The U.S. Food and Drug Administration is investigating claims that many soft drinks continue to contain benzene, a known carcinogen, some 15 years after the soft drink industry promised to police itself on this matter.

Benzene is formed when two common soft drink ingredients—sodium benzoate and ascorbic acid (vitamin C)—are used together. Sodium benzoate and vitamin C are common preservatives; the latter is also used for its antioxidant properties and to give products a tart taste. Natural and organic soft drink brands, such as Izze, Hansen's and Santa Cruz Organic, do not contain sodium benzoate. Energy drinks, which have greatly expanded their presence in recent years, often do.

In late 1990, Koala Springs and Cadbury Schweppes alerted the FDA to the presence of benzene in their products, but the FDA did not notify the public. "The Food and Drug Administration and the Health Protection Board of Canada agree that [the] low [parts per billion] level of benzene found in these products do not constitute an imminent health hazard," according to minutes from the meeting.

In a January 1991 meeting with the National Soft Drinks Association (now known as the American Beverage Association), the FDA agreed to allow the industry to resolve the problem. According to a memo, the NSDA had conducted its own investigation and discovered certain processes and ingredients that encourage or inhibit benzene formation. For example, "sweeteners like high fructose corn syrup influence benzene formation in the presence of benzoic acid," the memo said.

"At that time there was an effort to reformulate many of these beverages," said an FDA spokesman. "We did some testing to see that benzene levels had dropped to an acceptable level."

The FDA spokesman said the agency "recently became aware that this had shown up again in some beverages." According to other published reports, a former chemist for Cadbury Schweppes commissioned new tests, and found that some products had benzene levels up to five times that permitted by the World Health Organization for drinking water.

"FDA is in the process now of sampling some products … to determine if there is benzene present," the spokesman said, noting that "a couple samples" had "levels of benzene a little bit higher than we'd like … but that was in a small percent of samples."

The FDA doesn't believe there's any nefarious intent among soft drinks manufacturers. Because the problems with benzene formation were published in open scientific literature, food safety specialists would've been aware of the issues. "It is [the companies'] responsibility to ensure their product is safe and properly labeled," the FDA spokesman said. "What we're seeing now is maybe some companies who were unaware of this before or forgot there was a reason we didn't formulate a certain way and have drifted back. It could be a new cast of people who just weren't in the industry before."

Meanwhile, the FDA will complete its sampling program. "We're really trying to gather some basic information. If we see a need to take action we'll consider the appropriate action once we have the data."