SAN FRANCISCO -- An expert witness testified Monday that average lead levels in baby foods fall below the threshold for warning labels mandated under a California reporting standard.

Barbara Petersen, a food safety scientist, was the first witness in the civil trial involving a lawsuit filed by an environmental group against the nation's largest baby food makers aimed at forcing the companies to alert consumers that some products contain low amounts of lead.

The trial will help determine if the products must carry warning labels in California.

Gerber Products Co., Del Monte Foods, Beech-Nut Nutrition Corp. and many other makers of baby foods and juices are selling products containing lead at levels that require warning labels under California Proposition 65, the Environmental Law Foundation asserts in the suit filed in Alameda County Superior Court in Oakland.

The plaintiffs countered that Petersen's analysis was misleading because it averaged the lead level in the entirety of the companies' products -- rather than looking at each individually.

"Under their argument you could consume the entire (legal limit) of lead in one day, so as long as you didn't eat any more of it over the rest of the month," Jim Wheaton, an attorney for the foundation, said after the proceedings ended for the day.

There is no jury in the trial. Instead, Judge Steven Brick will hear testimony from about a dozen lead and toxicology experts before issuing his ruling.

Lawyers for the food companies say the U.S. Food and Drug Administration tested products targeted in the lawsuit and decided levels were below the federal standards that require a warning.

FDA tests on products named in the lawsuit also found lead levels "below FDA's current tolerable intake levels for lead."

But both sides in the case agree on one fact: Baby foods containing carrots, peaches, pears and sweet potatoes have some lead. Also covered by the suit are grape juice and fruit cocktail.

Lead exposure can damage a child's developing brain and lead to a lower IQ. Overall, lead poisoning in the U.S. has declined significantly since it was removed from paints and gasoline formulas.

Still, more than 500,000 U.S. children are believed to have lead poisoning, according to the Centers for Disease Control and Prevention.

Old paint, contaminated drinking water and soil tainted by old leaded gasoline are primary sources of lead exposure for children in the U.S., the CDC reported. Some specialists describe children as having lead poisoning only when high levels of lead are present. Others use the term more broadly to describe any child with levels that can impact intelligence or cause other harm.

The companies also argue that the lead in fruit and vegetables used in the products is naturally occurring. If the trial judge finds that is true, it could exempt the companies from having to warn consumers.

"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," the companies' attorney, Michele Corash, wrote in court documents. "FDA recently reiterated its conclusion that the trace levels of lead in the products at issue in this case do not pose unacceptable health risks."

Corash would not comment on the first day's court testimony.

Christine Kuhinka, director of corporate communications for Gerber Products Co., which is owned by Nestle Nutrition U.S., said the company does not comment on pending litigation.

"We will, however, vigorously defend the allegations in this litigation," Kuhinka said in an email.

Wheaton said California law nonetheless requires food makers to warn consumers about a possible risk of certain toxins, including lead. He noted that Proposition 65 requires warning labels on foods that contain a toxin at 1/1000th of the levels considered dangerous to human health.

The plaintiffs also argue that no level of lead is considered safe, especially for newborns and pregnant women.

"Even when studying the low level of exposure that is typical in the dietary context, scientists have not been able to identify a level of exposure that is without any health risks," the plaintiffs wrote in court documents.

They hope public pressure or the judge will compel the companies to take steps to remove lead from their foods because a warning label would be devastating for business.

"Everyone assumes that a company selling foods for children will never offer a product for sale that carries a warning label," Wheaton said. "They will take the steps their competitors apparently already are taking to offer product with no lead or so low no warning is required