There's new pressure on the Food and Drug Administration to take action on the synthetic food coloring, red No. 3, after California passed a law to ban it last week.

California became the first state to ban four food additives, including red No. 3, and public health advocates are pushing to remove the dye from the food supply nationwide. "I think the passage of the bill in California creates undeniable pressure on the FDA," says Dr. Peter Lurie, president and executive director of the Center for Science in the Public Interest.

His group, joined by other consumer advocacy groups including Public Interest Research Group and Consumer Federation of America, filed a petition last October with the FDA to ban the use of red No. 3. Lurie says he thinks the action in California will "make it more likely that they grant our petition."

Back in 1990, the FDA halted the use of the red No. 3 in cosmetics and externally applied drugs, such as medicated ointments or lotions, based on a study that it can cause cancer in rats. But the FDA continued to allow the dye's use in foods and has become ubiquitous in the U.S. food supply. Since then, multiple studies have linked consumption of synthetic dyes to behavioral issues in children, including hyperactivity.

"For 33 years now, we have been waiting for the FDA to take common sense action that would remove [red No. 3] from the market and thereby better protect American consumers," says Lurie.

Synthetic food dyes, including red No. 3, give bright, flashy coloring to foods, making them more appealing to the eye.

"The vibrant colors are important to our industry," says Christopher Gindlesperger, of the National Confectioners Association, a trade group that includes companies that make candy. Red No. 3 is also found in many other foods and beverages, such as bright-colored sodas, juices, yogurts, snacks and frozen desserts. He says his industry does not use any ingredients that do not comply with FDA's safety standards.

But Gindlesperger says the California ban, slated to take effect in 2027, would create a patchwork of state requirements that could increase food costs and create confusion amongst consumers, including parents.

He says the FDA should use its regulatory authority to settle the issue of whether red 3 can continue to be used in food. In a letter to the FDA, the organization said that California was "out of its depths when it comes to national food safety standards."

"It's FDA's call to make," Gindlesperger says. "It's time for the FDA to lean into the discussion and, have a solid review, [and] evaluate all the available science," he says, so the agency can provide the guidance that food companies need.

When California's Environmental Protection Agency reviewed the body of evidence on synthetic dyes, they found evidence the dyes consumed in food can negatively impact children's behavior. Out of about 25 studies, more than half identified a positive association between artificial food coloring intake and behavioral outcomes.

They also reviewed dietary survey data and found a higher intake of synthetic dyes in lower income communities. "We also found Black Americans tended to have higher intake," says Asa Bradman, a public health scientist at the University of California, Merced, who helped with the state's analysis.

"I tend to err on the side of precaution," Bradman says. "I think there is good reason to remove [red No. 3] from the food supply."

In one double-blinded study, children, aged 3 to 9 years old, consumed a drink that contained synthetic dyes or a placebo drink that was dye-free. The researchers found artificial colors in the diet resulted in increased hyper-activity.

"I think the evidence is compelling from those human studies that children's consumption of synthetic food dyes can contribute to increases in symptoms like inattention, hyperactivity in some children," says Mark Miller, a scientist with California's EPA's Office of Environmental Health Hazard Assessment.

Miller has also published a review of animal studies, which indicate synthetic food dyes can affect memory and learning. He says the state's review suggests it's time for the FDA to re-evaluate synthetic dyes, based on the newer evidence.

An FDA spokesperson told NPR the agency is "actively reviewing" the petition from the consumer advocacy groups to halt the use of red No. 3 in foods. The agency will assess whether there's "sufficient data" to revoke its use, according to the spokesperson.

In regard to a potential cancer risk identified back in 1990, the agency argues this risk is specific to rats. "These data have been carefully studied and widely understood among the scientific community to not be relevant to human cancer," the spokesperson wrote in an email to NPR.

Scientists with the Center for Science in the Public Interest disagree. "By our evaluation, the mechanism by which Red 3 causes cancer has still not been clearly established," wrote Thomas Galligan, a scientist who focuses on food additives for the group. He says establishing a mechanism would require long-term studies, which have not been conducted, to his knowledge.

He points out that red dye 3 is already severely restricted in the European Union, among other countries.

The FDA spokesperson also says a state-initiated ban on food ingredients, referring to the new California law, could potentially disrupt the food supply. "The U.S. depends on a unified food system," the spokesperson says, adding that "the science-based FDA approach to oversight of the food system is the best way to ensure safety."

Though synthetic food dyes in general have been linked to behavioral issues in kids, red dye 3 has been singled out for a ban due to the evidence on cancer. "It's very low-hanging fruit that FDA really should have picked decades ago," Galligan says, but the evidence that synthetic colors can influence children's behavior extends to a range of colors. His group has been urging the FDA to ban eight synthetic food dyes since 2008, including Yellow 5, Red 40, Blue 1, Blue 2, Green 3, Red 3, Yellow 6, and a rarely used orange hue.

Meanwhile, California EPA's Miller says it may be difficult to conduct more trials on the effects of synthetic food dye on humans, since there's already a body of evidence pointing to a link. Better evidence could come from a controlled study, where kids are dosed with dye, but "it might be considered unethical to perform those tests," Miller says.

"That's really important because, the data that we have now may be the all the data we're really going to have in human children," he adds.

The American Academy of Pediatrics has long advised parents to limit sugary drinks, juices and candy, the types of foods that often contain synthetic dyes, and the group concluded these dyes may have effects on children's behavior and attention.

Some candy manufacturers have already removed red No. 3 from their products. For instance, M&M's and Skittles no longer contain the dye. But it can still be found in many other products, including certain snacks, cake and cookie icing and decorations and some Halloween treats.

Edited by Carmel Wroth. The radio version of this story was edited by Diane Webber.