The Dietary Risk Index Data and Methods

The Environmental Protection Agency (EPA) has developed detailed science policies describing the methods it uses to quantify pesticide dietary risks. The “Dietary Risk Index” (DRI) is based on these methods and EPA’s evaluation of pesticide toxicity. The pesticide residue data required to calculate DRI values for a specific pesticide-food combination comes from the U.S. Department of Agriculture’s (USDA) Pesticide Data Program (PDP) (for more, see Sources of Pesticide Residue Data). The PDP is widely regarded as one of the premier pesticide residue datasets available for use in pesticide dietary risk assessment.

Pesticide dietary risk is a function of the level of exposure in a given day and pesticide toxicity. A person’s age, health status, and genetics also can markedly alter risk levels. It is widely acknowledged that exposures during pregnancy and in the first few years of life pose the most significant risks, because of the potential to trigger birth defects or epigenetic changes in gene expression and/or regulation during development (NRC/NAS, 1993).

Pesticide dietary risks are typically measured – and regulated – on a daily basis. The maximum “safe” amount is the upper bound of exposure levels that are deemed by the EPA as still representing a “reasonable certainty of no harm.” Exposure to a specific pesticide in a given day, and all pesticides collectively, is determined by how frequently residues appear in the foods consumed by a person, the residue levels in those foods, the number of residues in different foods, and how much of a given food an individual eats in that day.

An elaborate and data-intensive process is used by EPA to determine the toxicity of a food-use pesticide to humans. Dozens of toxicology studies must be carried out and submitted by registrants in response to the agency’s data requirements. After reviewing all submitted studies, the EPA identifies the acute and chronic adverse effects observed that occur at the lowest dose levels. For the acute and chronic study demonstrating an adverse effect at the lowest dose, the EPA identifies the next-lower dose where no statistically significant adverse effects were observed. These become the acute and chronic “no observable adverse effect level,” or NOAEL. A 100-fold safety factor is then applied to the NOAEL to establish an acute and chronic Reference Dose (aRfD, cRfD).

Calculating the DRI

For a given pesticide-food combination, the DRI value is the average residue level in the food divided by the pesticide’s “chronic Reference Concentration” (cRfC).

The cRfC is the maximum concentration of a pesticide that can be present in a daily serving (or servings) of a food/beverage, without exposing an individual of known weight to a dose of the pesticide that exceeds his or her personal, “reasonable certainty of no harm” limit.

Chronic RfCs are expressed in milligrams (mg) pesticide per kilogram (kg) food (parts per million) and are calculated for each pesticide using a formula driven by cRfDs or cPADs. For this reason, the bigger and heavier an individual is, the higher is or her cRfC will be.

The cRfC for a given food-pesticide combination is derived from the equation:

cRfC Pesticidex (mg/kg food) X Serving Size Foodx (grams/day) = Weight of Person (kg) X cPAD for Pesticidex (mg/kg bw/day)

After dividing both sides of the equation by serving size:

cRfC Pesticidex (mg/kg food) =
[Weight of Person (kg) X cPAD for Pesticidex (mg/kg bw/day)] ÷ Serving Size Foodx (kg/day)

Chronic RfCs are based on the diet of a child weighing 16 kg. The selection of 16 kg corresponds to children around 3.5 years old at the 50th percentile of growth based on the Centers for Disease Control and Prevention (CDCP) growth chart (CDCP, 2013). By or around age 3.5 years, children are consuming their largest portions of individual foods per kilogram of body weight. Other bodyweights could be used, but the bobyweight parameter does not change the relative values of DRIs for one pesticide-food combination in contrast with others.

Applying the DRI to Monitor Dietary Risks

For each pesticide-food combination, the arithmetic mean of all positive residues found by PDP in a given year of testing is used in calculating the DRI-Mean (DRI-M). This measure of relative pesticide dietary risk is best applied to crops/foods known to contain, or likely to contain residues of a particular pesticide.

PDP also reports the percent of samples of a given food that test positive for each pesticide. This parameter – “Percent Positive” — is used in computing the “Food Supply DRI” (FS-DRI). The FS-DRI is simply the DRI-Mean multiplied by the percent of samples tested that were found to contain a residue (i.e., percent positive).

The DRI-M is the appropriate metric when comparing risk levels in a serving of two foods, both known to contain residues of a single pesticide or multiple pesticides. The FS-DRI, on the other hand, reflects average pesticide risk levels across many servings of a given food, or many servings of different foods, and takes into account the frequency of residues in food, addition to mean residue levels.

As a general rule of thumb, single food-pesticide DRI-M values at or below 0.1 are regarded as consistent with the EPA’s “reasonable certainty of no harm” standard, because very few pesticides are likely to be present in 10 different foods consumed by a person in a given day, each with DRI-M values of 0.1 or higher. DRI-M values greater than one do not mean a high probability of harm, but rather represent a reduction in the typical 100-fold to 1,000-fold margin of safety that the EPA incorporates in its setting of pesticide tolerances. “DRI-M = 10” means, in effect, that EPA’s typical 100-fold safety factor has been reduced to a 10-fold safety factor.

It is worth noting, however, that since the DRI-M is based on the mean of all positive residues of a given pesticide in a food, several samples will contain residues above the mean level. Indeed, analyses of the distribution of pesticide residue levels show that the mean residue level is usually five- to 10-fold lower than the 99th percentile of the residue distribution, a level EPA strives to assure is consistent with the FQPA’s basic “reasonable certainty of no harm” standard.

The Rule of 10

The PDP typically tests 600 to 750 samples of a given food in an annual program cycle. On average, about 40% of fresh fruits and vegetables are imported. PDP identifies the country of origin, so that country-specific DRIs can be computed, as well as DRIs covering “combined imports.” Whenever there are less than 10 samples of a food from a given country, the “rule of 10” prohibits the calculation of a DRI because of an inadequate number of samples to produce a reliable DRI value.

The program also classifies samples according to market claims, with the vast majority falling into two groups – “no claim” (and presumed to be conventionally grown) and organic. But for some foods, the number of organic samples is limited and the rule of 10 is invoked. In the case of imported organic food, there are rarely 10 or more samples in a given year, and hence the rule of 10 makes it difficult to assess differences in DRI values across organic food from multiple countries. This is why most comparisons in DRI values between domestically grown and imported foods are based on “combined imports.”

History of the DRI

The seminal 1993 National Academy of Sciences report Pesticides in the Diets of Infants and Children (NRC/NAS, 1993) set forth the reasons why pesticides can lead to significantly heightened risks when infants, children, and pregnant women are exposed. It called for improved data on pesticide residues in key children’s foods, a need fulfilled in large part by the PDP. The primary recommendations of the 1993 NAS report were incorporated in the historic 1996 Food Quality Protection Act (FQPA), legislation that directed the EPA to dramatically alter the way it sets tolerances governing pesticide residues in food. The FQPA provided the EPA key new tools to assure a “reasonable certainty of no harm” stemming from pesticide exposures via all routes for all vulnerable population groups, and in particular pregnant women, infants, and children.

In cases where the EPA determines that an added safety factor (usually 3-X or 10-X) is required in accord with the provisions of the FQPA, a pesticide’s RfD is reduced 3-X or 10-X and becomes a Population Adjusted Dose (PAD). Dietary risk assessments are then typically based on a pesticide’s cRfD or cPAD, because in most cases chronic-risk benchmarks are lower than acute RfDs or PADs.

The first version of the DRI focused on cancer as the toxicological endpoint, and was developed and applied under the direction of the National Academy of Sciences/National Research Council (NAS/NRC) committee that wrote the 1987 report Regulating Pesticides in Food: The Delaney Paradox (NRC/NAS, 1987). The DRI concept was further developed and applied to chronic risks in the 1996 book Pest Management at the Crossroads published by Consumers Union (Benbrook et al., 1996). Dietary risks associated with potato pesticide use has been tracked as part of a collaborative project involving the World Wildlife Fund, the Wisconsin Potato and Vegetable Growers Association, and the University of Wisconsin-Madison (Benbrook et al, 2002). The EPA’s Office of Inspector General sponsored an independent analysis of the impacts of the FQPA on dietary risks through 2002, based on a version of the DRI designed to track, as closely as feasible, EPA dietary risk assessment policies (OIG, 2006a and 2006b). The DRI has been applied in multiple other analyses for non-profit organizations, companies, and government agencies.


Benbrook, C. M. et al. “Developing a pesticide risk assessment tool to monitor progress in reducing reliance on high-risk pesticides.” American Journal of Potato Research, 79 (2002): 183-99.

Centers for Disease Control and Prevention. 2013. “2 to 20 years: Boys Stature-for-age and Weight-for-age percentiles,”, accessed 4/23/12.

Office of Inspector General (OIG). 2006a. Measuring the impact of the Food Quality Protection Act: Challenges and opportunities. Report No. 2006-P-00028, August 1, U.S. EPA, Washington, D.C.

Office of Inspector General (OIG). 2006b. Supplemental Report: Details on dietary risk data in support of Report No. 2006-P-00028, Measuring the impact of the Food Quality Protection Act: Challenges and opportunities. U.S. EPA, Washington, D.C.

National Research Council/National Academy of Sciences. Regulating Pesticides in Food: The Delaney Paradox. 1987. Washington D.C., National Academy Press.

National Research Council/National Academy of Sciences. Pesticides in the Diets of Infants and Children. 1993. Washington D.C., National Academy Press.