Pesticides are natural or synthetic agents that are used to kill unwanted plant or animal pests. While the term pesticide is now often associated with synthetic chemical compounds, it was not until relatively recently that synthetic pesticides came into use. Naturally occurring compounds or natural extracts have been used as pesticides since ancient times. The earliest pesticides were most likely salt, sulfurous rock, and extracts of tobacco, red pepper, and the like.
It is rumored that the Napoleonic army used crushed chrysanthemums to control lice, with limited effectiveness. Petroleum oils, heavy metals, and arsenic were used liberally to control unwanted pests and weeds until the 1940s, when they were largely replaced for many uses by organic synthetic pesticides, the most famous of which is DDT.
Pesticides in Agriculture
The use of synthetic chemical pesticides in agriculture around the world began in the 1950s, though the types and diversity of chemicals used have changed over time. Organochlorine, organophosphate, carbamate and pyrethroid pesticides were introduced onto the world market at that time, marking the beginning of industrial agriculture or the ‘Green Revolution’. In the decades since then, other types of pesticides (e.g. neonicotinoids) have been introduced onto the world market and industrial agriculture has come to rely more and more on the use of synthetic chemical pesticides to protect crops from pests and diseases and to secure or boost yields.
• ORGANOCHLORINE PESTICIDES (OCPs)
Used for agriculture and public health purposes since 1950, though the uses of some have since been severely restricted or banned altogether because of their recognized toxicity to non-target species, including humans. Some organochlorine pesticides are very stable compounds and, therefore, extremely persistent in the environment because of their resistance to natural breakdown processes. For this reason several of the chemicals listed as Persistent Organic Pollutants (POPs) under the 2001 Stockholm Convention are OCPs. Although environmental levels of some organochlorines have fallen over time, many can still be found as contaminants in a wide range of ecosystem compartments, including soils, river sediments, coastal marine sediments reaching as far as the deep oceans and the poles (Willet et al. 1998).
Examples: OCPs include: carbon tetrachloride; chlordane; DDT; DDE; dieldrin; heptachlor; β-HCH; γ-HCH
• ORGANOPHOSPHATE PESTICIDES (OPPs)
The insecticidal properties of certain organophosphate compounds were discovered during military nerve gas research and, since World War II, a number of organophosphate pesticides (OPPs) have been commercialized for agricultural use. OPPs include a diverse range of chemical structures. Their mode of toxicity makes them effective as pesticides as the chemicals inhibit a critical enzyme (acetylcholinesterase) in the central and peripheral nervous system, a property which also accounts for some of their observed toxicity to non-target species.
Examples: acephate; chlorpyrifos; coumaphos; diazinon; dichlorvos; fonofos; parathion; malathion; methyl parathion; phosmet.
Generally neurotoxic and are also acetylcholinesterase inhibitors. Some have been associated with adverse effects on human development, affecting both babies and children (Morais et al. 2012).
Examples: aldicarb; carbaryl; methiocarb; pirimicarb; maneb and mancozeb (both dithiocarbamates); EPTC (S-Ethyl-N,Ndipropylthiocarbamat).
• SYNTHETIC PYRETHROIDS
These interfere with cell signaling (ion channels). Some have been associated with adverse effects on male reproductive health and are suspected endocrine disruptors (effects on hormone function) (Koureas et al. 2012).
Examples: cyhalothrin; cypermethrin; deltamethrin; permethrin.
Newer class of pesticides; for example, imidacloprid was first commercially available in 1985. These substances are structured very like nicotine and block certain cell signaling pathways. They also have negative impact on neurodevelopment (Kimura-Kuroda et al. 2012). Due to suspected toxicity to wild and managed bees, some restrictions on use have been emplaced by the European Commission.
Examples: clothianidin; imidacloprid; thiamethoxam.
The active constituent of Roundup acts by inhibiting a particular enzyme in plants. The health impacts remain disputed, however, the International Agency for Research on Cancer (IARC) recently classified glyphosate as Class 2A ‘probably carcinogenic to humans (Guyton et al. 2015). This classification was based on limited evidence in humans (particularly on links to non-Hodgkins lymphoma) but a robust body of evidence in animals. Also potentially causes endocrine disrupting effects in human cells lines, and reproduction effects (Gasnier et al. 2009, CassaultMeyer et al. 2014). Glyphosate is widely used globally and is the active ingredient in more than 750 different products relating to agriculture, forestry, urban and home application. Its use has increased sharply in conjunction with Roundup Ready crops that are genetically engineered to be resistant to the effects of glyphosate.
How Are We Exposed to Pesticides?
Pesticides in fruits and vegetables
Pesticides are used extensively in the commercial production of fruit and vegetables. Residues of the pesticides applied can persist within the tissues or on the surface of crops when they are brought to market. Over many years, scientists have developed a variety of techniques to quantify levels of pesticides in food, and results have suggested that continuous monitoring is necessary to ensure as far as possible that limit values set for pesticide residues are not exceeded in produce reaching the market place (Wilkowska and Biziuk 2011; Li et al. 2014). Most countries, either on a national or regional basis, maintain a threshold Maximum Residue Level (MRL) for each substance, above which the foodstuff is thought unacceptable for human consumption. For example, the European Union designates MRL limits that apply across the region.
Various literature published between 2007 and 2014 suggests that legumes, leafy greens and fruits such as apples and grapes frequently contain the highest levels of pesticide residues (Bempah et al. 2012; Jardim et al. 2012; Fan et al. 2013; Yuan et al. 2014). There is consistent evidence that these substances are regularly present as mixtures of multiple residues and, in many cases, at levels above MRL limits in certain countries (Latifah et al. 2011; Jardim et al. 2012). Among many other pesticides, cypermethrin, chlorpyrifos, iprodione, boscalid, dithiocarbamates and acephate are regularly detected in our food (Claeys et al. 2011; Lozowicka et al. 2012; Yuan et al. 2014). Whilst extensive research suggests that washing and cooking vegetables does reduce some of these residues that are on the surface of the plant, in some cases food preparation can actually concentrate levels (Keikotlhaile et al. 2010).
Pesticides in fish
Organotins have been widely used as fungicides and biocides in agriculture since the 1970s. Organotin compounds (primarily tributyltin, or TBT) were also commonly used as antifouling agents on boats and ships, contributing to widespread pollution of many coastal waters and leading to a global ban of this application by the International Maritime Organisation under the 2001 International Convention on the Control of Harmful Antifouling Systems on Ships (the AFS Convention, which entered into force in 2008).
A study of organotin pollution in global marine environments found that a triphenyltin compound (TPT), used as a pesticide on land, was also a common contaminant of sediments (Yi et al. 2012). Phenyltin compounds are not easily biotransformed by marine organisms and therefore bioaccumulate and potentially biomagnify through marine food web systems. Concentrations of organotins are particularly high in the blood of those people who consume greater amounts of seafood and it has been suggested that regular monitoring of the levels of these substances be carried out for public health purposes (Yi et al. 2012).
Pesticides in animal products
Farmed animals can also accumulate pesticides from contaminated feed and from veterinary pesticide application. Whilst these substances are generally stored in the fat and muscles of the animals, some can also be found in the brain, liver, lungs and other offal (LeDoux 2011).
Insecticides and acaricides are often used to control ectoparasites such as red mites in poultry and egg production. Consequently, some of these pesticides accumulate in the muscle, fat and liver and can be detected in eggs even long after the chemicals have been eliminated from other tissues (Schenck and Donoghue 2000). Milk and other dairy products also similarly contain a range of substances through bioaccumulation and storage in the fatty tissues of the animals. This is of particular concern as cows’ milk is often a staple component of human diets, and is particularly widely consumed by children.
Exposure from Agricultural and Urban Pesticide Spraying
Pesticides sprayed on agricultural land, and in urban areas, become airborne during application and can drift for great distances in the air. For example, a study in the US found that several commonly used pesticides can be detected far beyond the sites of agricultural application. At distances of 10 m to 150 m away from the application sites some pesticides, such as diazinon and chlorpyrifos, still exceeded government safety levels (Reference Exposure Levels for air) (Sutton et al. 2011). People living in agricultural areas may therefore have a high exposure to pesticides because of inhalation of pesticide spray drift. Similarly, when pesticides are sprayed in parks and urban areas or in the home, people can be exposed if they breathe in the contaminated air.
Exposure via Household Dust, Spraying and Garden Soil
Household dust has been found to be contaminated with many chemicals including some pesticides, particularly if they are commonly used to control household pests (Naeher et al. 2010). The key substances used in domestic pest control are the pyrethroids permethrin and cyfluthrin, and in some cases chlorpyrifos. Ingestion, inhalation and skin contact with contaminated dusts can lead to continued and varied exposures to pesticides (Morgan et al. 2007, 2014; Starr et al. 2008). Homes situated in agricultural areas, particularly those that are in close proximity to pesticide-sprayed land, have been shown to be more contaminated (Harnly et al. 2009). However, contaminated dust is also a potential problem in urban areas where residues persist as a result of household application (Naeher et al. 2010; MuňozQuezada et al. 2012).
Health Impacts linked to Pesticide Exposure
Effects of Prenatal (Fetal) & Infant Exposures
Human development is particularly vulnerable to effects of toxic chemicals, including pesticides. Exposure of pregnant mothers to pesticides, and in some cases exposure of young children themselves, has been linked to adverse health outcomes for the children, including:
1. Reduced birth weights and lengths and occurrence of abnormalities
2. Lower intelligence
3. Altered behavior
4. Higher incidence of leukemia and other cancers
5. Higher incidence of miscarriage
These adverse impacts on children’s health have been reported for children born to mothers who worked with pesticides whilst pregnant, though the health effects of pesticide exposure are a concern also for children from the general population, living both in agricultural regions and in the city.
Several studies, including research on agricultural workers, suggest that increased risk of prostate cancer may be associated with pesticide use, particularly OCPs (Band et al. 2010). The risk of prostate cancer was greater for those exposed to OCPs who also had a family history of the disease (Alavanja et al. 2003; Alavanja and Bonner 2012; Mills and Shah 2014).
As most of all lung cancer is related to cigarette smoking, it is particularly difficult to study the impact of other substances. Therefore, studies must adjust for the effect of smoking to identify the contribution of other chemicals. In general, although it is suspected that agricultural workers smoke less than other population groups as a result of their outdoor and physical lifestyle, in the case of prolonged exposure to certain pesticides (e.g. chlorpyrifos), there is some evidence to suggest that these workers suffer a higher incidence of lung cancer (Lee et al. 2004a; Lee et al. 2004b; Alavanja and Bonner 2012).
There are certain, more rarely diagnosed, cancers for which there is some evidence of association with various occupational health hazards. Amongst the many other substances that people are exposed to, long-term occupational use of pesticides is thought to be associated with increased risk of multiple myelomas, bone sarcoma and Ewing sarcoma that develops in the bone and surrounding tissues (Merletti et al. 2006; Perrota et al. 2008; Vinson et al. 2011; Pahwa et al. 2012; Charbotel et al. 2014). Some incidence of Hodgkin’s disease (a lymphomal cancer) may also be related to pesticide exposure, particularly from chlorpyrifos (Khuder et al. 1999; Orsi et al. 2009; Karunanayake et al. 2012).
The mechanisms by which pesticides can cause cancer are numerous. Direct damage to the DNA (genotoxicity) is thought to occur in agricultural workers in contact with organophosphates, carbamates, pyrethroids and complex pesticide mixtures, though there may also be other mechanisms involved (Bolognesi 2003; Bolognesi et al. 2011). Some people within a population may be at a greater risk than others due to variations in their genetic characteristics. There are several genes which code for enzymes known to detoxify pesticides, and others that are specifically involved in DNA repair. Some individuals carry variants of these genes that code for enzymes that are not as effective and, as a consequence, their bodies are less able to cope with these chemicals when exposure occurs. This is thought to be part of the mechanism involved in putting some individuals at a greater risk of developing cancer than others, though uncertainties in this field remain high.
Parkinson’s disease is a common neurodegenerative disease that is characterized by neuron loss in the mid-brain. Movement-regulating cells in this area of the brain are disabled, causing the person to suffer from tremors and slow movement, balance problems and sometimes behavioral changes (Chhillar et al. 2013). The causes of Parkinson’s disease are complex – it is associated with aging, gender and genetic factors, upon which environmental factors such as pesticide exposures are superimposed (Wang et al. 2014).
Several studies have, nevertheless, found that exposure to pesticides in farm workers and pesticide sprayers is statistically associated with an increased risk of developing Parkinson’s disease (Van Maele-Fabry et al. 2012). Van der Mark et al. (2012) reviewed 46 studies on the association between pesticides and Parkinson’s disease, concluding that summary risk estimates strongly suggest that the risk of developing Parkinson’s disease is increased by exposure to pesticides, particularly herbicides and/or insecticides.
Dementia and Alzheimer’s disease
Alzheimer’s disease (AD) is the most common form of dementia. Genetic factors account of up to 70% of the risk associated with contracting AD, as well as obesity, smoking, inactivity, hypertension and diabetes (Ballard et al. 2011). In addition to these well-known factors, there is an emerging body of evidence to suggest that exposure to certain pestcides, particularly chronic exposure to OPPs, may contribute to the risk of developing AD (Zaganas et al. 2013). For example, some studies have shown that there is an increase in cognitive, behavioural and psychomotor dysfunction with increased long-term exposure (Costa et al. 2008).
AMYOTROPHIC LATERAL SCLEROSIS (ALS)
ALS is a rare condition affecting 1-2 people per 100,000 and is a rapid neurodegenerative disease where the motor neurons of the brain and spinal cord are affected. Around 10% of cases have a family history, but environmental factors such as exposure to solvents, metals and OCPs are thought to increase the risk of developing the disease (Kamel et al. 2012). Acute poisoning by OPPs may also be linked to the development of ALS, and further research is required in order to focus on quantifying the exposure of people to different pesticide classes and to test the strength of correlation with development of the disease (Baltazar et al. 2014).
Experimental research has indicated that many pesticides are endocrine disruptors that can disturb the functioning of various hormones throughout the body (Mnif et al. 2011; Mandrich et al. 2014). Thyroid hormone production is thought to be inhibited by substances such as amitrole, cyhalothrin, fipronil and pyrimethanil. Floriculture workers who were exposed to various OPPs, showed altered levels of thyroid hormones in their bodies (Lacasaña et al. 2010).
Pesticides and Sex Hormones
Experimental studies conducted in vitro (in a test-tube or cell-line culture) support observations that the balance of sex hormones can be disrupted by exposure to certain pesticides (Kjeldsen et al. 2013). Andersen et al. (2008) reports that the sons of women that have been exposed to pesticides whilst pregnant during their work in greenhouses can suffer impaired development. Conversely, girls whose mothers worked in greenhouses in Denmark during the first trimester of pregnancy have been shown to develop breasts earlier than in other populations, even though hormone levels appeared similar when the girls reached school age (Wohlfahrt‐Veje et al. 2012).