The Dirty Dozen is a low blow to growers, we agree. But is the science behind it suspect? Here is a scientific perspective on the Environmental Working Group’s Dirty Dozen methodology.
The report, found in the Journal of Toxicology, is called “Research Article Dietary Exposure to Pesticide Residues from Commodities Alleged to Contain the Highest Contamination Levels” and was authored by Carl K. Winter and Josh M. Katz.
The report looked at the 2010 EWG rankings. From the research article:
In June 2010, the EWG released its most recent “Dirty Dozen” list . Topping the list as the most contaminated commodity was celery, followed by peaches, strawberries, apples, blueberries, nectarines, bell peppers, spinach, cherries, kale, potatoes, and grapes (imported). According to an EWG news release, “consumers can lower their pesticide consumption by nearly four-fifths by avoiding conventionally grown varieties of the 12 most contaminated fruits and vegetables.”
It is unclear how the EWG could make such a statement since the methodology used to rank the various fruits and vegetables did not specifically quantify consumer exposure to pesticide residues in such foods. Instead, the methodology provided six separate indicators of contamination, including (1) percentage of samples tested with detectable residues, (2) percentage of samples with two or more pesticides detected, (3) average number of pesticides found on a single sample, (4) average amount of all pesticides found, (5) maximum number of pesticides found on a single sample, and (6) total number of pesticides found on the commodity. Each of these indicators was normalized among the 49 most frequently consumed fruits and vegetables, and a total score was developed to form the basis for the rankings. Since none of these indicators specifically considered exposure (the product of food consumption and residue levels), it is difficult to see how the EWG could substantiate the claim that consumers could lower their pesticide consumption by nearly four-fifths by avoiding conventional forms of the “Dirty Dozen” commodities. Additionally, the toxicological significance of consumer exposure to pesticides in the diet is also not addressed through an appropriate comparison of exposure estimates with toxicological endpoints such as the reference dose (RfD) or the acceptable daily intake (ADI).
To more accurately assess the potential health impacts from consumer exposure to pesticide residues from the “Dirty Dozen” commodities, this study utilized a probabilistic modeling approach to estimate exposures. The exposure estimates were then compared with toxicological endpoints to determine the health significance of such exposures.
I encourage readers to consider the full report. What were the results of the study? The authors say this:
The methodology used to create the “Dirty Dozen” list does not appear to follow any established scientific procedures. Only one of the six indicators used by the EWG crudely considers the amount of pesticide residue detected on the various commodities, and that indicator fails to relate exposures to such residues with established health criteria. Another indicator considers the percentage of samples found to be positive for pesticide residues. The remaining four indicators seem related as all appear to focus upon the existence of residues of multiple pesticides (percent of samples with two or more pesticides, average number of pesticides found on a single sample, maximum number of pesticides found on a single sample, and total number of pesticides found on the commodity) which suggests that the commodity rankings are significantly skewed to reflect instances of multiple residues. While research has demonstrated that the toxicity of a single chemical may be modulated by the presence of another chemical, such effects still require exposure to the modulating chemical to be at a level high enough (above a threshold dose) to cause a biological effect. Results from this study strongly suggest that consumer exposures to the ten most common pesticides found on the “Dirty Dozen” commodities are several orders of magnitude below levels required to cause any biological effect. As a result, the potential for synergistic effects resulting from pesticide combinations is negligible, and the EWG methodology which skews rankings due to the presence of multiple residues is not justified. The EWG methodology also does not appear to be capable of justifying the claim that “consumers can lower their pesticide consumption by nearly four-fifths by avoiding conventionally grown varieties of the 12 most contaminated fruits and vegetables” since no effort to quantify consumer exposure was made.
It should also be mentioned that consumption of organic produce should not be equated with consumption of pesticide-free produce. Winter and Davis  summarized pesticide monitoring results from the PDP, the California Department of Pesticide Regulation, the Consumers Union, and a study in Belgium. While conventional produce was between 2.9 and 4.8 times more likely to contain detectable pesticide residues than organic produce, samples of organic produce frequently contained residues. The PDP data, in fact, indicated that 23 percent of organic food samples tested positive for pesticide residues.
In summary, findings conclusively demonstrate that consumer exposures to the ten most frequently detected pesticides on EWG’s “Dirty Dozen” commodity list are at negligible levels and that the EWG methodology is insufficient to allow any meaningful rankings among commodities. We concur with EWG President Kenneth Cook who maintains that “We recommend that people eat healthy by eating more fruits and vegetables, whether conventional or organic” , but our findings do not indicate that substituting organic forms of the “Dirty Dozen” commodities for conventional forms will lead to any measurable consumer health benefit.