Every year the Environmental Working Group figures out which of our foods are most highly contaminated with 
Only buy these if they are organic:
The Dirty Dozen Plus*** 2015
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Apples
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Celery
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Cherry Tomatoes
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Cucumbers
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Grapes
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Nectarines
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Peaches
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Potatoes
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Snap Peas
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Spinach
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Strawberries
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Sweet Bell Peppers
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**Hot Peppers and Kale/Collard Greens
Buy conventional for these and save yourself a little money:
The Clean Fifteen 2015
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Asparagus
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Avocados
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Cabbage
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Cantaloupe
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Cauliflower
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Eggplant
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Grapefruit
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Kiwi
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Mangoes
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Onions
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Papayas
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Pineapples
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Sweet Corn
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Sweet Peas (frozen)
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Sweet Potatoes