If strawberries, spinach or kale is on your shopping list, you might want to opt for the organic versions the next time you are at the grocery store. The three items were found to have the highest levels of pesticide residue, according to an annual analysis from the Environmental Working Group (EWG). 

The annual EWG list ranks popular fruits and vegetables based on levels of contamination. The resulting lists – the "dirty dozen" and "clean 15" – outline the produce found to have the highest and lowest levels of pesticide contamination in the United States. 

Strawberries topped this year's dirty dozen list for the fourth year in a row. Kale – a nutrient-rich cabbage – was a surprising newcomer at number three on the list. Ninety-two percent of the kale samples had two or more pesticide residues. Nectarines, apples, and peaches were also on the "dirty" list. 

At the top of the "clean 15" list are avocado, sweet corn and pineapple. Popular items such as onions, mushrooms and cabbage were also among the cleanest produce items. "More than 70 percent of 'clean 15' fruit and vegetable samples had no pesticide residues," said the report. 

Pesticides are commonly used to control weeds, insects or unwanted vegetation in agriculture. People are exposed to pesticides through the food they eat because of their broad use. 

The World Health organization says pesticides can be hazardous and have toxic effects. However, scientists still do not have a clear understanding of the health effects of pesticide residues, writes the National Institute of Environmental Health Science. 

Rankings are based on an analysis of more than 40,000 samples taken by the United States Department of Agriculture (USDA) and the U.S. Food and Drug Administration (FDA). 

EWA says the report is intended to help shoppers decide when to buy organic versus conventional.

The report cites a French study published in JAMA Internal Medicine last year that found that among almost 69,000 participants, those who ate organic more often had 25 percent fewer cancers than those who didn't eat organic food.

Because the items on the clean list have low pesticide residue, shoppers can buy standard items, while organic might be a better option for produce on the dirty dozen list. Recognizing this isn't always an option for people, the report acknowledges that eating any fresh fruit or vegetable  – organic or not – is beneficial to one's health.