Strawberries remain the dirtiest of the Environmental Working Group's 2019 Dirty Dozen list, but they've been joined by a vegetable not seen on the list since 2009: kale.

It's not that kale growing practices have changed that much. The popular leafy green hasn't been included in the regular produce tests upon which these lists are based. The last time it was included, kale ranked eighth on the nonprofit's dirty list. The lists are based on results of more than 40,900 samples of produce tested by the U.S. Department of Agriculture and the Food and Drug Administration. All produce is rinsed and peeled before the tests are conducted.

Each year since 2004, EWG has released the lists to make it easier for consumers to avoid pesticide residue. Although fruits and vegetables are an important part of a healthy diet, the pesticides left behind on conventionally grown produce can affect health in subtle ways.

In 2018, EWG also released a video (above) that explains how the list is created and why certain fruits and vegetables frequently end up on the Dirty Dozen list while others consistently land at the bottom of the other list, what the organization calls the Clean Fifteen.

The Dirty Dozen

The high level of residual pesticides left on conventional strawberries and spinach mean you will be better off making this classic spinach salad with organic produce. (Photo: Lesya Dolyuk/Shutterstock)

These are the fruits and vegetables EWG suggests buying organically to eliminate the highest risk of pesticide exposure:

Strawberries Spinach Kale Nectarines Apples Grapes Peaches Cherries Pears Tomatoes Celery Potatoes

It's worth noting that kale and spinach samples had, on average, 1.1 to 1.8 times as much pesticide residue by weight than any other crop.

The Clean Fifteen

Conventionally grown papaya is likely to have a lower pesticide residue, according to EWG's commissioned tests. (Photo: Amallia Eka/Shutterstock)

Purchasing only organic produce is cost-prohibitive for most people, so it helps to know where it can do the most good. These 15 foods were found to have the least amount of residual pesticides on them. If you aren't going to buy all organic produce, these are the conventionally grown fruits and vegetables that are your safest bets.

Avocados Sweet corn Pineapples Frozen sweet peas Onions Papayas Eggplants Asparagus Kiwis Cabbages Cauliflower Cantaloupe Broccoli Mushrooms Honeydew melon

In this year's testing, more than 70 percent of Clean Fifteen fruit and vegetable samples had no pesticide residues.