5 June 2018

‘Dirty Dozen’ List Contaminated with Non-Science

Carl K. Winter, Ph.D.

Extension Food Toxicologist
University of California - Davis

The Environmental Working Group (EWG), a U.S.-based environmental advocacy organization, released its annual “Shopper’s Guide to Pesticides in Produce” report in April 2018 amidst widespread media coverage and replete with unsubstantiated consumer advice derived from dubious methodology.

The EWG report includes its listing of the “Dirty Dozen” fruits and vegetables alleged to contain the greatest relative levels of pesticide residues. The most contaminated foods, according to the EWG, are strawberries, spinach, nectarines, apples, grapes, peaches, cherries, pears, tomatoes, celery, potatoes and sweet bell peppers. The EWG urges consumers to avoid purchasing conventional forms of these Dirty Dozen commodities and recommends they purchase organic forms of these foods or to substitute conventional forms of other fruits and vegetables.

The three pillars of risk assessment – actual levels of pesticides detected, amount of foods consumed, and, importantly, the toxicity of the individual pesticides – are conveniently ignored in development of the rankings.

Questionable Methodology

As has been the case since the EWG began releasing its annual Shopper’s Guide in 1995, the methodology used to rank fruits and vegetables for pesticide contamination is arbitrary and incapable of supporting its recommendation that consumers should avoid eating conventional forms of the Dirty Dozen. Using results from the U.S. Department of Agriculture’s Pesticide Data Program, the report considered six metrics regarding pesticide residues:

  • Percent of samples tested with detectable pesticides
  • Percent of samples with two or more detectable pesticides
  • Average number of pesticides found on a single sample
  • Average amount of pesticides found, measured in parts per million
  • Maximum number of pesticides found on a single sample
  • Total number of pesticides found on a crop

Individual fruits and vegetables were ranked within each metric and an unweighted summation of each food’s “score” for each metric was used to determine the overall rankings. What is most notable from the list of metrics is that none have any use in a legitimate assessment of risk; the simple focus on “detection” of residues is insufficient to quantify risk. The three pillars of risk assessment – actual levels of pesticides detected, amount of foods consumed, and, importantly, the toxicity of the individual pesticides – are conveniently ignored in development of the rankings. Ironically, the report’s Executive Summary references the 1993 National Academy of Sciences report that provided methodological recommendations (subsequently adopted by the United States Environmental Protection Agency) to appropriately assess the risks from pesticide residues from food, water and residential settings.

In summary, we found that the risks of the most frequently detected pesticides from the 2010 Dirty Dozen list pose negligible risks to consumers and the substitution of organic forms would not appreciably impact consumer risk.

Science-Based Risk Assessment

Fortunately, more sophisticated methods have been used to assess dietary risks from pesticide residues on the Dirty Dozen list. In a peer-reviewed scientific paper published in the Journal of Toxicology in 2011, graduate student Josh Katz and I looked at specific exposure to the 10 most frequently detected pesticides on each fruit and vegetable included on the 2010 Dirty Dozen list.1 Exposures were calculated by identifying actual amounts of specific residue levels found on the fruits and vegetables and incorporating estimates of consumption. Exposure estimates were then compared with toxicological indicators of safety – the Reference Dose (RfD) that represents levels not considered to pose any health concern for consumers). Findings demonstrated that consumer exposure to the most frequently detected pesticides on the 2010 Dirty Dozen commodities were extremely low and well below the RfD values in all cases. In 75 percent of the cases, exposure was below 0.01 percent of the RfD, representing exposures at least one million times lower than those that do not demonstrate any toxicological effects in laboratory animals exposed to the pesticides on a daily basis throughout their entire lifetimes. In summary, we found that the risks of the most frequently detected pesticides from the 2010 Dirty Dozen list pose negligible risks to consumers and the substitution of organic forms would not appreciably impact consumer risk.

The 2018 Shopper’s Guide includes a new wrinkle conflating a questionable epidemiological study with its Dirty Dozen rankings. The study looked at women who were using assisted reproductive techniques at a fertility clinic and surveyed their fruit and vegetable consumption through a questionnaire.2 Using a fairly arbitrary ranking system similar to that used by the EWG, the study considered the percentage of samples detected with pesticide residue, the percentage of samples with residues over the tolerance level (an extremely rare occurrence), and the percentage of samples with three or more detectable residues. Fruits and vegetables were considered to be either “high” or “low” pesticide residue foods. The study reported that women eating two or more servings per day of produce with “high” pesticide residues were 26 percent less likely to have a successful pregnancy than those who ate fewer servings of these foods. Interestingly, the study also pointed out that women who consumed more “high” pesticide residue fruits and vegetables were also more likely to report regular organic food consumption. Put another way, it is quite likely that had the researchers simply related organic food consumption to reproductive outcomes, it may have concluded that organic food consumption reduced the likelihood of a successful pregnancy. This illustrates the difficulties in deriving “cause-and-effect” relationships from epidemiological findings that simply indicate correlations.

Strong science, regardless of what the EWG says, indicates that all conventionally produced fruits and vegetables should be part of a healthy diet, and those singled out as “dirty” based upon arbitrary and methodologically unsound approaches can and should be consumed without stress or fear.

Over the years, in the face of criticism that the Shopper’s Guide might lead many consumers to reduce their consumption of fruits and vegetables, the EWG has reluctantly mentioned in its annual Shopper’s Guide that “eating conventionally grown produce is far better than skipping fruits and vegetables.” Unfortunately, subsequent alarmist rhetoric in its summaries of pesticide residue findings for specific crops sends a distinctly different message.

The impact of such messaging is not trivial; a recent paper surveying low-income consumers and their fruit and vegetable consumption behavior indicated that 15 percent of those surveyed planned to consume fewer fruits and vegetables after becoming aware of the Dirty Dozen list.3

Consumers should feel confident when they purchase fruits and vegetables, whether organic or conventional, and feed them to their families. Strong science, regardless of what the EWG says, indicates that all conventionally produced fruits and vegetables should be part of a healthy diet, and those singled out as “dirty” based upon arbitrary and methodologically unsound approaches can and should be consumed without stress or fear.

Carl K. Winter, Ph.D., is vice chair and extension food toxicologist in the Department of Food Science and Technology at the University of California in Davis, Calif., USA. His research and outreach activities focus upon chemical contaminants in food. He has authored two books and more than 150 publications in scientific journals, books and popular media.


1 Carl K. Winter and Josh M. Katz, 2011.  Dietary exposure to pesticide residues from commodities alleged to contain the highest contamination levels.  Journal of Toxicology, Article ID 589674, doi:10.1155/2011/589674.
2Yu-Han Chiu, Paige L. Williams, Matthew W. Gillman, et al., 2017.  Association between pesticide residue intake from consumption of fruits and vegetables and pregnancy outcomes among women undergoing infertility treatment with assisted reproductive technology.  JAMA Internal Medicine, doi:10.1011/jamainternmed.2017.5038.
3Yancui Huang, Indika Edirisinghe, and Britt M. Burton-Freeman, 2016.  Low-income shoppers and fruit and vegetables:  What do they think?  Nutrition Today 51(5): 242-250.

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