The old saying that an apple a day keeps the doctor away may need an asterisk, in the wake of a new report says some of the most popular produce contains the highest levels of pesticides.

The seventh annual Shopper's Guide to Pesticides in Produce report from the Environmental Working Group lists what it calls the "Dirty Dozen," the 12 fruits and vegetables found to be most contaminated by pesticides and fungicides.

According to researchers at Purdue University in Indiana, apples now top the list as the most contaminated by pesticides, even after they are peeled and washed. Apples jumped three spots from last year, bumping celery to number two. Strawberries round out the top of the most contaminated list, with grapes, blueberries and lettuce not far behind.

In contrast, onions top the report's "Clean 15," or least-contaminated, list, followed at number two by corn, pineapple at number three, asparagus at number five and watermelon at number 12.

According to the group, eating five servings a day of the most contaminated fruits and vegetables is akin to ingesting 14 different pesticides per day. While the chemicals keep both bugs and bacteria at bay from crops, they are also linked to various health problems, from nervous system disorders to cancer.

The agency said the goal of the report is to help consumers make smarter choices when they are cruising the produce aisle.

"Pesticides are toxic," Sonya Lunder, a senior analyst at EWG, said in a statement. "They are designed to kill things and most are not good for you. The question is, how bad are they?"

The researchers came to their conclusions after analyzing data from the U.S. Department of Agriculture and the Food and Drug Administration from 2000 to 2009. Each produce item received a score based on how many pesticides were found in the testing and at what levels. Most fruit and vegetable samples were washed and peeled, if necessary, before testing, and so chemical readings closely reflect the levels present when the produce is consumed, the report said.

Apples gained the top spot this year after pesticides were found on 98 per cent of the more than 700 samples tested.

While the researchers recommend that consumers get around the problem by choosing organic, one food sciences professor cautions that a natural or organic label on produce doesn't automatically make it safe.

"The (E. coli) outbreak in Germany clearly shows locally grown organic produce was able to kill 35 people," Rick Holley of the University of Manitoba told CTV News.

The Canadian government monitors fruits and vegetables for pesticide levels and what makes it to store shelves is considered safe to consume.

With a report from CTV's Jill Macyshon