The Myth of Pesticide Breakdown

“Modern pesticides rapidly biodegrade” is a myth — an excerpt from The Myths of Safe Pesticides by André Leu

One of the major pesticide legends is the belief that most modern agricultural chemicals rapidly biodegrade and leave few if any residues. We are misled into believing that they break down and do not persist in our food like older chemicals such as DDT. 

The following is a claim by the main food regulator in Australia and New Zealand, FSANZ, and is typical of the claims by many nations’ regulators. They state, “Organophosphorous pesticides, carbamate pesticides, are mostly biodegradable, and therefore do not concentrate in the food chain. Synthetic pyrethroids . . . are generally biodegradable and therefore tend not to persist in the environment.”¹ These types of statements give the false impression that few agricultural pesticides persist in our food and environment. In fact, most agricultural and veterinary chemicals leave residues in food. That is the reason why tolerances for maximum residue limits (MRLs) and the acceptable daily intake (ADI) are set for these poisons. 

The data presented in the United States President’s Cancer Panel 2010 report indicating that only 23.1 percent of food samples had zero pesticide residues is reasonably consistent with the data from testing in most countries. This means that the overwhelming majority of foods contain pesticide residues.

Many of the current chemicals, including some of the synthetic pyrethroids, organophosphates, carbamates, and herbicides such as atrazine, are as residual as the mostly banned older chemicals such as the organochlorine group that includes dieldrin, DDT, chlordane, heptachlor, lindane, and aldrin.

METABOLITES OF PESTICIDES

One of the biggest myths is the assumption that once a chemical degrades it disappears and is harmless. Most agricultural poisons leave residues of breakdown products or daughter chemicals when they degrade.² These breakdown products of chemicals are also called metabolites. Where there is research, it shows that many of the metabolites from agricultural poisons cause health and reproductive problems.

A substantial number of agricultural pesticides—such as organophosphates like diazinon, malathion, chlorpyrifos, and dimethoate—become even more toxic when they break down. These metabolites are known as oxons. Scientists at the Cooperative Wildlife Research Laboratory at Southern Illinois University and the Western Ecology Research Center of the U.S. Geological Survey in Point Reyes, California, found that the oxons can be up to one hundred times more toxic than the original pesticide.

“In this study, the oxon derivatives of chlorpyrifos, malathion, and diazinon were significantly more toxic than their respective parental forms. Chloroxon killed all of R. boylii tadpoles and was at least 100 times more [toxic] than the lowest concentration of chlorpyrifos, which resulted in no mortality. Maloxon was nearly 100 times more toxic than malathion, and diazoxon was approximately 10 times more toxic than its parent. This is consistent with other studies that have compared parent and oxon forms.”³ 

Dimethoate is a good example. Dimethoate is a systemic pesticide because it is absorbed into all the tissues of the plant, including the edible portions such as all the flesh of fruits, stems, tubers, and leaves. 

Contrary to popular belief, because systemic poisons are absorbed into the flesh—and consequently every part of the plant is toxic—washing or peeling the surface of the food only removes a small percentage of the poisons on the surface. It will not remove the bulk of poison, which is inside the food. 

Dimethoate is widely used as a fruit fly treatment because it is so residual that even after two weeks, any maggots that hatch from eggs inside the fruit will be killed by the poison residues in the edible portion of the flesh. Dimethoate breaks down to an even more toxic metabolite called omethoate. Omethoate is also used as a pesticide, and consequently, unlike the vast majority of metabolites, it has been researched and has an LD₅₀. According to the WHO, omethoate has an LD₅₀ of 50 milligrams per kilogram, whereas dimethoate has an LD₅₀ of 150 milligrams per kilogram. This means that as the dimethoate decays within the treated food, it becomes 300 percent more toxic as omethoate. Under the WHO classification of hazards, it goes from being a moderately hazardous to a highly hazardous pesticide. Several countries have withdrawn or are in the process of withdrawing omethoate from use as a pesticide due to its high toxicity and its persistence. Other countries are still debating whether to ban dimethoate. All food that is treated with dimethoate will end up with residues of the more toxic and persistent omethoate, as well as a number of other toxic metabolites that are generated as the dimethoate breaks down.

In her article “A Case for Revisiting the Safety of Pesticides,” Dr. Theo Colborn gives the example of research into paraoxon, the main metabolite of parathion, showing that it is very toxic and causes a range of negative health effects. “Chronic paraoxon exposure (0.1, 0.15, or 0.2 mg/kg subcutaneously) during a stage of rapid cholinergic brain development from PND8 to PND20 [various stages of prenatal development] in male Wistar rats led to reduced dendritic spine density in the hippocampus without obvious toxic cholinergic signs in any of the animals (Santos et al. 2004). Some animals in the two highest dose groups died in the early days of the study. All doses caused retarded perinatal growth, and brain cholinesterase activity was reduced 60% by PND21.”⁴

Glyphosate is another example. It breaks down into the more persistent aminomethylphosphonic acid (AMPA) that has been linked to liver disease.⁵ 

A scientific study published in the journal Annals of Allergy, Asthma & Immunology found that exposure to dichlorophenols was linked to an increase in food allergies. Dichlorophenols are metabolites of chlorinated pesticides such as 2,4-D, dichlorvos, and chlorpyrifos, and they are found in chlorinated drinking water. The researchers concluded that “High urine levels of dichlorophenols are associated with the presence of sensitization to foods in a US population. Excessive use of dichlorophenols may contribute to the increasing incidence of food allergies in westernized societies.”⁶

IMPURITIES IN PESTICIDES

Pesticide testing is done with pure, laboratory-grade active ingredients and not with actual ingredients from the mass-manufacturing process. Manufacturing processes can result in the creation of a number of by-products, many of which can be toxic. “Other industrial chemicals or processes have hazardous by-products or metabolites. Numerous chemicals used in manufacturing remain in or on the product as residues, while others are integral components of the products themselves.”⁷ These by-products are largely ignored by regulatory authorities based on the assumption that, because they are at such low levels, they are safe. However, where there has been testing, some of these impurities have been found to be highly toxic.

Dioxins, or more correctly polychlorinated dibenzodioxins (PCDDs), are examples of some of the most common impurities. PCDDs are commonly called dioxins because their primary molecules have dioxin skeletal rings. There are potentially hundreds of dioxins, most of which have had limited testing. Dioxins are one of the major groups of metabolites that result from chemical processes that use chlorine. These can include chlorine bleaching fibers for paper or textiles, the wood preservative pentachlorophenol, herbicides such as 2,4-D, and pesticides such chlorpyrifos. 

Dioxins can be generated by burning or heating substances that contain chlorine, as in municipal and hospital wastes and crop residues that have been treated with pesticides containing chlorine. Some of the major emitters are sugar mills that burn the crop residues that have been treated with chlorinated herbicides and pesticides such as 2,4-D and chlorpyrifos as the energy source to boil the sugar cane juice in the first stage of sugar production. 

Some forms of dioxins are among the most toxic chemicals known to science and can cause a wide variety of illnesses, especially cancers and birth defects. Chlorine is a common ingredient in many pesticides due to its toxicity and its residual persistence. 

Dioxins are also endocrine disrupters, and according to the study by the WHO and UNEP they cause sex ratio imbalances in humans and wildlife, resulting in fewer males. “EDC-related sex ratio imbalances, resulting in fewer male offspring in humans, do exist (e.g., in relation to dioxin and 1,2-dibromo-3-chloropropane), although the underlying mechanisms are unknown. The effects of dioxin on sex ratio are now corroborated by results obtained in the mouse model.”⁸ 

Agent Orange, an herbicide that was widely used to destroy the highly biodiverse rainforests in Vietnam and Laos during the Vietnam War, was the best known of the chemicals contaminated with dioxins. Agent Orange was a combination of two herbicides: 2,4-D and 2,4,5-T. The manufacturing process of 2,4,5-T resulted in very high levels of dioxins, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This was the reason it was banned. However 2,4-D continues to be widely used despite being contaminated with TCDD, one of the most toxic dioxins. It also contains other dioxins. These dioxins are present as impurities from the manufacturing processes; however, they can also be formed as metabolites as the 2,4-D decays. 

Dioxins are very persistent in the environment, so consequently Vietnam still has extremely high levels of the environmental contamination resulting in birth defects, immune diseases, cancers, and many other problems more than forty years after the widespread use of Agent Orange was stopped. 

Dioxins are pervasive throughout the global environment and are found in the tissues of most living species, especially in species at the top of the food chain, such as humans, as they bioaccumulate. In some cases, they can come from natural causes, such as active volcanoes and forest fires; however, the bulk of dioxins are by-products of the chemical industry.

Some of the most infamous cases are Love Canal, New York; Times Beach, Missouri; and the massive release from an industrial accident in Seveso, Italy. The attempted assassination of President Viktor Yushchenko of Ukraine by poisoning with dioxins in 2004 resulted in permanent health problems. Due to a lack of research, however, the full extent of the contribution of the numerous chlorinated pesticides to the widespread global contamination by dioxins in the environment and the tissues of most living species has not been determined.

The greatest concern, according to the U.S. National Institute of Environmental Health Sciences (NIEHS), is that most people are exposed to dioxins from food, “in particular animal products, contaminated by these chemicals. Dioxins are absorbed and stored in fat tissue and, therefore, accumulate in the food chain. More than 90 percent of human exposure is through food.”⁹ Examples include dioxins being found in mozzarella cheese in Italy and in pork in Ireland in 2008 and in animal feed in Germany in 2010.

LACK OF COMPLETE TESTING

To the knowledge of this author, no country in the world has tested food for every pesticide used, and most only test for a “representative sample” of commonly used pesticides. For example, very few if any national residue monitoring programs test for glyphosate due to the difficulty posed by testing for it, despite the fact that it is the most commonly used herbicide. There is virtually no testing to detect the residues of the metabolites and by-products of agricultural poisons in our food and water. The 23 percent of food in the United States that was found with no residues could still be toxic for two reasons. 

Firstly, the 23 percent of food with no residue is largely meaningless if the testing does not include 100 percent of pesticides used in food production. How can the researchers claim that the food is free of residues if they have not tested for every possible residue? Secondly, just because the testing didn’t find the parent chemical does not mean that it is free of the toxic residues of the metabolites or the toxic by-products that can result from the manufacturing of pesticides. All it means is that there has been no testing for them. 

MORE RESEARCH NEEDED TO DETERMINE METABOLITE SAFETY

Very little research has been done to determine safe intake levels for the metabolites or the by-products of agricultural poisons. Consequently, there are virtually no safety levels to determine the average daily intake (ADI) for the numerous toxic metabolites and by products that contaminate our food.

The toxicity and health effects of pesticide metabolites and impurities are mostly ignored on an assumption that they are safe. The regulation of pesticides is supposed to be based on science and evidence. Until research is conducted into the toxicity and persistence of the metabolites of pesticides and published in peer-reviewed journals, regulatory authorities have no peer-reviewed, science-based evidence to show that any of the current residue levels in food or in the environment are safe. Ignoring them or assuming that they are safe cannot be regarded as an acceptable regulatory practice. Regulatory authorities have a duty of care to ensure that the general population is not harmed by these toxic chemicals.

Footnotes