Cost is certainly important. But there are essential numbers that go beyond the price tag of a peach, or any other item from the produce aisle.

Which contain the highest levels of pesticides?

Preliminary 2008 U.S. Department of Agriculture tests obtained by the Chicago Tribune show that more than 50 pesticide compounds showed up on domestic and imported peaches headed for U.S. stores. Five of the compounds exceeded the limits set by the Environmental Protection Agency, and six of the pesticide compounds present are not approved for use on peaches in the United States.

These are the types of findings that have landed peaches on one environmental group's "Dirty Dozen" list — 12 fruits and vegetables that retain the highest levels of pesticide residues — and give many consumers pause as they shop grocery aisles. It seems that peaches' delicate constitutions, fuzzy skins and susceptibility to mold and pests cause them to both need and retain pesticides at impressive rates.

Although most pesticides in peaches were found at levels well below EPA tolerances, some scientists and activists remain concerned about even low-level exposure, especially to pregnant women and children. They point to studies, for example, that show cognitive impairment in rats after dietary exposure to chlorpyfiros, a pesticide that showed up in 17 percent of conventional peaches tested by the USDA.

For assurance, some shoppers turn to farmers markets, which don't guarantee reduced pesticide use but do allow shoppers to discuss pesticide practices with the farmer. Organic, meanwhile, does come with the expectation that the fruit will be free of synthetic pesticides. Yet no government agency ever tested that promise until this year — and so far those tests have been limited to lettuce, with no published results.

To get some hard facts and new insights, the Tribune paid for lab tests on California organic peaches bought here and local farmers market peaches from Illinois and Michigan.

The newspaper sent these samples to the same federal lab where the USDA does its pesticide testing and found promising results. Of the 50 compounds the Tribune had tested for, one showed up on the organic peaches and three or fewer pesticides were detected on the Michigan and Illinois peaches.

"Our growers (in southwest Michigan) pride themselves on being very careful," said William Shane, district fruit educator for Michigan State University, when he learned how Michigan peaches fared in the test. "We also tend to have smaller operations and it's easier to keep track of pesticide use."

The better results in the Tribune's small sample may also be attributable to the fact that the wider 2008 USDA conventional tests included peaches imported from Chile.

Chilean peaches have, in the past, shown a higher incidence of certain pesticides than U.S. peaches. The conventional samples, taken from more than 700 sites, also included peaches from areas like Georgia and South Carolina where a broader range of pesticides are often needed to control pests and fungus.

More surprising, however, was the presence of the unapproved pesticide fludioxonil on the organic peaches from California. According to Shane, the pesticide is often used on conventional peaches postharvest to slow rot and extend shelf life.

University of Illinois entomologist and extension specialist Rick Weinzierl suggested that the unapproved pesticide could have come from drift or cross-contamination at processing facilities. "But there is always the chance that a farmer is not doing what he is saying," he added.

Rayne Pegg of the USDA's agriculture marketing service confirmed that fludioxonil is not an approved compound for organic farming but added, "as long as the concentrations don't exceed 5 percent of EPA tolerances, it can be sold as organic." In fact, the USDA allows such levels of any legal pesticide to be present on organic produce. In the wake of recent allegations about slipping standards in the USDA's National Organic Program, Congress has widened a probe into the NOP and recently USDA announced an independent audit of the program. The organic world was further rocked last month by a controversial British review of nutrient studies that challenged the nutritional benefits of organic produce.

Supporters of organic foods complained that many important studies were left out of the review, and the debate on nutrition ignores the question of pesticide residue.

Although the 1996 Food Quality Protection Act sets pesticide tolerances at levels that offer "a reasonable certainty that no harm will result from aggregate exposure to the chemical residue," some scientists worry about exposure among children and pregnant women.

Alex Lu, who teaches environmental exposure biology at Harvard, has studied a particularly troubling class of pesticide called organophosphates, or OPs, which showed up consistently in the systems of Seattle-area children ages 3 to 11 who ate non-organic diets. When the children switched to an organic diet for five days, these pesticide levels became nearly undetectable, the study found.

The professor acknowledged the importance of fresh produce in a young diet but is concerned that conventional produce consumption translates too easily into the presence of OPs in these developing systems. He advises against giving children conventionally farmed produce from any items on the Environmental Working Group's "Dirty Dozen," which is culled from FDA and USDA test results. Other produce on that list are strawberries, apples, nectarines, cherries, lettuce, bell peppers, celery, pears, kale, imported grapes and carrots.

Lu is even more concerned about the dietary habits of pregnant women.

"Don't eat conventional peaches while you are pregnant," he said. "It's a critical time. Spend a little bit more money to buy organic just to be safe."

Dr. Catherine Karr, who serves on the American Academy of Pediatrics Environmental Health committee, stopped short of advising against conventional peaches for children altogether.

"You want to maximize the healthfulness of children's diets by making sure they get plenty of fruits and vegetables," she said. "But ... you want to minimize their exposure to pesticides, and we know that the best way to do that is by giving them as much organic produce as possible."

According to the USDA, when its Pesticide Data Program discovers the use of unapproved pesticides or pesticide residues that exceed federal tolerances, it reports them to the FDA and EPA. Because of the length of the complicated screening and reporting process, these violation reports are not used for enforcement but rather to highlight potential problem areas.

"Consumers should feel confident that we collect this data and provide it to the proper regulatory agencies for enforcement," said USDA spokesman Justin DeJong.