If you just came home from the grocery store with a fresh package of spinach or kale, your first thought might be to start making a salad. But there is one important thing you should do first.

While the fruits and vegetables can be some of the healthiest the foods you eat, some have higher cases PESTICIDES than others. Might be worth a wash before slicing or peeling and eating. Trace amounts of pesticides will not usually cause major health problems, but consumed in larger amounts can.

Before you get too worked up about pesticides in non-organic produce, consider that US Department of Agriculture Pesticide Data Program (PDF) found that more than 99 percent of the foods sampled measured residue levels that met the safety standards set by the Environmental Protection Agency, with 27 percent having no detectable pesticide residue at all.

In short: some leftovers are okay, and most products found in grocery stores can be safely eaten without washing. But if you’re concerned about getting rid of pesticides or other chemicals that your food may have been exposed to, you might take a better approach than being sorry and washing your produce before you eat.

To help sort the dirtiest produce from the not-so-bad, the Environmental Working Group, a non-profit food safety organization, publishes a list of produce most likely to contain pesticides. It’s called “The Dirty Dozen” and it’s a cheat sheet of which fruits and vegetables you should always wash.

The group analyzed 46,569 samples of 46 fruits and vegetables tested by the US Food and Drug Administration and the Department of Agriculture.

The No. 1 pesticide offender in the group’s latest study? Strawberries. Popular fruits had more total instances of chemicals found on them than any other fruit or vegetable included in the overall analysis.

Below you’ll find the 12 foods most likely to contain pesticides — and the 15 least likely to be contaminated.

The dirty dozen: fruits and vegetables to always wash

Foods most likely to contain pesticides, according to FDA and USDA data.

1. Strawberries
2. Spinach
3. Cabbage, cabbage and mustard greens
4. Peaches
5. pears
6. NECTARINES
7. The apples
8. The grapes
9. Bell peppers and hot peppers
10. Cherries
11. Blueberries
12. Green beans

The Dirty Dozen is a good indicator designed to alert consumers to the fruits and vegetables that need the most thorough washing. Even a quick rinse with water or a splash of washing products help.

You can also avoid much of the potential risk by buying certified organic fruits and vegetables that do not use agricultural pesticides. Knowing which foods are more likely to contain pesticides can help you decide where to spend that extra cash on organic produce. As I learned in a organic and non-organic price analysisthey are not as expensive as you might think.

More takeaways from the Dirty Dozen study

• Over 90% of samples of strawberries, apples, cherries, spinach, nectarines and grapes tested positive for residues of two or more pesticides.
• A total of 210 different pesticides were found on Dirty Dozen items.
• Of the 210, more than 50 different pesticides were detected on every crop type on the list except cherries.
• Cabbage, collards and mustard, as well as hot peppers and bell peppers, had the most pesticides detected of any crop — 103 and 101 pesticides in total, respectively.

Instead, EWG found these 15 fruits and vegetables East likely to contain pesticides.

The Clean 15: Fruits and vegetables you can skip washing

These are the fruits and vegetables least likely to contain pesticides, according to the study:

1. Avocado
2. Sweet corn
3. Pineapple
4. Onion
5. Papaya
6. Sweet peas (frozen)
7. Asparagus
8. Cantaloupe
9. Kiwi
10. Cabbage
11. Mushrooms
12. Mango
13. Sweet potatoes
14. Watermelon
15. Carrots

Clean 15 was found to have the lowest levels of pesticide contamination of all the samples tested, but that doesn’t mean they aren’t contaminated with pesticides at all. Of course, that doesn’t mean the fruits and vegetables you bring home aren’t contaminated with pesticides either. You’d be safer eating unwashed foods from the Clean 15 than the Dirty Dozen, but it’s still a good rule of thumb to rinse all your fruits and vegetables before eating them.

The EWG methodology involves six measures of pesticide contamination. The analysis focuses on fruits and vegetables that are most likely to contain one or more pesticides, but does not measure how much of a pesticide is on a given piece of produce. You can read more about EWG’s Dirty Dozen in the published study Here.

Frequently asked questions about washing fruits and vegetables