(CNN) -- Should baby food makers be required to post lead warnings on their products in California?
That's the question at stake in a civil trial that kicked off Monday in Alameda County Superior Court in Oakland.
Some of the country's biggest baby food makers are defendants in the case, including Beech-Nut Nutrition, Del Monte Foods, Dole and Gerber.
In a 2011 lawsuit, the Environmental Law Foundation alleged that many of those companies' foods and juices contain lead and that under California law, they are required to warn consumers.
The suit targets products marketed and intended for babies and toddlers, including grape juice, packaged peaches and pears, carrots and sweet potatoes.
It seeks to require the companies to put warning labels on their products, and face penalties of up to $2,500 per violation per day.
"Our hope is that rather than put the labels on, these companies will get off their duffs and do what all their competitors are doing and just get the lead out," said Jim Wheaton, president of the Environmental Law Foundation, a California-based environmental group.
"We banned lead in paint, we banned it in gasoline. What is it doing in baby food?"
According to the Centers for Disease Control and Prevention, some 500,000 U.S. children ages 1 to 5 have unsafe levels of lead in their blood. Exposure to lead can affect virtually every system in the body and cause intellectual and behavioral deficits.
Representatives for Del Monte and Gerber declined to discuss the case Monday, both saying they do not comment on pending litigation. Messages left with Dole and Beech-Nut were not immediately returned.
Attorneys for the defendants have previously argued they will show no warnings are required.
"Despite the trace amounts of lead in the products at issue, the federal government has determined that Americans need to eat more -- not less -- of these nutritious foods," they said in court documents.
The Grocery Manufacturers Association, a trade group, released a statement stressing that Food and Drug Administration tests on products in the lawsuit found they do not pose any health concerns.
"Many minerals, including lead, are found naturally in soil and water throughout the world. As a result, virtually all foods grown in nature, including fruit -- whether fresh or packaged -- contain trace levels of such naturally occurring minerals," it said.
"Recognizing this, the FDA regularly tests a wide variety of foods, including fruit juice and packaged fruit products, for lead and other minerals. The FDA has repeatedly concluded that these categories of products do not contain lead in amounts that would pose an unacceptable health risk to adults or children."
The trial, which is expected to last three to four weeks, has no jury. It's being heard by Judge Steven Brick.
CNN's Amanda Watts contributed to this report.