by Jeff Gillman
At the beginning of the war, a number of new chemicals were introduced that were highly effective and relatively safe, especially when compared with something like lead arsenate. These included the insecticide DDT, introduced in 1939, which had a profound effect on lice and the transmission of diseases among troops, and the herbicide 2,4-D, a wartime secret that targeted broadleaved plants, such as dandelions and clover, but left grasses relatively unscathed. Though it was never used during World War II, it did find a place in the Vietnam War as an ingredient in the defoliant Agent Orange. The organophosphate insecticides—chemicals based on the chemistry of nerve gases that were used against humans—were also first identified around the time of World War II. As a group, these chemicals were far superior to their predecessors in terms of efficacy and, in most cases, safety, which led to their widespread use. But this use was often uncontrolled and overly enthusiastic, and it was just a matter of time before our irresponsible application of these poisons caught up with us.
Dangers Emerge
The potential dangers of pesticides were first brought to the attention of the United States, and to a lesser extent the world, by Rachel Carson’s 1962 book, Silent Spring. The book was required reading in nearly every high school, and it’s a shame that it isn’t more popular today. The author dramatically described the negative effects of DDT on majestic birds of prey and on humans, and warned of a bleak future if aggressive action wasn’t taken to ban the most dangerous pesticides and to reduce the use of pesticides in general. Rachel Carson was not completely opposed to pesticide use, just their gross misuse. As she wrote, “It is not my contention that chemical pesticides must never be used. I do contend that we have put poisonous . . . chemicals into the hands of persons largely or wholly ignorant of their potentials for harm.”
Human exposure to pesticides and the associated health risks occur in a number of ways, including in their application or misapplication, or simply by eating a fruit or vegetable from a farm where these poisons were used. Agricultural workers, who are exposed to the chemicals regularly, and infants and children, who have lower tolerances to poisons than adults, are particularly at risk. Consequently, since the 1940s, the government has been actively involved in trying to determine which chemicals are safe, which are safe if used under specific and limited conditions, and which are so risky that they should be banned entirely.
Rachel Carson was not completely opposed to pesticide use, just their gross misuse.
Though many people would like to see pesticides disappear, a somewhat less vocal but extremely powerful group considers them necessary. The government’s attempts to ban or limit their use are viewed warily by chemical manufacturers and farmers, who see these chemicals as critical to producing mass quantities of food at low cost. They are concerned that their livelihoods are threatened by scares whipped up by the media or environmental groups rather than by scientific evidence of the effects of a chemical on humans or the environment.
The Science
Pesticides offer one simple thing to their users: they get rid of pests by poisoning them. The poisons present in pesticides vary widely and have many different effects on the intended target. With herbicides, which kill plants, the poison may shut down photosynthesis, stop the plant from producing an amino acid, supply an overdose of a plantspecific hormone, or destroy any cells that the herbicide touches (as is the case with salt). Insecticides may kill insects by interfering with their nervous systems (these tend to be very dangerous chemicals), by preventing the insect from eating, by interfering with the ability to grow, or through a variety of other devious means. Fungicides can change the pH of the surface of a leaf, prevent a cell from dividing or a spore from germinating, or do a host of other things to knock out a disease.
While there are certainly other ways to get rid of pests, nothing else is quite as quick and efficient as a shot of poison. Over the years, pesticides have served us in a variety of ways, from insecticides that have helped us control malaria and yellow-fever-carrying mosquitoes, to herbicides that have helped us control the weeds on our property, some of which—water hemlock and nightshade, for example—are deadly poisons. Conservative estimates of how many lives have been saved by controlling disease-carrying insects with insecticides are in the millions. Pesticides also allow us to produce the copious amounts of food needed to feed our country. Many of the crops that we enjoy, such as apples and peaches, have so many potentially devastating pests that it would be almost impossible to produce them in any quantity without using pesticides. Even crops like corn and wheat depend on applications of herbicides to control weeds, insecticides to control insects, and fungicides to stop dangerous diseases. Without these pesticide applications, we would be left with only a fraction of the food that we currently grow.
The United States spends about 13 billion dollars on pesticides every year, which saves about 52 billion dollars worth of crops and keeps us rolling in unblemished foods. Still, there are many potential dangers involved with using pesticides. These dangers are usually classified and measured in three ways. First, acute toxicity, which is a measure of how much of the poison is needed in a single dose to kill you. Second, chronic toxicity, which is a measure of how repeated exposure to sublethal doses of the poison will affect you. And finally, environmental effects, which is an overview of how the poison will affect nontarget organisms such as bees or fish. We will look at each of these in turn.
Acute Toxicity
Of the dangers posed by pesticides, the most obvious and easiest to measure is acute toxicity. How toxic is a single dose of a poison? It is usually calculated by exposing rats, mice, or rabbits to various concentrations of the poison and then expressing the results as the milligrams of the poison needed per kilogram of the animal’s weight to have a 50 percent chance of killing it. This is called an LD50 (LD stands for lethal dose and 50 stands for 50 percent). LD50 ratings can be further categorized by how the dose is administered. In other words, what is the difference if the poison is administered by mouth, to the skin, or, in some cases, through the air? Regardless of how the dose is administered, the lower the LD50 is the more dangerous the chemical is. It is unfortunate that one of the fundamental ways to determine the toxicity of pesticides is by using them on animals. Currently there are no other testing options that are as universally accepted.
While there is quite a range of LD50s among pesticides, most of the pesticides that you are likely to encounter on garden center shelves will have LD50s that are quite high. In fact, the caffeine in your morning coffee probably has a lower LD50 (in other words, it is more acutely toxic) than any chemical that you or your fastidious neighbors have ever sprayed on a lawn. Caffeine has an oral LD50 of 192 mg/kg in rats, while four of the most common lawn herbicides—2,4-D, dicamba, mecoprop, and triclopyr—have rat-based oral LD50s of 375, 757, 431, and 630 mg/kg. Of course you still need to examine the concentration at which these chemicals are applied. After all, it would take somewhere in the neighborhood of 100 cups of coffee to kill someone. The caffeine in your coffee is more diluted than the herbicidal chemicals we named above, right? Wrong. Actually, right out of the spray tank, these herbicides are at roughly the same concentration as the amount of caffeine in your coffee. Of course, these are just some examples. There are pesticides that are much more and much less toxic (and more and less concentrated coffee), but it is worth noting that one of the most toxic pesticides ever known, the now-banned Black Leaf 40, an insecticide that worked by affecting an insect’s nervous system, was based on the insecticide nicotine (the “40” stood for 40 percent nicotine sulfate), which is still legal for you to inhale or put between your cheek and gum.
Chronic Toxicity
If our only concern about pesticides had to do with their acute toxicity, then the stigma currently associated with their use probably wouldn’t exist. But what happens to a person when they are repeatedly exposed to a poison again and again over months or years? The fear is that pesticides will have chronic effects, the mo
st disturbing of which is cancer. There are two ways to establish the likelihood of a pesticide causing cancer. The first is through animal testing. This testing involves feeding test animals the highest dose that they can handle without showing an obvious effect (called the NOEL, for No Observed Effect Level) repeatedly over days, weeks, or months. Frequently, this involves force-feeding, as the NOEL dose is often quite high and may be distasteful to the animals. Notes are taken on how the animals fare (sometimes including their ability to reproduce) and then, at the end of the experiment, the animals are dissected. Evidence of cancer is the most significant of the problems that researchers are looking for, but malfunctioning or oversized or undersized organs, such as the kidneys or liver, are assessed as well. Generally, if a pesticide causes cancer in animals during these tests, that pesticide never reaches the garden center shelves. (Pesticides already on the shelves will be removed if tests demonstrate they cause cancer.)
Epidemiological studies are the second way that a pesticide is examined to see whether it causes cancer or other terminal illnesses. These studies examine people who have been exposed to a pesticide, and then track their health over several years. Some of them are pretty scary. In one 2000 study examining pesticide use around homes, it was established that exposing children to pesticides may increase the likelihood of that child developing non-Hodgkin’s lymphoma. More recent research found a relationship between the herbicides pendimethalin and EPTC and pancreatic cancer. While these links may not show the remarkable correlation that tobacco, for example, has with cancer, they are still statistically significant and need to be acknowledged. Other problems with pesticides may include a slight increase in risk for Parkinson’s disease, as indicated by one large study that included well over 100,000 participants. Another study saw a correlation between low-birth-weight babies and women who were pregnant when they were exposed to pesticides, particularly chlorpyrifos, an insecticide (now illegal) for use around the home.
The problem with just accepting these studies and others like them as proof positive that pesticides are killing us is that epidemiological studies don’t always agree. The Agricultural Health Study, which followed (and is continuing to follow) more than 80,000 pesticide applicators and their spouses, is run by the EPA and compares these subjects to the general population. In fact, this is the study that demonstrated the link between pancreatic cancer and some herbicides. Despite that correlation, on the whole the pesticide applicators fared well when compared to the general population. They have a tendency to survive longer than the population at large, a result that perhaps can be attributed to the daily exercise of many pesticide applicators (who are often farmers).
Epidemiological studies are valuable tools, but they just don’t provide the unambiguous science that proves that pesticides, as a group, are bad. For example, while one study showed that chlorpyrifos has a weak relationship to the onset of rectal cancer, the same study showed that dichlorophenoxyacetic acid (2,4-D)—one of the most commonly used herbicides around the home for weeds like dandelion and clover—was associated with a slight decrease in colon cancer.
In general, studies into the carcinogenicity of pesticides show only slight, insignificant increases in the risks of certain cancers as they are associated with the use of pesticides, if they show anything. Even when these risks are found to be significant, they are usually linked to one particular pesticide, rather than pesticides in general, and they usually show that people who are exposed to more of a particular pesticide are more likely to be affected.
Environmental Effects
Acute and chronic toxicity of pesticides represent just the beginning of the problems posed by pesticide use. Certain pesticides have demonstrated themselves to be quite harmful to the environment around us. Many pesticides that we use are known as broad-spectrum poisons, meaning they kill the pest that has been targeted as well as many other innocent bystanders. Over the years, numerous pesticides have been banned because of their effects on creatures they were never meant to kill. DDT, for example, was banned in large part because of its effects on the ability of birds to reproduce (exposed birds produced very thin-shelled eggs), while the insecticide diazinon, once a very effective ant killer, was banned in large part because of its deadly effects on birds, fish, and mammals. One of the most widely used pesticides in the world, atrazine, is an herbicide that kills weeds before they emerge from the ground. This poison is also well known for its propensity to get into groundwater, where it appears to have hormonal effects on frogs and potentially other amphibians, causing them to become hermaphrodites.
Misapplication or accidents add another layer of potential problems. We know that if the popular pesticide glyphosate (Roundup), for example, is applied to a small pond or even a puddle where tadpoles are playing, there is a good possibility that those creatures will be killed. With many insecticides—even if they are properly applied—beneficial insects such as honeybees and predator insects are likely to be killed. Many of the pesticides we use are also highly toxic to aquatic organisms if the pesticides find their way into a pond, lake, or stream. Rotenone, once a popular organic insecticide, has now been relegated to use as a fish poison. Some pesticides, such as the organic fungicide copper sulfate and various synthetic herbicides, may even build up in the environment as they are used over time, potentially causing future problems. But we are now discovering that some of the scariest problems, environmental and otherwise, are those that occur even at very low concentrations.
Low Doses
Most of us are willing to take the responsibility inherent in our use of pesticides when we spray them or apply them to our yards, just as we take responsibility for other risky pursuits, such as driving a motorcycle or taking a drink of alcohol. But pesticides aren’t quite the same as motorcycles or alcohol because most of us are exposed to pesticides every day in ways that we aren’t aware of and can’t control. Everything from the food we eat, to the grass we walk on, to the water we drink may contain pesticides of one sort or another, at one concentration or another. As we discovered in the previous chapter, even organic food may be contaminated with synthetic pesticides. Fruits and vegetables are notorious for having large amounts of insecticides and fungicides, both organic and synthetic, applied to them to ward off insects and disease, and lawns—our neighbor’s, if not our own—are sprayed with herbicides to control dandelions. Pesticides used on farmland can leach through the soil to reach our drinking water, or run off into nearby streams and ponds. The more we test the world around us, and the more sensitive our testing equipment gets, the more it becomes obvious that we are constantly in contact with low doses of pesticides.
How are we to know, when a piece of fruit contains 5 parts per million of an insecticide, whether this level of poison is sufficient to cause harm to someone who eats it? Even if it doesn’t cause us any obvious harm, what if we continue to eat fruit with this pesticide for days, weeks, and years? In other words, what does chronic exposure to these extremely low doses of poison do to us? There is no data proving that any of our currently used poisons are dangerous at the levels that we would typically find them in fruits and vegetables or on our lawns. But how can we really know? Likewise, while we get extremely concerned when an herbicide such as atrazine is discovered in our water supplies, there isn’t actually any hard data available that will tell us exactly what that atrazine will do. Rather, we’re worried about it simply because it seems reasonable to worry about it. The government provides information on what it considers to be unsafe levels of various contaminants, but can we be certain that lower levels are really safe? After all, it is impossible to know exactly how a pesticide will affect any given animal in any given circumstance. There is no data on how eating an apple a day with five parts per million of a new pesticide that was just registered for use is going to affect us in thirty years. On one hand, it seems better not to have these poisons at any level in our food and drinking water. But on the other hand, if no evidence can be found that these poisons
are dangerous, and there are compelling beneficial reasons to use them, such as a weed-free lawn or the ability to grow valuable crops, then why should we lose the use of these poisons?
There is no data proving that any of our currently used poisons are dangerous at the levels that we would typically find them in fruits and vegetables or on our lawns. But how can we really know?
Although there is little data on the effects on humans of extremely low doses of pesticides, there is some data from amphibians that should make us sit up and take notice. Experimenting with frogs, Tyrone Hayes, a researcher at the University of California at Berkeley, found that atrazine, an herbicide used on many crops, especially corn, will cause frogs to change their sex. This occurs at an extremely low dose, which might be found under typical conditions in a pond, or even in groundwater located close to an agricultural field. Other researchers have also found that very low concentrations of commonly used pesticides may dramatically affect aquatic communities. Rick Relyea, a researcher at the University of Pittsburgh, discovered that a mixture of very low doses of ten commonly used pesticides would kill leopard frogs but not gray tree frogs, though the tree frogs grew twice as large as they normally would. The mixture included the insecticides malathion and carbaryl (nerve toxins used around homes and schools as well as on commercial farms), and the herbicides 2,4-D (the most commonly used herbicide on lawns, best known for its ability to kill dandelions) and glyphosate (Roundup). Similar mixtures of commonly used insecticides and herbicides at concentrations likely to be found in streams and rivers are also known to affect the ability of salmon to sense the world around them, damaging their olfactory sensors (basically their noses), with the potential for drastically affecting their lives. And these are just some of the findings that show the effects of low concentrations of pesticides on living creatures.
