WASHINGTON, D.C. — The Environmental Working Group (EWG) has released its latest revision of its “Dirty Dozen” and “Clean Fifteen” lists, in which pesticide content in fruits and vegetables are ranked from most to least.

Apples were determined to be the most contaminated out of 53 crops tested. This suggests a steady increase in apple orchardists’ use of pesticides over the past decade, since apples were ranked third-most contaminated in 2003 tests, and second-most contaminated in 2006 and 2009.

According to the EWG, the data behind the list are obtained entirely from information released by the USDA’s Pesticide Data Program, which routinely tests levels of pesticide contaminants in produce at distribution centers around the nation. The EWG’s stated mission “is to use the power of public information to protect public health and the environment.” It is a 501(c)(3) nonprofit organization, founded in 1993.

Some of the food items are partially prepared in order to remove the parts not “commonly eaten” from the analysis. For example, potatoes are peeled before testing, but apples are not.

“Samples tested by (Pesticide Data Program) … are taken randomly from large distribution centers (that reach) approximately 600 wholesalers,” said a spokesman for USDA Agricultural Marketing Service agent Michael Jarvis, who is partly responsible for coordinating the program.

“These sites are located in 11 states representing 50 percent of the U.S. population, and in all four (geographic) regions … The program also tests for environmental contaminants, pesticides no longer used but due to their persistence in the environment, (they) continue to be found in some crops.”

Its Dirty Dozen list refers to the 12 fruit and vegetable crops most often found to be contaminated by toxic residues. The current list, starting with the “dirtiest,” is as follows: apple, celery, strawberry, peach, spinach, nectarine, grape (especially if imported), bell pepper, white potato, blueberry, lettuce and kale/collard.

The significance of the list, according to the EWG, is that for consumers it can serve as a “shopper’s guide list.” The EWG recommends shoppers avoid buying conventional supermarket versions of the so-called Dirty Dozen, and instead seek to buy organic versions of those crops or obtain them from local farms that are known to not use petrochemical-based pesticides on their vegetables.

For farmers and market gardeners, the list is also helpful because it lets them know which vegetables would be more profitable to grow as “organic” or “certified naturally grown” rather than without such certifications.

The study did not include peanuts, which are often called the most pesticide-treated crop in the United States, because they are a dry non-perishable commodity. The Pesticide Data Program (PDP) only tests fresh-marketable fruits and vegetables, such as those listed above.

Data are compiled for the crops in terms of “pesticide/crop combinations”, meaning individual pesticide levels, such as “methoxychlor level in spinach.”

These are measured for approximately 400 chemicals on 53 crops, which theoretically involves 21,000 pesticide/crop combinations to analyze.

The EWG then creates a meta-analysis, or summary, of what crops have the most total pesticide residual burden and which have the least. Factors involved in what ranks where on the list include susceptibility of the crop to pests, current pest outbreaks affecting particular plant species and varying state laws regarding pesticide legality.

Although the testing was broad-spectrum, it has not yet included any pesticides belonging to the class known as EBDCs, or ethylene bisdithiocarbamates. Those substances require more involved processes and specialized equipment, and cannot easily be detected through a simple analysis.

The PDP tests only for those contaminants that can be detected through “multiple-residue methods.” In other words, it only tests for chemicals that can be detected simultaneously with a wide range of other chemicals in the same crop sample. This allows the testing to be conducted using a much smaller number of actual sample analyses than would be needed if the program included tests for EBDCs and other contaminants, which must be detected and measured one at a time.

The “clean” list was also revised slightly. Starting with the least pesticide-contaminated, the list, excluding tropical fruits, is: onion, sweet corn, asparagus, snap pea, aubergine (eggplant), cantaloupe, kiwi fruit, cabbage, watermelon, sweet potato and mushrooms.

Although sweet corn ranked near the top of the “Clean Fifteen” list, according to the EWG, this may be because of the use of Bt bred into the corn through technology, which sometimes serves as an equivalent substitute for topical pesticides but may still pose health risks.

“We at the Environmental Working Group advise those with concerns about GMOs (genetically modified) to buy organic sweet corn,” a representative from the group said in a telephone interview.