Those peaches at the grocery store might look nutritious, but they might also be loaded with pesticide residues, suggests a new report from the Environmental Working Group.

The non-profit environmental group, based in Washington, D.C., has updated its list of the produce items that are heaviest in pesticide residues and those that it says are "cleanest."

It found that of the 12 most contaminated foods, seven are summertime fruits: peaches, strawberries, U.S.-grown blueberries, nectarines, cherries, apples and imported grapes.

Many of these foods have soft skin that make them vulnerable to pests. But those are also the fruits and veggies that tend to absorb more pesticides and that are most likely to be eaten unpeeled.

This year, the worst of the worst items -- or what the EWG likes to call its list of the "Dirty Dozen" -- looks like this:

Dirty Dozen

Buy these organic

Clean 15

Lowest in pesticides

 1. Celery  1. Onions
 2. Peaches  2. Avocados
 3. Strawberries  3. Sweet corn
 4. Apples  4. Pineapples
 5. U.S.-grown blueberries  5. Mango
 6. Nectarines  6. Sweet peas
 7. Sweet bell peppers  7. Asparagus
 8. Spinach, kale and collard greens  8. Kiwi fruit
 9. Cherries  9. Cabbage
 10. Potatoes  10. Eggplant
 11. Imported grapes  11. Cantaloupe
 12. Lettuce  12. Watermelon
  13. Grapefruit
  14. Sweet potatoes
  15. Sweet onions

To come up with this year's list, EWG looked through early 100,000 produce pesticide reports from the U.S. Department of Agriculture and the U.S. Food and Drug Administration to determine what fruits and vegetables have the highest, and lowest, amounts of chemical residue.

Nearly all the studies tested the produce as the foods would be eaten. So bananas were peeled before they were tested, while apples were simply washed.

They found that more than 96 per cent of peaches tested positive for pesticides, followed by nectarines (95.1 per cent) and apples (93.6 per cent).

Nearly 86 per cent of peaches contained two or more pesticide residues‚ followed by apples (82.3 per cent) and nectarines (80.6 per cent). Strawberries and domestic blueberries each had 13 pesticides detected on a single sample. Peaches and apples were second, with nine pesticides on one sample.

On the other hand, asparagus, sweet corn, and onions had no detectable pesticide residues on 90 per cent or more of samples. And fewer than 10 per cent of pineapple, mango, and avocado samples showed detectable levels of pesticide residues.

While it's not always easy to buy organic produce all the time, the EWG lists can help cash-strapped families decide which items are worth the extra cost and which are not. The EWG estimates that consumers can reduce their pesticide exposure by 80 per cent by only buying organic versions of the "Dirty Dozen".