toxic, and among them, a small number of Highly Hazardous Pesticides, cause disproportionate harm to the environment and human health. Highly Hazardous Pesticides means pesticides that are acknowledged to present particularly high levels of acute or chronic hazards to health or environment according to internationally accepted classification systems such as WHO or Global Harmonized System (GHS) or their listing in relevant binding international agreements or conventions. In addition, pesticides that appear to cause severe or irreversible harm to health or the environment under conditions of use in a country may be considered to be and treated as highly hazardous.

Around a thousand active ingredients are used to manufacture the wide array of pesticides in countries all over the world. Pesticide ingredients come in many thousands of different formulations, including nanopesticide formulations in which an active ingredient is carried on or within a nanoparticle. All these formulations degrade over time although some are more persistent in the environment than others. The chemical by-products that form as the pesticide deteriorates can be more toxic than the original product (FAO, no date b).

Hazardous pesticide contamination in soils often results from improper storage of (obsolete) agrochemicals, as a result of which pesticides are spilled in the surroundings of the storage site, where they seep into the soil or are dispersed by wind. In some cases, pesticide spillage has been ongoing for many years. Such spillage may cause serious soil or groundwater contamination. In addition, highly toxic and persistent compounds have been used in agriculture for decades to control pests and diseases, which are proven to cause harm to non-target species. Although international agreements are put in place to regulate the production and use of those highly toxic and persistent compounds they will still remain in soils for several more decades. Moreover, in some countries, the international agreements are not yet being implemented or fully implemented, and therefore toxic pesticides are still being used. When soil and groundwater are contaminated, crops, livestock and drinking water may become affected and, when they are consumed by people, health risks may occur (FAO, 2000).

Once pesticides enter soil, their fate depends on the physico-chemical characteristics of the soil, such as moisture, texture, and soil organic matter content, as well as the pesticide properties. A relatively small amount of spilled pesticides can therefore create a much larger volume of polluted soil. For example, approximately 30 tonnes of pesticides buried at a site in Yemen in the 1980s contaminated over 1500 tonnes of soil. This can pose a serious health and environmental threat to nearby communities (FAO, no date c).

Pesticide residue refers to the pesticides that may remain on or in food after they are applied to food crops or owing to their persistence in the environment or because of other uses such as vector control to combat endemic pests such as mosquitos transmitting malaria. Risk of pesticide residues mainly concerns occupational health of field workers, food safety (consumer health) and the environment. The maximum allowable levels of pesticide residues in foods are often stipulated by national regulatory bodies and by the Joint Food and Agriculture Organization of the United Nations (FAO) / World Health Organization (WHO) Food Standards Programme (Codex Alimentarius Commission).

The following definitions are relevant to the Codex Alimentarius (FAO, no date):

Pesticide: means any substance intended for preventing, destroying, attracting, repelling, or controlling any pest including unwanted species of plants or animals during the production, storage, transport, distribution and processing of food, agricultural commodities, or animal feeds or which may be administered to animals for the control of ectoparasites. The term can also include substances intended for use as a plant growth regulator, defoliant, desiccant, fruit-thinning agent, or sprouting inhibitor and substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport. The term normally excludes fertilisers, plant and animal nutrients, food additives, and animal drugs.

Metrics and numeric limits

Available data are too limited to estimate the overall global health impacts of pesticides; however, the global impact of

self-poisoning (suicides) from preventable pesticide ingestion was estimated to be 155,488 deaths and 7362,493 Disability Adjusted Life Years (DALYs) for 2016 (WHO, 2019).

The WHO provides maximum residue limits (MRLs) for pesticides in food and levels in drinking-water as follows (WHO, 2019):

• Maximum residue limits in food: The Joint Food and Agriculture Organization of the United Nations (FAO) / WHO Meeting on Pesticide Residues (JMPR) evaluates those pesticides likely to contaminate food. MRLs are published by the Codex Alimentarius Commission. Guidance for individual pesticides or pesticide components - including for a number of highly hazardous pesticides - can be accessed via the FAO, WHO, Codex Alimentarius Commission or National Framework for Chemicals Environmental Management (INCHEM) websites and in hard-copy publications.
• Drinking-water: WHO water quality guidelines exist for some pesticides used in agriculture and public health - including for some highly hazardous pesticides - where there is a likelihood of drinking-water contamination. Many pesticides have a maximum contaminant level given by the US Environmental Protection Agency of approximately 0.2 ppb (EPA, no date).