Health effects of pesticides

Route of exposure

People can be exposed to pesticides, which include insecticides, herbicides, fungicides, by a number of different routes including: occupation, in the home, at school, in the air, water, soil, and in food. Almost all humans are exposed to some level of pesticides. For example, pesticide drift, may be a potentially significant source of exposure to the general public. Exposure can occur via ingestion, inhalation, or contact with skin. Some pesticides can remain in the environment for prolonged periods of time.

There are concerns that pesticides used to control pests on food crops are dangerous to people who consume those foods. Many food crops, including fruits and vegetables, contain pesticide residues even after being washed or peeled. Chemicals that are no longer used but that are resistant to breakdown for long periods may remain in soil and water and, thus, in food. For example, most people in the United States still have detectable levels of dichlorodiphenyltrichloroethane (DDT), an insecticide, despite its ban in the US in 1972. These concerns are one reason for the organic food movement. In California, 92% of farmworkers are Latino and exposure to pesticides in majority-Latino counties of the state to pesticides is 906% higher than counties in which the Latino population is fewer than 24%. This has raised concerns over environmental justice.

Because of the common use of pesticides in agriculture, the United Nations Codex Alimentarius Commission has recommended international standards for maximum residue limits (MRLs), for individual pesticides in food. In the United States, levels of residues permitted to remain on foods are limited based on tolerance levels considered to be safe as established by the U.S. Environmental Protection Agency (EPA). The EPA sets the permitted levels of pesticide residues based on the toxicity of the pesticide, its breakdown products, the amount and frequency of pesticide application, and how much of the pesticide (i.e. the residue) remains in or on food by the time it is marketed and distributed. Tolerance levels are obtained using scientific risk assessments that pesticide manufacturers are required to conduct—assessments include toxicological studies, exposure modeling and residue studies before a particular pesticide can be registered. However, the effects are tested a single pesticide at a time and there is little information on the possible synergistic effects of exposure to multiple pesticide traces in the air, food and water on human health.

While pesticide use is commonly associated with agriculture, pesticides are also used as part of public health interventions to control vector-borne diseases (e.g. malaria and Dengue fever) and unwanted plants in the landscaping of parks and gardens.

Mechanism of action

Pesticides are designed to kill living organisms and vary in their mechanisms of action, depending on their class. The major classes of pesticides are organochlorines (OCs) or persistent organic pollutants (POPs), organophosphates, carbamates, phyrethroids, and triazines. While all pesticides have been shown to have effects on human health, OCPs are notable for significant risk for adverse effects as they dissolve in fatty tissues and can, thus, accumulate to harmful levels in these tissues. For example, some OCPs are structurally similar to estrogen and can mimic the effects of endogenous estrogen via binding to the estrogen receptors. These pesticides exhibit their toxic effects by interfering with hormonal homeostasis, resulting in hormonal dysregulation. This promotes abnormal growth and development of reproductive tissues and can lead to cancer or harmful effects on reproductive health. OCPs or POPs, which were used in agricultural practices in the 1950s, have now been banned in most countries. However, their breakdown products are persistent and can still be found in soil.

Pesticides can also exert their adverse effects on human health by acting on cell receptors and ion channels, suppressing key signal pathways in cells, and by affecting DNA methylation and histone modifications, thus resulting in changes in gene expression and cellular function.

Acute effects

Main article: Pesticide poisoning

Acute health problems may occur in workers that handle pesticides, such as abdominal pain, dizziness, headaches, nausea, vomiting, as well as skin and eye problems. In China, an estimated half-million people are poisoned by pesticides each year, 500 of whom die. Pyrethrins, insecticides commonly used in common bug killers, can cause a potentially deadly condition if breathed in.

Long-term effects

Cancer

Pesticide residues on food do not cause cancer. Some medical research has suggested glyphosate exposure may cause cancer, but there is no good evidence of this, even with high-level exposure.

Agricultural workers exposed to high levels of synthetic pesiticide have an increased prevalence of some cancers (including lymphoma and prostate cancer), and a decreased prevalence of others.

Neurological

A review of multiple studies that looked at high pesticide exposure, mainly organophosphates, among agricultural workers suggested an association with various neurological disorders, but the evidence was weak.

The United States Environmental Protection Agency finished a 10-year review of the organophosphate pesticides following the 1996 Food Quality Protection Act, but did little to account for developmental neurotoxic effects, drawing strong criticism from within the agency and from outside researchers. Comparable studies have not been done with newer pesticides that are replacing organophosphates.

In-utero and early-childhood exposure

There is accumulating evidence of neurological effects secondary to pesticide exposure. Acute exposure to high levels of pesticides that affect the central nervous system can cause neurotoxicity, including cognitive and motor changes. In-utero and early-childhood exposure to organophosphates can cause neurodevelopmental impairment, in particular because some pesticides and their metabolites cross the placenta and fetal blood-brain-barrier, which has not fully developed in a fetus.

Parkinson's disease and Alzheimer's disease

An accumulation of chronic exposure has been associated with an increased risk of developing neurodegenerative disease later in life. There is strong evidence that chronic exposure to pesticides increases risk of developing Parkinson's disease, potentially through direct toxic effects on dopaminergic neurons (which are depleted in Parkinson's disease). A review paper implicated several pesticides in Parkinson's disease, including rotenone, PQ (paraquat), MB (methyl benzoate), organochlorines and pyrethroids. In addition, there is increasing evidence that chronic exposure increases risk of Alzheimer's disease.

Autism and ADHD

There is some suggestive research, but no good evidence, potentially linking pesticide exposure with autism and ADHD. Organochlorines specifically have been linked with autism. Furthermore, it is thought that early disruptions to the gut microbiome caused by pesticides can lead to symptoms of autism.

Reproductive effects

Many pesticides act as endocrine-disrupting chemicals (EDC) or substances that interfere with normal hormonal activity. As of 2013, 101 pesticides have been listed as proven or possible endocrine disruptors. As such, high levels of and/or long-term exposure to pesticides can impact reproductive health and is associated with decreased fertility, increased rates of miscarriage, and changes in pattern of maturity. Specifically, triazines, organs-chlorine, and carbamate insecticides have anti-androgenic effects impacting males, resulting in the lack of development of male characteristics including decreases in testicular size, sperm production, and androgen production. A number of pesticides including dibromochlorophane and 2,4-D has been associated with impaired fertility in males. Pesticide exposure resulted in reduced fertility in males, genetic alterations in sperm, a reduced number of sperm, damage to germinal epithelium and altered hormone function.

The effects of endocrine disruption is dependent on the timing of pesticide exposure (for example, during embryogenesis in early pregnancy or in infancy) as windows of varying susceptibility dictate disease manifestation. Several studies suggest that higher levels of pesticides in the blood of the mother is associated with longer time to pregnancy and greater infertility rates. For example, in mothers and/or their partners who reported pesticide exposure, there was an increased risk of miscarriage with the strongest association with exposure during the first three weeks of pregnancy. This is perhaps linked to the possible negative impact of pesticides on oocyte development and fertilization. Similarly, studies evaluating the short-term impact of occupational exposure to a variety of pesticides on reproductive health suggest that pesticides can have deleterious effects on sperm—pesticide exposure, associated with decreased sperm motility, defects in sperm morphology and semen volume. However, the long-term impacts of pesticide exposure on spermatogenesis and fertility are unknown.

Fetal death and birth defects

Because some pesticides and their resulting metabolites can cross the placenta and, therefore, the blood-brain-barrier, they can also impact development of the fetus; strong evidence links pre- and post-natal exposures to pesticides to congenital disorders, including physical and/or mental disabilities, fetal death and altered fetal growth. Perhaps the more widely known health effect of pesticides is the elevated rate of birth defects in areas in Vietnam sprayed with defoliant or Agent Orange, a 50:50 mixture of 2,4,5-T and 2,4-D, which has been associated with bad health and genetic effects in Malaya and Vietnam. It was also found that offspring who were at some point exposed to pesticides had a low birth weight and had developmental defects. Maternal exposure to pesticides has also been linked to higher incidence of hypospadias in the newborn, which is the abnormal opening of the urethra in males.

Respiratory problems

Studies have indicated that pesticide exposure is associated with long-term respiratory problems. A significant association was found between exposure to pesticides and decreased lung function along with related airway symptoms. Decreased lung function was associated with occupational exposure to pesticides. Studies have suggested a correlation between inhibition of cholinesterase by pesticides like carbamate and organophosphate and reduction or impairment of lung function. In addition, exposure to pesticides was also reported to be linked with obstructive and restrictive lung conditions. Specifically, organophosphate exposure was associated with lung function decline driven by a restrictive process.

Other

Some studies have found increased risks of dermatitis in those exposed.

There is increasing evidence that possibly suggests increased risk of development of type 2 diabetes with exposure to pesticides and their metabolites.

Prevention

Pesticides exposure cannot be studied in placebo controlled trials as this would be unethical.

The American Medical Association recommend limiting exposure to pesticides. The utility of applicator certification and public notification programs are also of unknown value in their ability to prevent adverse outcomes.

Epidemiology

The World Health Organization and the UN Environment Programme estimate that each year, 3 million workers in agriculture in the developing world experience severe poisoning from pesticides, about 18,000 of whom die. According to one study, as many as 25 million workers in developing countries may suffer mild pesticide poisoning yearly. Detectable levels of 50 different pesticides were found in the blood of a representative sample of the U.S. population.

Research conflicts of interest

Concerns regarding conflict of interests regarding the research base have been raised for some research into toxicity of pesticides. For example, Richard Doll of the Imperial Cancer Research Fund in England was found to have undisclosed ties to industry funding.

Other animals

A number of pesticides including the neonicotinoids clothianidin, dinotefuran, imidacloprid are toxic to bees. Exposure to pesticides may be one of the contributory factors to colony collapse disorder. A study in North Carolina indicated that more than 30 percent of the quail tested were made sick by one aerial insecticide application. Once sick, wild birds may neglect their young, abandon their nests, and become more susceptible to predators or disease.

References

References

1. (30 August 2007). "Pesticides: Health and Safety. National Assessment of the Worker Protection Workshop #3". U.S. Environmental Protection Agency.
2. (February 2008). "Management of acute organophosphorus pesticide poisoning". Lancet.
3. (October 2007). "Non-cancer health effects of pesticides: systematic review and implications for family doctors". Canadian Family Physician.
4. (2008). "Prenatal and childhood exposure to pesticides and neurobehavioral development: review of epidemiological studies". International Journal of Occupational Medicine and Environmental Health.
5. (January 2017). "Exposure to pesticides and the associated human health effects". The Science of the Total Environment.
6. "What are POPs?". Pops.int.
7. (2010). "Pesticides and health risks". Journal of Obstetric, Gynecologic, and Neonatal Nursing.
8. (December 1999). "Spray drift of pesticides". [[U.S. Environmental Protection Agency]].
9. (March 1999). "Consumer concerns about pesticides in food". Cornell University, College of Veterinary Medicine.
10. (October 2013). "Farmworkers in California: A Brief Introduction". CRB Short Subjects.
11. (21 January 2021). "Pesticide use in California remains at record high, new data show {{!}} Pesticide Action Network".
12. "Code of Ethics for International Trade in Food". Codex Alimentarius Commission.
13. (27 March 2007). "Pesticides and food: What the pesticide residue limits are on food". U.S. Environmental Protection Agency.
14. (4 July 2007). "Setting tolerances for pesticide residues in foods". U.S. Environmental Protection Agency.
15. (2001). "Pesticide Mixtures Induce Immunotoxicity: Potentiation of Apoptosis and Oxidative Stress". Virginia Polytechnic Institute and State University.
16. (March 2020). "Endocrine disrupting chemicals: exposure, effects on human health, mechanism of action, models for testing and strategies for prevention". Reviews in Endocrine & Metabolic Disorders.
17. (2021). "Pesticides: formulants, distribution pathways and effects on human health - a review". Toxicology Reports.
18. Ecobichon DJ. 1996. Toxic effects of pesticides. In: Casarett and Doull's ''Toxicology: The Basic Science of Poisons'' (Klaassen CD, Doull J, eds). 5th ed. New York:MacMillan, 643–689.
19. (13 February 2007). "Chinese develop taste for organic food: Higher cost no barrier to safer eating". Bloomberg News.
20. (17 May 2006). "Medical Encyclopedia: Insecticide". U.S. National Library of Medicine.
21. (20 December 2024). "Do pesticides cause cancer?".
22. (March 2020). "Synthetic Pesticides and Health in Vulnerable Populations: Agricultural Workers". Curr Environ Health Rep.
23. (October 2006). "Registering skepticism: does the EPA's pesticide review protect children?". Environmental Health Perspectives.
24. (26 May 2006). "EPA workers blast agency's rulings on deadly pesticides: Letter sent to EPA administrator Stephen L. Johnson by unions representing 9,000 EPA scientists". The Oregonian.
25. (August 2013). "Growing up with pesticides". Science.
26. (September 2019). "Neurotoxicity of pesticides". Acta Neuropathologica.
27. (2021-02-10). "An extensive review on the consequences of chemical pesticides on human health and environment". Journal of Cleaner Production.
28. (June 2021). "Reproductive Health Risks Associated with Occupational and Environmental Exposure to Pesticides". International Journal of Environmental Research and Public Health.
29. (November 2003). "Measurement of pesticides and other toxicants in amniotic fluid as a potential biomarker of prenatal exposure: a validation study". Environmental Health Perspectives.
30. (April 2013). "Pesticides and human chronic diseases: evidences, mechanisms, and perspectives". Toxicology and Applied Pharmacology.
31. (September 2022). "Neurotoxicity of pesticides - A link to neurodegeneration". Ecotoxicology and Environmental Safety.
32. (January 2022). "Association Between Exposure to Pesticides and ADHD or Autism Spectrum Disorder: A Systematic Review of the Literature". Journal of Attention Disorders.
33. (July 2023). "Impact of pesticides exposure during neurodevelopmental period on autism spectrum disorders - A focus on gut microbiota". Ecotoxicology and Environmental Safety.
34. (July 2016). "Endocrine-Disrupting Chemicals and Reproductive Health". Journal of Midwifery & Women's Health.
35. (April 2003). "Effect of occupational exposures on male fertility: literature review". Industrial Health.
36. (29 August 2022). "Environmental Impacts on Reproductive Health: Pesticides".
37. (October 2006). "Association between Agent Orange and birth defects: systematic review and meta-analysis". International Journal of Epidemiology.
38. (2010). "Paternal exposure to Agent Orange and spina bifida: a meta-analysis". European Journal of Epidemiology.
39. (May 2006). "Pesticide exposure: the hormonal function of the female reproductive system disrupted?". Reproductive Biology and Endocrinology.
40. (June 2014). "Is pesticide exposure a cause of obstructive airways disease?". European Respiratory Review.
41. (November 2013). "Occupational pesticide exposures and respiratory health". International Journal of Environmental Research and Public Health.
42. (December 2000). "Pesticides and parkinsonism: is there an etiological link?". Current Opinion in Neurology.
43. (1997). "Educational and informational strategies to reduce pesticide risks. Council on Scientific Affairs". Preventive Medicine.
44. Miller GT (2004), ''Sustaining the Earth'', 6th edition. Thompson Learning, Inc. Pacific Grove, California. Chapter 9, Pages 211-216.
45. (1990). "Acute pesticide poisoning: a major global health problem". World Health Statistics Quarterly. Rapport Trimestriel de Statistiques Sanitaires Mondiales.
46. (March 2007). "Secret ties to industry and conflicting interests in cancer research". American Journal of Industrial Medicine.
47. (8 December 2006). "Renowned cancer scientist was paid by chemical firm for 20 years".
48. (2010). "Insect Nicotinic Acetylcholine Receptors".
49. (May 2008). "Colony Collapse Disorder: Many Suspects, No Smoking Gun". BioScience.
50. (1992). "Wildlife and Pesticides-Corn.". North Carolina Cooperative Extension Service.