As a nearly $8 billion business in the U.S. alone, according to 2000 data from the U.S. Department of Agriculture, organic fruits and vegetables have moved rapidly from a fringe business at the local food co-op to a mainstream supermarket staple. A key reason consumers buy organic is to avoid pesticide residues, but a small study suggests that organic produce may not be quite as clean as shoppers expect. Banned pesticides like DDT were found in organic carrots and potatoes at levels as high as or higher than conventionally grown produce, according to a screening study conducted by a college undergraduate and presented at the Society of Toxicology and Chemistry annual meeting. Under federal law, crops labeled organic must be grown without the use of synthetic pesticides, chemical fertilizers, or sewage sludge. Such treatments must not have been used on a field for at least three years prior to planting of the organic crop. Those three years are meant to cleanse the soil of pesticide residues. But many long-used—and now-banned—toxic organochlorine pesticides can take decades to break down. Because root crops, such as carrots, grow directly in the soil, they represent a worst-case scenario for evaluating whether crops acquire such lingering pesticide residues.

Organic produce has lower levels of pesticides overall, according to agricultural scientist Brian Baker, who co-authored a widely cited paper that demonstrates this point (Food Addit. Contam. 2002, 19, 427–446). The new results attest to the persistence of organochlorine pesticides, adds Baker, who is research director for the Organic Materials Review Institute, a nonprofit organization that specializes in the review of substances for use in organic production, processing, and handling.

Beth Wolensky, a senior at Chatham College in Pittsburgh, Pa., bought 20 batches of carrots—half labeled organic and half grown conventionally. She washed the carrots as if she were cooking them for dinner and peeled some of them. Every carrot she tested harbored traces of p,pN-DDE, a breakdown product of the insecticide DDT, which has been banned for more than 30 years. Many of the carrots also carried residues of chlordane, a common pesticide that was banned in 1983. Some samples also contained small amounts of heptachlor, once popular as an agricultural pesticide and residential termite treatment. She plans to publish the study.

In all the carrot samples, concentrations of these chemicals were very low, in the low parts-per-trillion (ppt) range. The chemicals concentrated in the skin of the vegetables. In conventionally grown whole carrots, the mean concentration of p,pN-DDE was 40 ppt, but organic carrots had mean concentrations of 340 ppt. However, the skin of the conventionally grown carrots had concentrations of 588 ppt, compared to 3050 ppt for the organic ones. Renee Falconer, the analytical chemist who served as Wolensky’s faculty adviser, notes that the study lacks the statistical power to determine whether the organic carrots actually contain higher levels of the banned pesticides. She thinks it instead reflects the variability of the data.

In 2004, another Chatham student, Tanieka Motley, found similar results for potatoes, Falconer notes. At the concentrations detected, none of the chemicals in the carrots or potatoes is harmful. “But these low levels add to the overall pesticide load entering our bodies from all sources,” she says. Falconer notes that organic produce does have lower overall levels of pesticides that are currently in use. To reduce the pesticide load to her family, she buys organic and peels her root vegetables.

  (Januari 2006) (Opm. Kijk bij Wist U dat? voor ons advies.)