Nature: The Original Chemist

We frequently see a contrast drawn between what is “natural” and what is “chemical.” Sometimes products are described as “chemical-free” even though every physical object is made of chemicals. As much as this suggests a problem with our science education, it speaks to a missed opportunity for wonder. Nature is not some sort of cosmic mother figure; on the contrary, nature is composed of diverse biological and physical processes, including some pretty amazing examples of chemistry continually taking place. If we indulge the human personification of nature and it’s “children” a bit, we could say the following about these “chemists:”

  • They are extremely creative.
  • They can make really complex molecules.
  • Some of their chemicals last a really long time – which is sometimes good and sometimes bad.
  • They are really good at making polymers.
  • They make some extremely toxic things.


I’ll give a few examples below.


Creative Natural Chemistry


The diversity of naturally occurring chemicals is staggering. Humans regularly take advantage of this, particularly when we need ideas for things like pharmaceuticals or crop protection products. Sometimes we extract the chemicals from a plant or other living thing. Often we grow tanks of microbes to harness their ability to make a chemical we find useful. In cases where the amounts of the chemical are too small to be practical from the natural source, human chemists can synthesize the same compound to fulfill the quantity needed. An example of this is a new potato sprout inhibitor. In many other instances, a natural chemical serves as the inspiration for human chemists to experiment with similar structures leading to the discovery of particularly useful drugs, fungicides, etc.


Taxol structure image by Calvero. Pacific Yew tree image by Jason Hollinger via creative commons. Azoxystrobin fungicide structure by Yikrazuul.   Strobilurus tenacellus mushroom picture by Tatiana Bulyonkova at Mushroom Observer.



Complex Natural Chemistry


Some of the most abundant chemicals in nature are simple. Nearly 80% of the air we breathe is nitrogen in the form, N2 – just two nitrogen atoms bonded together. Nitrogen goes through natural cycles that are important to all living things but often stays in relatively uncomplicated forms like ammonia (NH3) or nitrate (NO3). On the other hand, natural chemicals can be complex, so much so that it would be challenging for even a skilled human chemist to make them.


One of these complex examples is called spinosad and it is produced by a microbe called an actinomycete. We have found this to be a particularly effective insecticide for use on crops yet quite benign for the environment and not toxic to people. The chemical company that produces this for farmers relies on the natural microbe to produce this complicated bit of chemistry.


Structure of Spinosyn image by Capaccio via creative commons.


Long-Lived Natural Chemicals


Most naturally occurring chemicals are part of a cycle in which chemicals combine, making a material, but then eventually break back down into basic constituents to begin the cycle again. Some naturally produced chemicals are relatively long-lived. This can be a good thing in the case of the chemicals that are found in the organic matter of a healthy, undisturbed soil. These are not just any plant or microbial product; they are specific compounds that slowly cascade through a series of breakdown products.


For instance, plants make a group of complex, phenolic chemicals, called lignin, which are important for strengthening their cell walls. Lignin is quite resistant to microbial breakdown, although some fungi can and do destroy it, even as they decompose wood. Lignin is a major component of what is termed humus – the component of soil that helps to buffer nutrients and retain moisture. When soils are converted from wild land to cultivation, there is a dramatic increase in the rate of breakdown of these chemicals and thus the release of the carbon dioxide.


Some long-lived, natural chemicals, however, are less desirable. Under low oxygen conditions, soil-dwelling microbes can interconvert forms of nitrogen (e.g. ammonia to nitrate or nitrate to nitrogen gas). In that process, they “accidentally” make some nitrous oxide (N2O). Nitrous oxide is around 300 times more potent than carbon dioxide as a greenhouse gas because it lasts longer in the atmosphere. Unfortunately, human activity can exacerbate the production of this naturally generated chemical from farmed soils. Adjustments in farming practices can lead to a better balance of the production of natural chemicals that help or hurt greenhouse gas levels.



Fancy Polymeric Natural Chemicals


In the 1967 movie The Graduate, the character played by Dustin Hoffman is lectured about how the future is going to be all about plastics. Indeed, many people were excited in that era about polymers that chemists were developing, like nylon and polyester. These are based on long chains of monomers attached end to end.


Many of the most abundant natural chemicals on earth are also polymers, which are long chains made of simple sugar molecules. Depending on which sugar and how the sugars are linked together, the polymers result in anything from the cellulose that makes cotton fiber to wood or even the alginate from seaweed we use for thickening foods or the starch that is the primary energy source in foods like pasta, bread, rice or potatoes. Increasingly, we are tapping in to the enzymatic tools found in microbes in order to make polymers from renewable resources.


Variously Toxic Chemistries


Most people associate the term natural with the terms safe and wholesome. This impression has been created by decades of marketing, not by any understanding of the chemicals in nature. Many natural chemicals are perfectly benign; however, nature’s assortment of chemicals also includes many that are toxic by various mechanisms.  Lots of plants make chemicals to protect themselves from being eaten or otherwise bothered. We have all heard about nasty plants like poison ivy or even lovely plants like the Colorado Columbine which are dangerous to eat.


Cut Granny Smith apple image from Wikimedia. Cauliflower image from Calliope via creative commons. Hot pepper image by Andre Karwath via creative commons. Capsaicin structure by Jurgen Martens. Nicotine structure by NEUROtiker. Cyanide structure via Wikimedia.

Food plants also make some fairly toxic chemicals. The seeds of many familiar crops, including apples, cherries and peaches to name a few, contain a chemical storage component called a cyanogenic glycoside. When the seed is damaged, enzymes release hydrogen cyanide from the glycoside. Hydrogen cyanide is very toxic! It is a good reason not to eat those seeds, although it would take a lot of such seeds to hurt a person. The capsaicin that we enjoy in hot sauce is an insect protection chemical made by the pepper plant to defend itself. It is moderately toxic to us but not at the doses we normally consume. Quite a few plants make nicotine to ward off insects including tomatoes, cauliflower and eggplant. Nicotine is very toxic but not at the doses these crops produce. As with any toxic chemical, natural toxins are only an issue to humans at a certain dose.


Some natural chemicals, however, are extremely dangerous and we don’t want those in our food. Mycotoxins are a particularly nasty category of natural chemicals produced by certain fungi. One such chemical, called aflatoxin, is among the more toxic chemicals in existence and is also a potent carcinogen. Unfortunately, under certain circumstances, fungi can produce aflatoxin in food crops. In the developed world, a system of controls and testing keeps us well protected from this; in the developing world, though, aflatoxin is a major cause of death both through acute and chronic effects because it contaminates staple foods like corn or groundnuts.


Aspergillus infected groundnut image from International Institute of Tropical Agriculture. Aflatoxin structure by Ju


Some natural chemicals are elegantly selective in their toxicity. A soil bacterium, called Bacillus thuringiensis (usually called “Bt”), makes proteins that are specific in their toxicity to only certain categories of insects. One strain of Bt makes proteins that only effect beetles while another’s toxin only affects caterpillars. None of these Bt proteins are toxic to humans or almost anything else. We have made excellent use of these natural chemical toxins as sprayable insect controls and by genetically engineering plants to make their own supplies of the protein resulting in the plant being insect resistant.




Yes, nature does a great deal of chemistry. For us, these chemicals can be a source of good things, a source of good ideas, and sometimes a hazard or problem.


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Do You Need to Worry About Pesticide Residues on Your Food?

Dr. Steve Savage, Crop Protection Benefits Research Institute (CPBRI)

fresh fruits and vegetables
Some of the healthy fruits and vegetable we can enjoy (Image from Wikimedia)

Many Americans have concerns about pesticide residues on food – particularly for fruits and vegetables. In contrast with that oft-communicated perception, the safety of our food supply is well documented. One reason for this disconnect is that there are activist groups (non-governmental organizations) that consistently promote the idea that consumers should buy organic versions of certain crops in order to avoid pesticides. A recent study documented how that sort of message induces some lower income Americans to simply avoid fruits and vegetables all together. The truth is that our food supply is extremely safe because farmers are careful to use pesticides in ways that don’t lead to residue problems at the consumer level and because of rigorous regulation followed by farmers over the last several decades.

The common perception of organic as a safer option in this regard is also at odds with reality. The United States Department of Agriculture (USDA), which oversees organic certification, clearly states on its National Organic Program website: “Our regulations do not address food safety or nutrition.” Organic farmers can and do use pesticides from an approved list, but that list is not based on safety criteria. Organic growers are limited to natural chemicals and to a limited list of synthetic materials. As with any crop protection material, the EPA has the responsibility to evaluate and regulate their safe use. That oversight is why consumers can confidently enjoy both conventional and organic foods.

In this post I will describe the testing, regulatory and training systems that are in place in the US to protect consumers from risks associated with pesticide residues. I will also describe the intense monitoring system that demonstrates year-after-year that this system is working.

All farmers face challenges from a variety of pests and although they use a number of methods to manage those threats, pesticides are a critical part of that “toolbox.” The broad category “pesticide” includes certain chemicals that occur in nature as well as various synthetic chemicals. There are also pesticide products based on living biological agents. The responsibility for pesticide regulation is with the Environmental Protection Agency or EPA. It determines how pesticides can be used safely, based on their particular intrinsic properties, and by restrictions on how and when they can be used.


EPA Risk Assessments

Before any new pesticidal product can be sold in the United States, an extensive list of toxicological tests must be performed and reported to the EPA. The company that makes or which will sell the product is responsible for the cost of this testing, but most of the work is performed in contract labs that are closely audited by EPA. The tests evaluate many different facets of potential toxicity for human and environmental health, both in terms of short-term effects (acute toxicity via consumption, by skin exposure, by inhalation exposure…) and long-term effects on development, organ health, reproduction, and potential carcinogenicity. In addition, a great deal of data has to be generated to show what happens to the chemical over time on the food, and in the environment in terms of its persistence, movement, and breakdown into innocuous ingredients. It costs on the order of $286,000,000 and can take more than 10 years to generate all of this required data. EPA then uses these data to conduct an extensive “risk assessment.” Based on that assessment, EPA develops “label requirements” specifying how, on which plants, when, and how much of the pesticide can be used. These risk assessments cover issues of worker safety, environmental impact and also what sort of residues might be left by the time the crop is harvested, and any potential risk to human health.

Some safe, delicious apples ready for harvest in western Washington this summer

Pesticide Tolerances (or MRLs)

With regard to pesticide residues at harvest, EPA designs the label requirements to make sure that any residues still present when the food gets to the consumer are below what is called a “tolerance.” (Outside the US this is called an MRL or maximum residue limit). The tolerance is set to insure that there is a substantial margin of safety (typically 100-fold) between the allowed residue and any level to establish reasonable certainty of no harm to humans. EPA then sets limits on how much of the pesticide can be applied and how close to when the crop is going to be harvested so that the tolerance is unlikely to be exceeded when farmers use the product.

These tolerances are very conservative limits and represent such small amounts that they can be difficult to envision. For instance, a tolerance might be five (5) parts per million. That can be visualized as to two drops of water in a five (5) gallon carboy. Some tolerances are set as low as one part per billion (e.g. one drop in 528 carboys). In summary, tolerances are extremely small levels of pesticide residue, set as a conservative standard for human safety, and customized to the specific properties of the each chemical.


In order to be allowed to apply pesticides, farmers have to be trained and certified about how to comply with the chemical-specific label requirements. They have to maintain that training through on-going classes.

Is the System Working?

Every year, as part of a USDA effort called the Pesticide Data Program (PDP), thousands of food samples are randomly gathered from normal food channels and consumer markets. The samples are taken to labs where each sample is screened for the presence of hundreds of different chemical residues. The data that the USDA generates is transparently published both in raw and summarized form. Year after year, what the data show is that the system is working! The vast majority of samples have either no detectable residues or residues that are below the assigned tolerances – mostly far below. On 11/29/16 another year of results (2015) were released and once again documented the safety of our food supply.  The fact that a small residue can be detected does not mean it is of concern. Modern analytical chemists have the ability to detect chemicals at very low levels. The reason that the numbers below tolerance are still published is not that they are of concern, but rather as transparent documentation that these products should be of little concern to consumers and regulators.  Several governmental agencies evaluate this information each year and confirm that consumers can confidently enjoy their food supply without concern about pesticide residues.  The FDA also has a residue testing program from which it concludes, “Results in these reports continue to demonstrate that levels of pesticide residues in the U.S. food supply are well below established safety standards.”  California does its own residue testing and concludes, “California tests show low or no pesticide levels in many fruits and vegetables.” Similar residue testing is conducted in Canada and the EU with equally encouraging results.  With this overwhelming body of evidence, how can the fear of residues persist?

What About the “Dirty Dozen List?”

Unfortunately, each year there is an organization called the Environmental Working Group (EWG) that takes the USDA PDP data and grossly misuses it to create a “Dirty Dozen List.” Instead of looking at how detections relate to carefully developed tolerances, EWG essentially treats all detections as significant – an approach that has been completely rejected by independent experts in the field of toxicology. EWG then recommends that certain crops be sought out as organic. Similarly misguided recommendations to purchase organic are published Consumer Reports. This makes no sense, since organic is not a safety certification. In fact, organic crops often have the same sort of low-level, detectable residues of pesticides as conventional (example data from the US and Canada). This point is conveniently ignored by these organizations.

In conclusion, we have a system in the US that both enables farmers to control pests and which protects consumers so that they can enjoy healthy foods without worrying about pesticide residues.


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