(CNN) Once again, strawberries top the list of the 12 "dirtiest" fruits and vegetables, according to the Environmental Working Group.

Every year since 2004, the group-- a nonprofit, nonpartisan environmental organization -- ranked pesticide contamination in 47 popular fruits and vegetables for its Shopper's Guide to Pesticides in Produce

Pesticides include a wide array of chemicals that kill unwanted insects, plants, molds and rodents.

Spinach is the second dirtiest item on the "Dirty Dozen" list, followed by (in order of contamination) nectarines, apples, grapes, peaches, cherries, pears, tomatoes, celery, potatoes and sweet bell peppers. Each of these foods tested positive for pesticide residues and contained higher concentrations of pesticides than other produce.

In fact, nearly 70% of conventionally grown -- non-organic -- produce samples were contaminated, the tests indicated.

The 13th suspect

The shopper's guide is based on results of tests by the US Department of Agriculture and the Food and Drug Administration on more than 38,800 non-organic samples. The Environmental Working Group looks at six measures of contamination including the average number of pesticides found on samples and the average amount of pesticides found.

When testing samples, the USDA personnel wash or peel produce to mimic consumer practices.

A single sample of strawberries showed 20 pesticides, the report indicated. More than 98% of strawberries, spinach, peaches, nectarines, cherries and apples tested positive for at least one pesticide residue. And, on average, spinach samples had 1.8 times as much pesticide residue by weight than any other crop.

This year, the Dirty Dozen list is actually a "baker's dozen" and includes a 13th suspect: hot peppers. These were found to be contaminated with insecticides toxic to the human nervous system, according to the organization. Anyone who frequently eats hot peppers should buy organic, it says.

"If you cannot find or afford organic hot peppers, cook them, because pesticide levels typically diminish when food is cooked," the authors of the report noted.

'Chronic health implications'

Children are of special concern as younger bodies have greater susceptibility to pesticides than adult bodies, the report emphasizes.

Research "suggests that pesticides may induce chronic health complications in children, including neurodevelopmental or behavioral problems, birth defects, asthma, and cancer," noted the authors of a 2012 American Academy of Pediatricians report quoted by the Environmental Working Group.

Other studies indicate that a child's earliest exposure to pesticides -- through the mother during pregnancy -- may also be harmful.

Consumers who want to eat the dirty dozen fruits and veggies should buy organic, according to the organization.

Rinsing produce under tap water is an effective way to eliminate pesticide residues from produce, according to the Connecticut Agricultural Experiment Station , a government-run scientific group. Scientists there advocate rinsing all fresh produce under tap water for a minimum of 30 seconds before using.

Water is enough, the scientists say, as mild detergents or commercial vegetable washes do not increase the amount of pesticide residues you are able to wash away. However, a recent study from the University of Massachusetts, Amherst, suggests that soaking produce in a solution of baking soda and water is a more effective way to rid fruits and veggies of pesticides.

Tom Stenzel, president and CEO of the United Fresh Produce Association, says consumers should not rely on a shopping guide when deciding which fruits and vegetables to purchase. The industry group represents growers, shippers, fresh-cut processors, wholesalers, distributors and retailers.

"Consumers have more choices now than ever before when it comes to the fruits and vegetables they consume," Stenzel said.

"Food safety is a top priority for the industry, from field to fork," he said. "The fresh produce industry seeks to ensure a safe, efficient and timely supply chain, allowing consumers to experience fresh fruits and vegetables at the peak of their performance."

He encourages consumers to continue educating themselves about food safety and consult the Safe Fruits and Veggies website from the Alliance for Food and Farming, which represents both organic and conventional farmers.

"Empowering consumers with knowledge is key to helping them make healthy choices for their diets and that of their family," Stenzel said.

'Clean 15'

Photos: 2018's 'Clean 15' Avocados – Fewer than 1% of avocados tested positive for pesticides. Best of all, only one pesticide of any kind was found on all the avocados tested. For these reasons, the Environmental Working Group ranked avocados as the No. 1 cleanest produce. Hide Caption 1 of 15 Photos: 2018's 'Clean 15' Sweet corn – Less than 2% of sweet corn, the second-cleanest produce, showed detectable levels of pesticides. Because a small portion of corn is grown from genetically modified seeds, the group suggests that those who wish to avoid genetically altered foods buy organic corn. Hide Caption 2 of 15 Photos: 2018's 'Clean 15' Pineapples – Nearly all the pineapples tested -- 90% -- showed no residual pesticides, while just five pesticides could be detected on any of the samples. For these reasons, pineapples fill position three on the clean list. Hide Caption 3 of 15 Photos: 2018's 'Clean 15' Cabbage – Overall, 86% of cabbage samples contained no detectable pesticides. Additionally, just two of more than 700 samples tested contained more than one pesticide. For these reasons, cabbage ranked fourth on the list of cleanest produce. Hide Caption 4 of 15 Photos: 2018's 'Clean 15' Onions – Fifth-placed onions contained three or fewer pesticides overall, while fewer than one in 10 contained any pesticides. Hide Caption 5 of 15 Photos: 2018's 'Clean 15' Frozen sweet peas – Frozen sweet peas ranked sixth on this year's clean produce list due to the fact that none of the tested samples contained more than two pesticides. Overall, about eight out of every 10 frozen sweet pea samples tested negative for pesticides. Hide Caption 6 of 15 Photos: 2018's 'Clean 15' Papayas – With no samples testing positive for more than three pesticides, papayas take seventh place on the list of clean fruits and veggies. The Environmental Working Group found that eight out of every 10 had no pesticide residues. Hide Caption 7 of 15 Photos: 2018's 'Clean 15' Asparagus – Asparagus samples mostly tested negative for pesticides: Ninety percent were clean, according to the group, and no more than three pesticides were found on any one sample of this vegetable. Asparagus ranks eighth among the cleanest produce. Hide Caption 8 of 15 Photos: 2018's 'Clean 15' Mango – Ninth-cleanest among all the different kinds of produce, mango samples showed no more than two pesticides when tested, while just over three-quarters tested negative for any and all chemical residue. Hide Caption 9 of 15 Photos: 2018's 'Clean 15' Eggplant – Eggplant showed themselves guilty, at worst, of contamination with three types of pesticides, according to the group. Best of all, almost three out of every four tested eggplants contained no pesticides whatsoever. Thus, eggplants ranked last on the top 10 within the "Clean 15" list. Hide Caption 10 of 15 Photos: 2018's 'Clean 15' Honeydew Melon – Half of honeydew melons tested negative for pesticides, while no more than four pesticides were found on any of the honeydew samples. And so, honeydew melon slips into position 11 on the clean list. Hide Caption 11 of 15 Photos: 2018's 'Clean 15' Kiwi – Lovely green kiwis were mostly pesticide free when tested: Sixty-five percent of all samples showed no chemicals, while only six pesticides could be found on any of the samples. Hide Caption 12 of 15 Photos: 2018's 'Clean 15' Cantaloupe melon – Overall, one out of 10 cantaloupe samples contained more than one pesticide, and more than 60% contained no pesticide residues. This orange melon ranked lucky 13 on the clean list. Hide Caption 13 of 15 Photos: 2018's 'Clean 15' Cauliflower – No pesticides were detected on about half of the cauliflower samples and none contained more than three separate chemicals. This is why cauliflower took the penultimate position on the "Clean 15" list. Hide Caption 14 of 15 Photos: 2018's 'Clean 15' Broccoli – Last but not least, broccoli found its way onto the list of cleanest produce. Seven out of 10 samples showed no pesticide residue, while only one in 10 contained more than one pesticide. Hide Caption 15 of 15

On a positive note, the Environmental Working Group also creates a lesser-known companion to the Dirty Dozen: the "Clean 15" guide to produce containing the least amount of pesticides.

Avocados lead 2018's clean fruits and veggies list, followed by sweet corn, pineapples, cabbage, onions, frozen sweet peas, papayas, asparagus, mangoes, eggplant, honeydew melon, kiwi, cantaloupe, cauliflower and broccoli.

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Veggies placed in the top two spots -- avocados and sweet corn -- both showed pesticides on less than 1% of tested samples, the new report indicated. And more than 80% of pineapples, papayas, asparagus, onions and cabbages tested negative for pesticide residues.

The organization cautions that a small portion of sweet corn, papaya and summer squash sold in the US is produced from genetically modified seeds. It says anyone wanting to avoid genetically modified produce should buy organic varieties of these crops.

Through its healthy eating reminder, MyPlate , the USDA recommends that half your plate be filled with fruits and vegetables.

"Everything you eat and drink matters," the agency says. "The right mix can help you be healthier now and in the future."