Most people choose organic produce for one simple reason: to avoid consuming pesticides. Organic farmers are prohibited from using virtually all synthetic chemicals, either to kill weeds or pests or to fertilize plants. Conventional farmers, on the other hand, can use around 200 approved synthetic chemicals—fertilizers, pesticides, herbicides, fungicides.

No one disputes the fact that at least some of those chemicals end up on the fruits and vegetables we take home from the market. The U.S. Department of Agriculture regularly tests hundreds of samples of fresh and processed foods for pesticide residues and posts the results on its website (www.ams.usda.gov).

The samples are gathered from a variety of markets around the U.S. and represent a cross-section of what’s available to consumers, including both organic and conventional produce. According to the latest results, which included 13,208 samples, 76 percent of fresh fruit and vegetables and 40 percent of processed fruits and vegetables had detectable residues.

All of the milk tested, both organic and conventional, showed residues. Overall, about 30 percent of the samples were pesticide-free, 30 percent contained one pesticide and 40 percent had traces of more than one chemical. To be sure, the detected levels were very low in most samples, measured in a few parts per billion. According to the USDA, only 0.2 percent of the contaminated samples exceeded tolerance levels set by the federal government.

Even organic produce isn’t necessarily pesticide-free. Synthetic chemicals can drift over from nearby fields or leach into groundwater. All the same, organic produce is consistently lower in residues.

In the USDA’s samples, for instance, 76 percent of conventionally grown fruits and vegetables had detectable pesticide levels, compared to only 23 percent of organically grown produce. California does its own testing, independent of the USDA, and its analysis found residues in 31 percent of conventionally grown samples and only 6.5 percent of organics.