By Johanzynn Gatewood

(CNN) -- An annual report by the Environmental Working Group found that nearly 70% of samples of 48 types of conventionally grown produce were contaminated with pesticide residues. That's down 6.6 percentage points from last year.

The EWG Shopper's Guide to Pesticides in Produce, released Wednesday, ranks pesticide contamination of popular fruits and vegetables based on more than 36,000 samples of produce tested by the US Department of Agriculture and the Food and Drug Administration.

This year, strawberries remained at the top of the list of produce with the highest concentration of pesticides, while sweet corn and avocados were ranked as having the lowest concentration.

What are pesticides?

Pesticides are widely used in producing food to control pests such as insects, rodents, weeds, bacteria, mold and fungus. In addition to their uses in agriculture, pesticides are used to protect public health by controlling organisms that carry tropical diseases, such as mosquitoes.

Pesticides are potentially toxic to humans, according to the World Health Organization. They may have negative effects on reproduction, immune or nervous systems, cause cancer and lead to other problems.

Pesticide residue can remain on fruits and vegetables even after they are washed and, in some cases, peeled, according to the report.

However, a report by the USDA in 2014 found that "overall pesticide chemical residues on foods tested were at levels below the tolerances established by the Environmental Protection Agency" and were not a safety concern to consumers.

The Dirty Dozen

Produce that tested positive for various pesticides and contained higher concentrations of pesticides than other produce is featured on the list, known as the "Dirty Dozen."

Starting with the highest amounts of pesticide residue, the list features strawberries, spinach, nectarines, apples, peaches, celery, grapes, pears, cherries, tomatoes, sweet bell peppers and potatoes.

Strawberries remained at the top of the list with at least 20 pesticides, while spinach jumped into the second spot with twice as much pesticide residue by weight than any other crop.

Americans eat nearly 8 pounds of fresh strawberries per person each year, and even when they are rinsed in the field and washed before eating, they are still most likely to be contaminated with pesticide residue, according to the Environmental Working Group.

In 2016, spinach was ranked eighth, but the latest numbers from the USDA showed a sharp increase in pesticide residues on non-organic spinach since the crop was last tested eight years ago.

The pesticides responsible for the residues included three fungicides and one insecticide called permethrin, which has been linked to tremors and seizures in the nervous systems of animals and insects.

The newest additions to the list were pears and potatoes, which replaced cherry tomatoes and cucumbers from last year.

The Clean Fifteen

Produce that had relatively fewer pesticides and lower total concentrations of pesticide residues was placed on the group's "Clean Fifteen" list.

This list included, in order, sweet corn (including corn on the cob and frozen corn), avocados, pineapples, cabbage, onions, frozen sweet peas, papaya, asparagus, mangoes, eggplant, honeydew melon, kiwis, cantaloupe, cauliflower and grapefruit.

Only 1% of samples showed any detectable pesticides in avocados and sweet corn, which were deemed the cleanest produce.

More than 80% of pineapples, papaya, asparagus, onions and cabbage that were sampled showed no pesticide residue.

Methodology

The Environmental Working Group, a nonprofit advocacy group, analyzed more than 36,000 samples taken by personnel at the USDA and the FDA who mimicked consumer practices by first washing or peeling the produce.

To compare the fruits and vegetables, the group came up with a composite score for each type of produce based on six measures of contamination. Some of the measures include the percent of the sample tested with detectable pesticides and the average number of pesticides found on a single sample.

Shopping smart

Nutrition experts support the findings and even use the list to make recommendations to their own patients.

"I believe that this is an important source of information," said Corinne Bush, a clinical nutritionist who was not part of the research.

Bush warns that some pesticides that do not exceed thresholds established by the EPA can stil lbe very harmful, since low-level exposure over time can have extremely damaging effects.

The Environmental Working Group recommends buying organic produce whenever possible to reduce exposure to pesticides.

"If you don't want to feed your family food contaminated with pesticides, the EWG Shopper's Guide helps you make smart choices, whether you're buying conventional or organic produce," Sonya Lunder, a senior analyst with the group, said in a news release.

"Eating plenty of fruits and vegetables is essential no matter how they're grown, but for the items with the heaviest pesticide loads, we urge shoppers to buy organic. If you can't buy organic, the Shopper's Guide will steer you to conventionally grown produce that is the lowest in pesticides."