Pesticides and health: farmers have been, are and will be the main victims of these substances
The harmful effects of pesticides on health, particularly on the health of farmers in Western countries and their families, are increasingly well documented. Several types of cancer are notably more prevalent in farming populations than in the general population. This is also the case for various neurodegenerative and respiratory diseases.
Here's what we know so far, and the questions that remain.
What is a “pesticide”?
The term "pesticides" covers a range of synthetic or natural products intended to combat, most often by destroying them, organisms deemed harmful to humans or their activities, particularly in agriculture.
These substances have four uses: they can be phytopharmaceutical products (the best known pesticides, those used on crops), certain biocides (used in livestock buildings or milking parlors, to treat wood to protect it from insects and mold, etc.), certain veterinary medicines (external antiparasitics or antifungals) and finally certain medicines intended for human health (anti-lice, anti-scabies, anti-fungals, etc.).
Pesticides therefore have a toxic effect on living organisms by nature. They are therefore subject to older and more stringent regulations than most other chemical products. These regulations, established at European level, are complex , because they aim to regulate the fourfold use of these substances.
Long-known health effects
The history of pesticides begins at the end of the 19th century. In France, as early as the 1880s, certain substances (arsenics, copper and sulfur derivatives) were used in regions where agriculture was becoming more intensive, particularly in viticulture and arboriculture. Already at that time, health doctors noted the emergence of new diseases related to agricultural workers .
But it was after the Second World War that the use of pesticides really took off, with the shift to industrial production in terms of quantity and variety of chemical families. As a result, several worrying observations were made from the 1950s to the 1970s.
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Acute poisonings are occurring in California orchards among organophosphate applicators and other workers in contact with plants after treatment. Alarming environmental contamination is being detected, and research reveals that human milk is also contaminated, particularly by certain organochlorine insecticides (such as DDT and lindane).
As early as the 1960s in France, some agricultural occupational physicians were concerned about the effects of pesticides on the health of farm workers. In the United States, criticism of their use fueled significant protests from that time on, denouncing their harmful effects on the health of seasonal agricultural workers, consumers, and wildlife.
After more than fifty years of epidemiological studies (1970-2020), it is now accepted that agricultural populations in high-income countries, in which most of the studies have been conducted, present particularities in terms of cancer risk.
Three cancers clearly more common among farmers
In Western countries, there is an excess of certain cancers in agricultural populations compared to the general population.
These are mainly prostate cancers (the most common male cancer in France, affecting nearly 60,000 men each year, resulting in the death of nearly 9,000 of them), non-Hodgkin's lymphomas and multiple myelomas.
For prostate cancer, at least 5 meta-analyses have been conducted on the link with occupational exposure to pesticides, and four of them concluded that there was an increased risk ranging from 13 to 33%. A few meta-analyses have focused on the link with specific chemical families of pesticides, such as that on organochlorine insecticides, which concluded that there was an increased risk ranging from 30 to 56% depending on the molecules studied. For lymphomas, a meta-analysis dating from 2014 showed an increased risk ranging from 30 to 70% for the 7 chemical families studied.
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In its first collective expert report published in 2013, the French National Institute of Health and Medical Research (Inserm) concluded that there was a strong presumption of a link between occupational exposure to pesticides and the occurrence of these three cancers. This conclusion was maintained when this collective expert report was updated in 2021 .
Due to these scientific data, these three cancers are the subject of occupational disease tables in France (table 59 of the agricultural scheme for non-Hodgkin's lymphomas including multiple myelomas and tables 61 (agricultural scheme) and 102 (general scheme) for prostate cancers).
Other cancers that have been the subject of less research (leukemia, central nervous system tumors, sarcomas, kidney and bladder cancers) are also more common among professional pesticide users. The 2021 Inserm collective expert report concluded that there was a moderate presumption of a link for these cancers.
Finally, many other cancers have been very little studied and could not be the subject of a detailed analysis by the Inserm expert assessments of 2013 and 2021 due to a lack of human resources and/or available data. These include bronchopulmonary cancers, digestive cancers (colorectal, stomach, pancreas, liver, esophagus), gynecological cancers (breast, ovaries, body and cervix), ENT or lip cancers and thyroid cancers.
Data are lacking to study all pesticides used
It should be noted that few epidemiological studies have analyzed the links between the occurrence of cancers or chronic diseases and exposure to specific pesticide families or molecules. Indeed, most of the studies conducted involved small numbers of people, making it impossible to explore the diversity of molecules.
It is believed that more than 1,000 molecules with pesticidal activity have been approved in Europe and have been used for agricultural purposes at one time or another. With some molecules being withdrawn while new ones are approved, today the number of authorized molecules is considered to be closer to 400.
However, it is important to also consider molecules that have been withdrawn from the market, due to the delayed effects they may have ( as in the case of lindane , banned in France since 1998 for agricultural and similar uses - but only in 2006 in anti-lice products, which nevertheless still persists in the environment).
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Thus, in the best case scenario, for highly studied cancers and for very old chemical families of pesticides (herbicides such as 2,4D or organochlorine insecticides such as DDT, used since the end of the Second World War), there are no more than ten studies available to document a link.
In the 2015 meta-analysis which concluded that there was an increased risk of prostate cancer of more than 50% for occupational exposure to lindane, due to a lack of data, the authors were only able to analyse 5 organochlorines out of the twenty which have been used extensively throughout the world since the 1950s...
The authors of the 2014 meta-analysis that established a link between non-Hodgkin's lymphomas and exposure to specific pesticides (21 chemical families and more than 80 active ingredients reported) identified only 12 studies providing data on phenoxy-herbicides (2,4D, MCPA, etc.).
In 2017, other authors focused on the link between these non-Hodgkin lymphomas and exposure to 2,4D based on 12 case-control studies and a historical cohort in a factory producing this herbicide. This meta-analysis concluded that there was a 70% increased risk among the most exposed workers.
Diseases other than cancer are also affected
Beyond cancer, a growing body of converging data indicates that pesticide exposure has other health consequences. Effects on the brain, for example, are increasingly well documented.
According to the collective expert assessments of 2013 and 2021 by Inserm, the level of presumption of the link between exposure to pesticides and the development of Parkinson's disease is strong . Knowledge of this link has been built up over time from the occurrence of a few cases observed in people who have been exposed to substances close to certain herbicides (drug addicts who have consumed drugs containing a substance, MPTP, very close chemically to paraquat and diquat, two widely used herbicides).
These findings were reinforced by geographical studies showing a higher prevalence of the disease in certain agricultural areas, then case-control studies and some cohort data. Ultimately, the numerous published studies highlight an almost doubled risk of Parkinson's disease in people who have been exposed to pesticides.
Toxicological data reinforce the understanding of this link: in animals exposed in the laboratory to certain pesticides (notably rotenone, a molecule derived from a plant and considered a biological insecticide), neurodegenerative damage has been highlighted.
Furthermore, more than fifty studies have also revealed alterations in cognitive performance (the brain's ability to process information) in people chronically exposed to pesticides, which also led Inserm's collective expertise to conclude that there is a high level of presumption for these disorders.
These results raise questions about a possible link with Alzheimer's disease, for which cognitive disorders can represent precursor symptoms. However, the number of studies on this disease remains limited. Therefore, the level of presumption of the link is considered "medium" .
Finally, it should be noted that certain chronic respiratory disorders have given rise to a large number of convincing studies over the last ten years, leading Inserm to conclude that there is a strong level of presumption between exposure to pesticides and the risk of developing chronic obstructive pulmonary disease , a serious inflammatory disease of the bronchi.
Accumulate and cross-reference data using large cohorts
The difficulty in documenting the effect of specific pesticide molecules has been partly resolved in some recent studies, which have relied mainly on large prospective cohorts.
This is the case, for example, of the Agricultural Health Study in the USA, which included more than 50,000 farmers using pesticides included at the end of the 1990s (the initial questionnaires asked farmers about the use of around fifty specific molecules).
In France, since the mid-2000s, the AGRIculture & CANcer ( AGRICAN ) cohort has followed more than 182,000 agricultural affiliates in 11 metropolitan French departments, including nearly 70% farmers/livestock farmers. These participants are pesticide users for more than 70% of men and more than 20% of women.
The Agricultural Health Study and AGRICAN cohorts are further combined with data from the Norwegian agricultural census within an international consortium of agricultural cohorts called AGRICOH.
At the same time, most of the more recent case-control studies allow for analysis of the link with specific pesticides. Moreover, some of these case-control studies – the oldest – are grouped into international consortia focusing on targeted diseases, generally uncommon, and benefiting from the pooling of cases on an international scale.
This is the case of the INTERLYMPH consortium: bringing together more than 20 case-control studies conducted in around ten different countries, including France, it covers more than 17,000 patients with lymphomas.
Harmfulness confirmed
Currently, AGRICAN has provided results on the effects of occupational agricultural exposure – including pesticides – on prostate, bladder, colon and rectal cancers, central nervous system cancers, ovarian cancers, as well as multiple myelomas and sarcomas.
For each of these cancers, several production sectors have been associated with harmful effects, as well as certain tasks associated either with direct exposure, during the application of pesticides to crops or in seed treatment, or with indirect exposure: reentry (in other words, returning to crops just after treatment, which leads to contact with treated surfaces and transfer of plant residue to the skin of workers), contact with coated seeds, harvests, etc.
To enable people who have worked in agriculture to estimate their exposure to certain pesticides, depending on the crops they worked on, an epidemiological tool ( PESTIMAT ) was developed. This made it possible to evaluate the influence, in the occurrence of central nervous system tumors, of specific pesticide molecules, such as carbamate herbicides, insecticides and fungicides.
Furthermore, in 2019, AGRICOH concluded that there was an association between exposure to glyphosate and the occurrence of a particular type of lymphoma, diffuse large B-cell lymphoma. This analysis also made it possible to detect an association between exposure to a pyrethroid insecticide, deltamethrin, and the occurrence of another lymphoid blood disease (chronic lymphoid leukemia).
Finally, in 2021, INTERLYMPH research showed (based on 9 case-control studies involving 8,000 lymphoma patients) that farmers' exposure to two insecticides, carbaryl and diazinon, was associated with a doubling of the risk of certain lymphomas. The following year, further research conducted by INTERLYMPH revealed that people who had used phenoxy herbicides such as 2,4 D for many years had a doubled risk of developing several specific lymphomas.
Questions still pending that also concern other professions
The impact of occupational exposure to pesticides on human health, particularly in terms of cancers and certain neurodegenerative diseases, is now largely unquestioned, thanks to a large and converging body of scientific literature. Arguments supporting a link between this exposure and other diseases, particularly respiratory and endocrine diseases, have also grown in number over the years.
However, knowledge needs to be further strengthened. Indeed, gray areas persist, particularly regarding the most critical exposure windows. The impact of pesticide exposure during fetal life and childhood is also a source of concern.
Furthermore, while agriculture is the professional sector using the largest quantities of pesticides, many other sectors of activity are also concerned, but are much less studied (green spaces, wood industry, public hygiene, firefighters, agri-food industries, etc.).
Pierre Lebailly is a lecturer in Public Health, a member of the Interdisciplinary Research Unit for the Prevention and Treatment of Cancers - ANTICIPE, a researcher in epidemiology at the François Baclesse Cancer Control Center in Caen, University of Caen Normandy
Isabelle Baldi is a University Professor – Hospital Practitioner, co-director of the EPICENE team (Epidemiology of cancer and environmental exposures) - INSERM U 1219 Research Center, University of Bordeaux
This article is an op-ed, written by an author outside the newspaper and whose point of view does not reflect the editorial staff's views.
By
Pierre Lebailly and Isabelle Baldi
