Caramel color is an additive that gives your bubbly soda that distinct amber hue.
But some of the artificial coloring also contains a potentially carcinogenic chemical called 4-methylimidazole or 4-MeI.
Although there are no federal limits on its use, California added the chemical to its Proposition 65 list of carcinogens in 2011. That required any food or beverage to carry a health warning label if it exposed consumers to more than 29 micrograms of 4-MeI a day.
A study released Thursday by Consumer Reports magazine says at least two soft drinks come close to breaking that rule: Pepsi One and Malta Goya.
Twelve ounce cans of Pepsi One, a low-calorie soda, contained as much as 43.5 micrograms of 4-MeI in California tests. Malta Goya, a nonalcoholic malt beverage, reached 352.5 micrograms in California.
By comparison, samples of Coca-Cola were found to contain 4.3 micrograms and Dr. Pepper, 10.1.
"We are concerned about both the levels of 4-MeI we found in many of the soft drinks tested and the variations observed among brands, especially given the widespread consumption of these types of beverages," said Urvashi Rangan, a toxicologist and executive director of the Consumer Reports Food Safety and Sustainability Center. "There is no reason why consumers need to be exposed to this avoidable and unnecessary risk that can stem from coloring food and beverages brown."
Consumer Reports said it tested 81 cans and bottles of popular sodas from April to September in California and New York. An additional 29 samples were tested in December.
Coca-Cola Co. and PepsiCo Inc. both pledged in 2012 to reduce the amount of 4-MeI in their drinks nationwide after the California rule went into effect.
In a statement, Pepsi challenged Consumer Reports' findings, saying all its products were below the threshold set in California.
[Updated 6:45 a.m.]
The soda giant said the levels of 4-MeI in its drinks did not amount to 29 micrograms per day because "the average amount of diet soda consumed by those who drink it is approximately 100 [milliliters] per day, or less than a third of a 12 [ounce] can."
Consumer Reports believes regular users of diet sodas drink more than just one-third a can a day.
Still, Pepsi said it was on target to reduce levels of 4-MeI by February nationwide.
"We are extremely concerned about Consumer Reports' allegation that one of our products exceeds the Prop. 65 standard and requires a warning label," the company's statement said. " We believe their conclusion is factually incorrect and reflects a serious misunderstanding of Prop. 65's requirements."
Goya Foods Inc. did not respond to a request for comment.
Consumer Reports asked the California attorney general Tuesday to investigate whether Pepsi One and Malta Goya should have warning labels.
The Food and Drug Administration said Thursday it was conducting new studies over the safety of caramel coloring, according to the Associated Press.
[Updated 6:45 a.m.]