Pesticides and Residuals: History and Food Safety

Most of the ancient and early pesticides were used as insecticides, but agriculture also created the need for biocides to target fungal, plant, or weed infestations. One of the earliest known agricultural fungicides is the Bordeaux mixture. In the late 19^th century, in the Bordeaux region of France, some vineyards began applying a mixture of copper sulfate and calcium oxide to grapes. The original intent was to give the grapes an unpleasant color and taste to discourage people from stealing them before they could be harvested. It was quickly discovered, however, that treating grapes with the mixture also killed mildew and fungi that plagued vineyards. The Bordeaux mixture is still used as a fungicide today, particularly in organic farming.

In addition to mineral-based pesticides, early civilizations turned to plant derivatives to help combat pests. The earliest recorded use of botanical pesticides was by the Romans who discovered that crushed olive pits yielded an oil called Amurea, an effective pesticide. Botanical-based pesticides take advantage of the chemicals plants naturally produce to avoid being eaten. In the 17^th century, for example, it was found that tobacco derivatives are effective insecticides, and in fact, nicotine-based insecticides were used in many regions until the early 2000s. One of the most important botanical-derived pesticides is pyrethrum. Extracted primarily from certain chrysanthemums, pyrethrum-based pesticides were widely used in agriculture throughout the 20^th century and are still used today.

The advent of modern chemistry led to a shift from mineral or botanical based pesticides to synthetic chemistry-based pesticides. Motivated by outbreaks of pest-related famine and disease, Swiss chemist Paul Müller dedicated his research to finding an insecticide that could be produced at an industrial scale, was easy to apply, and was safe for humans and farm animals. Pyrethrum-based pesticides, for example, could not be manufactured at the necessary scale due to the limitations of their natural origin. His research would earn him the Nobel Prize, but the insecticide he discovered, dichlorodiphenyltrichloroethane, better known as DDT, did not come without a cost.

Initially used as an insecticide on crops, DDT quickly gained widespread use during World War II to fight insect-borne diseases. In the post-war years, DDT became a ubiquitous insecticide, widely used for crop protection and to control pests that plagued people and animals. It was cheap to make, easy to administer, and killed insects and related pests in minute quantities. It even lingered in the environment, limiting future infestations. But by the late 1950s and early 1960s, mounting evidence indicated environmental and health risks associated with the use of DDT.

Silent Spring, the EPA, and Food Safety

It was Rachel Carson’s 1962 bestseller Silent Spring that popularized concerns about DDT. Although evidence of the risks of widespread pesticide use had been mounting for years, it was Carson’s book that led to widespread concern about the environmental and health effects of insecticides. Silent Spring illustrated the harm DDT was doing to birds and other wildlife. Additional studies indicated DDT may have caused chronic harm to people as well. Concerns over widespread pesticide use and other environmental disasters would eventually lead to the creation of the EPA. Although US pesticide regulations date back to the early 20^th century, the EPA helped drive the modern evaluation of residual pesticide risks.

Mitigating the unintended consequences of widespread pesticide use continues today. For example, although insecticides may be very effective at controlling pest insects, they do not discriminate and can cause considerable harm to helpful insects. Most notably, bee populations have plummeted because of widespread agricultural pesticide use, among other causes. Since bees are the most important natural pollinators of agricultural crops, their preservation is just as important as the control of plant-destroying insects. Current regulations on biocides now consider the unintended impact on non-pest insects and other species. Modern insecticides must be evaluated for their impact on pollinators and other non-pest species before they are authorized for use.

Beyond direct exposure, concerns over pesticides in the food chain were also investigated. Since pesticides are an integral part of modern farming, potential health effects from residual pesticides on food must be controlled. Strict limits have been imposed on pesticides due to concerns about chronic exposure, even from trace amounts, because pesticides such as DDT are recognized for their long-lasting presence in the environment. Now almost all global food regulatory agencies consider the risk of residual pesticides and establish maximum residue levels (MRLs) allowed in food. Food producers must test and demonstrate that their products are below relevant MRLs for likely pesticides. Recommended levels are reviewed periodically based on new data to confirm that any residual compounds do not present a health risk. Ethylene oxide (ETO) is a good example of the need to continually reevaluate potential sources of residual pesticides and, as necessary, update established MRLs.

Ethylene Oxide Fallout and Updates to Allowed Residue Levels

ETO is an effective biocide most typically used for sanitizing medical devices. It is highly volatile and quickly disperses from even the smallest parts of equipment. However, because it is a known carcinogen, its use in the food industry is strictly controlled. For example, while manufactured from ETO as a raw material, polysorbates are safe for consumption and are useful emulsifiers in food. Because of the potential for residual ETO in the final product, the EU set a strict limit of no more than 0.2 PPM of ethylene oxide in food-grade polysorbates.

In September 2020, ethylene oxide (ETO) residue was found on sesame seeds imported into Belgium. Shortly after, it was also found in locust bean gum, a common food additive used to add texture and sweetness to many foods. Concerns over ETO residue on food ingredients, primarily from the locust bean gum, led to the largest food recall in EU history, with >3000 different products removed from the market. Since ETO is not used in manufacturing these materials, it was initially thought the residual material was due to the application of ETO to crops as a fungicide. That was not the case, however. EU authorities eventually linked the ETO contamination to hygiene practices for ocean shipping containers. Some sea freight companies used ETO to sanitize large shipping containers to address potential biological risks to food transported via sea freight. Ironically, this move to help ensure food safety resulted in residual pesticides and a new food safety risk. As a result of the ETO investigation, EU authorities modified the MRL for ETO in food additives. The maximum level was revised from ≤0.2 PPM on select food additives where ETO was used as a raw material to ≤0.1 PPM for all food additives. Now all food additives marketed and used in the EU must be demonstrated to contain ≤0.1 PPM ETO, regardless of the source of the food additive.

Ensuring Pesticide Residue Safety

Today, there are dozens of pesticides available to growers as well as dozens of sanitizing agents for processing equipment. In depth safety studies have established MRLs for all authorized pesticides to help ensure public safety and environmental sustainability.

MRLs are determined based not only on the potential risk of the pesticide but also on the typical amount of the food or ingredient consumed and the consequent risk to consumers. Therefore, safe levels of residual pesticides vary depending on the type of food as well as the country or region where the food is consumed. Because health concerns from potential residual pesticides are based on long-term, chronic exposure, the exposure potential depends on the amount of a foodstuff a typical individual ingests. Differing regional diets sometimes contribute to varying MRLs for the same foods and pesticides due to consumption rate variation among those populations. Fortunately, many regions have extensive online databases to determine the necessary MRLs for many compounds of concern. Even where there is no country specific MRL guide, Codex Alimentarius also offers MRL standards[1] for global guidance on safe levels of residual pesticides.

The global population is estimated to reach 10 billion by 2050. Effective pesticides are essential for producing and transporting food at the scale needed to feed the globe. Pesticides have been critical to the growth of human civilizations, limiting deadly diseases and allowing large-scale farming and food storage and transport. By taking a risk-based approach to understanding the potential risks of residual pesticides and relying on established science for residual limits, the food industry can help ensure the continued safety of the global food supply chain.