Pesticides: Just How Bad Are They?

Harriet Hall

I think everyone would agree that it would not be a good idea to put pesticides in a saltshaker and add them to our food at the table. But there is little agreement when it comes to their use in agriculture. How much gets into our food? What are the effects on our health or in the environment? Is there a safer alternative?

Where should we look to find science-based answers to those questions? One place we should not look is books written by biased nonscientists to advance their personal agendas. A friend recently sent me a prime example of such a book: Myths of Safe Pesticides by André Leu, an organic farmer whose opinions preceded his research and whose bias is revealed in the very title.

The Foreword and Introduction: A Bad First Impression

The Foreword to Leu’s book is by Dr. Vandana Shiva, an environmentalist who blames Monsanto’s GMO seeds and pesticides for causing 284,000 Indian farmers to commit suicide since 1995, a claim that is almost certainly false (Kloor 2014). She says pesticides don’t even work in the first place: the brand Roundup is failing to control weeds and is creating superweeds, and “pesticides create pests.” She is an alarmist who cites the Bhopal tragedy and Rachel Carson’s Silent Spring. She says Leu’s book synthesizes the scientific evidence of pesticides’ harm to public health and provides evidence that pesticide-free alternatives are more productive.

Leu does collect a lot of evidence for the harms of pesticides, but he doesn’t balance it with other evidence of benefits and safety. And his evidence for pesticide-free alternatives is far from convincing.

His Introduction begins with his personal observation of “so much illness in our communities, especially cancers, behavioral disorders, and degenerative diseases” and with his personal testimonial of getting sick every spraying season even though no sprays are used on his farm. He set out to find evidence to validate his belief that pesticides were responsible. Predictably, confirmation bias kicked right in and had a field day.

His bias even interferes with his reading comprehension. He says the US President’s Cancer Panel report “clearly states that environmental toxins, including chemicals used in farming, are the main cause of cancers” [emphasis added] (President’s Cancer Panel 2010). In fact it states no such thing! It says “the true burden of environmentally induced cancer has been grossly underestimated [at 6%],” and even that statement has been challenged by the American Cancer Society, whose representative said the panel restated hypotheses as facts and its conclusion does not represent scientific consensus (Nelson 2010). Tobacco causes 30 percent of all U.S. cancer deaths and another 30 percent are attributed to diet, obesity, and physical inactivity; and there are many other causes of cancer including alcohol, sun exposure, radon, medical radiation, infections, and occupational exposures. Pesticides are low on the list.

The Five ‘Myths’

After finding such obvious errors in the Foreword and Introduction, I was tempted to drop the book in the wastepaper basket, but I persevered. Leu proceeds to address five “myths” in five chapters:

1. All agricultural poisons are scientifically tested to ensure safe use.

2. The residues are too low to cause any problems.

3. Modern pesticides rapidly degrade.
4. Regulatory authorities are reliable and trustworthy.

5. Pesticides are essential to farming.

“Myth” 1: All Agricultural Poisons Are Scientifically Tested to Ensure Safe Use.

Note the inflammatory choice of words: “poisons” rather than pesticides. It is true that many chemicals in use in the United States have not been adequately tested for safety, but even he himself admits that pesticides have been studied more than most other chemicals. Leu cites several of those studies. For instance, one study found 232 chemicals in the cord blood of newborns; some of those chemicals can harm brain development and the nervous system, but they have never been found to do so in the amounts detected. He criticizes current scientific testing methods as unsound and points out that the developing fetus may have special needs. Pesticide exposure has been linked to a number of health problems in children, including lower IQs, autism, ADHD, lack of physical coordination, loss of temper, bipolar illness, schizophrenia, and depression. These are reported correlations, not proven causations. He cites birth defects and gene alterations in mice exposed to pesticides, but there is no comparable evidence for humans. I agree that these studies are cause for concern and further research, but I think it is premature to draw firm conclusions or use that evidence to guide public policy.

“Myth” 2: The Residues Are Too Low to Cause Any Problems.

Leu says the lowest doses of some chemicals can be more toxic because they begin to act as hormones at those low levels. My colleagues and I have looked into endocrine disruptors and written about them on the Science-Based Medicine blog (Lipson 2009; Hall 2009; Hall 2011; Novella 2008); we were not particularly worried by what we found. Leu reproduces graphs showing increasing rates of diabetes, obesity, and thyroid cancer, and implies that endocrine disruptors are a major cause. He has a whole section on cell receptors that doesn’t make sense; he tries to tell us that “key” molecules can only fit into the “lock” receptors when they are present in small numbers(!?). Yes, there are legitimate concerns about hormone disruption, but so far it’s all hypotheses and no reliable conclusions. If there is any effect, it is small in magnitude.

“Myth” 3: Modern Pesticides Rapidly Degrade.

Dioxins are very persistent in the environment, and they can contaminate food. Pesticides can degrade into other chemical compounds that are harmful. Testing is not done on every possible pesticide and metabolite. Washing and peeling produce can reduce pesticide residues but can’t eliminate them. He says the bulk of pesticides are inside the food, but that is only true for systemic pesticides that are absorbed by the plant and circulated through its tissues; other pesticides remain on the plant’s surface and can be washed off or removed by peeling. For that matter, 99.99 percent of all the pesticides in our diet are natural components of the food—pesticides that plants produce naturally to protect themselves and that can’t be eliminated (Ames 1990). Any synthetic pesticide residue is a drop in the bucket compared to the much larger concentrations of natural plant pesticides.

“Myth” 4: Regulatory Authorities Are Reliable and Trustworthy.

Leu doesn’t trust authorities. He says, “History shows that regulatory authorities have consistently failed to prevent the contamination of the environment and human health by products previously designated safe, such as asbestos, lead, mercury, dioxins, PCBs, DDT, dieldrin, and other persistent organic pollutants.”

That’s horribly one-sided and unfair. He could have cited some of the many, many successes of government regulation in protecting public health and the environment; some of those successes involve the very contaminants he lists. He faults the government for continuing to allow mercury in fillings and vaccines, not acknowledging that mercury is no longer used in any vaccine except for the multidose injectable flu vaccine, and that there has never been any evidence of danger from the use of mercury in vaccines or fillings. He points out variations in regulations between different countries, and his simplistic explanation is “politics,” whereas I think it is clearly more complicated than that.

Leu particularly targets glyphosate, showing nine graphs with correlations between glyphosate, GMOs, and everything from diabetes to intestinal infection. Apparently he thinks correlation means causation; it doesn’t. He doesn’t seem to know about that now-famous graph showing an almost perfect correlation between autism and organic food.

“Myth” 5: Pesticides Are Essential to Farming.

Leu cites a number of studies where organic farming practices produced greater yields than conventional methods. He shows evidence that organic crops are more resilient in adverse conditions like droughts, and that good practice organic systems produce higher yields in traditional small farm settings.

Recent advances in organic farming techniques are very encouraging, and anything that reduces the need for pesticides sounds like a good idea. But it remains to be seen whether the techniques can be implemented everywhere, whether “industrialized” large-scale organic farming can adopt them, and whether they are really the most practical way to increase yields and improve safety. It may be possible for farmers to eschew pesticides, but is it feasible on a large scale? Is it really preferable to judicious use of pesticides following best practices and evidence-based guidelines? Those are questions that science will be able to answer, but the necessary studies have not been done yet.

Some Facts and Balances

How much pesticide residue is actually present on our foods? The Pesticide Data Program of the USDA has been asking that question for twenty-two years now. In 2012, they sampled nearly 12,000 foods and found that residues were below EPA tolerance levels in over 99 percent. They concluded that “residues found in agricultural products sampled are at levels that do not pose risk to consumers’ health (i.e., are safe according to EPA)” (USDA 2014). The World Health Organization and other organizations around the world are also monitoring pesticide residues and establishing tolerance levels, and there are international databases.

The obvious next question is how safe levels are determined. The EPA sets those levels and their website explains how they arrive at them (EPA 2007). They don’t just make stuff up; they go by evidence and expert reasoning. Levels change as new evidence warrants it. The system isn’t perfect, but it’s reasonably good. A perfect system would be prohibitively expensive if not impossible to carry out to Leu’s satisfaction.

One activist organization reports that eighty-six pesticides were found on cucumbers. That might sound scary, but what does it really mean? Modern analysis can find tiny trace amounts of lots of things, but remember the old adage “The poison is in the dose.” We know, for instance, that organophosphate pesticides are toxic and workers must take precautions, but we also know that ingesting one single molecule of even the most deadly poison will not result in any detectable harm. There is a legitimate concern that ingesting multiple chemicals at safe levels might add up to unsafe cumulative effects, and that infants and pregnant women might be at higher risk. This doesn’t mean we should reject all pesticides on the precautionary principle; it only means more studies should be done.

It’s important to put the harms of artificial pesticides into context. As Bruce Ames has pointed out:

Carrots, celery, parsley, parsnips, mushrooms, cabbage, Brussels sprouts, mustard, basil, fennel, orange and grapefruit juices, pepper, cauliflower, broccoli, raspberry, and pineapple contain natural pesticides that cause cancer in rats or mice and that are present at levels ranging from 70 ppb (parts per billion) to 4 million ppb-levels that are enormously higher than the amounts of man-made pesticide residues in plant foods. (Ames 1989)

Any discussion of harms from pesticides should weigh the harms against the benefits. There certainly are benefits: pesticides have increased crop yields and helped humans in other ways (EPA 2012):

• Pesticides are the only effective means of controlling disease organisms, weeds, or insect pests in many circumstances.

• Consumers receive direct benefits from pesticides through wider selections and lower prices for food and clothing.

• Pesticides protect private, public, and commercial dwellings from structural damage associated with termite infestations.

• Pesticides contribute to enhanced human health by preventing disease outbreaks through the control of rodent and insect populations.

• Pesticides are used to sanitize our drinking and recreational water.

• Pesticides are used to disinfect indoor areas (e.g., kitchens, operating rooms, nursing homes) as well as dental and surgical instruments.

• The pesticide industry also provides benefits to society. For instance, local communities and state governments may be partially dependent upon the jobs and tax bases that pesticide manufacturers, distributors, dealers, commercial applicators, and farmers provide.

Incidentally, when plants are not exposed to artificial pesticides they tend to increase their production of natural pesticides. This phenomenon has not been well studied. The “natural” pesticides allowed in organic farming have not been studied anywhere nearly as thoroughly as the artificial ones. It is a fallacy to assume they are safer just because they are natural.

The news about health effects of pesticides is not all bad. A review of the literature concluded:

When compared to the general population total mortality has been found to be consistently lower among pesticide manufacturers as well as among other groups of workers. This observation has been mostly attributed to the “healthy worker effect” or, in the case of agricultural workers, to the healthier lifestyle of farm families. With the exception of deaths by accidental causes, non-cancer causes of death (mainly represented by cardiovascular diseases), were generally found to be less frequent than expected among manufacturers or users of pesticides, in particular among farmers. No consistent evidence of global cancer mortality different from that of the general population has been reported among pesticide manufacturers or applicators. On the other hand, the papers examined have been strikingly consistent in reporting a low overall cancer risk among agricultural workers; life-style, clean air, low prevalence of smoking have been hypothesized so as to explain this observation. (Maroni and Fait 1993)


Pesticides are meant to harm weeds and insects, but are they also harming us? Leu demonizes pesticides as “poisons” and argues that the precautionary principle should make us eliminate their use entirely. I think he exaggerates the dangers, especially considering that evolution has equipped the human body to thrive while eating foods with levels of natural pesticides several orders of magnitude greater than the levels of artificial pesticides in today’s foods. I certainly agree with him that there is a need for more research. He says safe pesticides are a myth, but I have to wonder if any evidence of safety would ever satisfy him; he seems to be demanding “perfect” safety like some anti-vaccine activists do. There is a danger to invoking the precautionary principle before ensuring that the alternatives are not worse.

This is what typically happens when a layman with an ax to grind ventures into the scientific arena looking for evidence to support his prior beliefs. A good scientist would look just as carefully for any evidence that might prove his hypothesis wrong. Leu’s book stands in stark contrast to another book by a layman, Matt Fitzgerald, Diet Cults: The Surprising Fallacy at the Core of Nutrition Fads and a Guide to Healthy Eating for the Rest of Us. Unusually for a layman, Fitzgerald understands science better than many of the doctors who have written about nutrition. He approached his subject without bias, looked at the literature on both sides, and came to eminently reasonable conclusions. I reviewed his excellent book on the Science-Based Medicine blog (Hall 2014).

Leu’s book is useful, but only as a handy compilation of things we ought to be thinking about—things that scientists are already addressing.


This article was originally published on the Science-Based Medicine blog on December 9, 2014 (


Ames, B. 1989. Danger, Natural Pesticides. Online at

———. 1990. Dietary pesticides (99.99% all natural). Proceedings of the National Academy of Sciences 87: 7777–7781. Online at

EPA. 2007. Assessing Health Risks from Pesticides. Online at

———. 2012. Benefits of Pesticide Use. Online at

Hall, H. 2009. “The disappearing male”—A pinch of science, a pound of speculation. Online at

———. 2011. Phthalates and BPA: Of mice and men. Online at

———. 2014. Diet cults vs. science-based healthy eating. Online at

Kloor, K. 2014. The GMO-suicide myth. Online at

Lipson, P. 2009. Endocrine disruptors—the one true cause? Online at

Maroni, M., and A. Fait. 1993. Health effects in man from long-term exposure to pesticides. A review of the 1975-1991 literature. Toxicology 78(1–3):1–180.

Nelson, R. 2010. President’s Cancer Panel: Environmental Cancer Risk Underestimated. Online at

Novella, S. 2008. Bisphenol A in plastics: Should we worry? Online at

President’s Cancer Panel. 2010. Reducing Environmental Cancer Risk. Online at

USDA. 2014. Pesticide Data Program. Online at

Harriet Hall

Harriet Hall, MD, a retired Air Force physician and flight surgeon, writes and educates about pseudoscientific and so-called alternative medicine. She is a contributing editor and frequent contributor to the Skeptical Inquirer and contributes to the blog Science-Based Medicine. She is author of Women Aren’t Supposed to Fly: Memoirs of a Female Flight Surgeon and coauthor of the 2012 textbook Consumer Health: A Guide to Intelligent Decisions.