The Center for Science in the Public Interest (CSPI), a health watchdog group, is causing an uproar with its claim that several brands of soda contain sufficiently high levels of a carcinogen — five times the amount recommended by the state of California — to be dangerous. In a letter to the Food and Drug Administration (FDA), CSPI says industrial caramel coloring — which gives Coke, Pepsi, and similar sodas their distinctive brownish hue — is made with a chemical that has been shown to cause various types of cancer in mice and rats. The American Beverage Association says CSPI's claims are "outrageous," and nothing more than "scare tactics." Here, a guide to the controversy:

What does the watchdog claim?
The caramel coloring used in sodas contains the chemical 4-methylimidazole, or 4-MI, a known carcinogen. "Coke and Pepsi, with the acquiescence of the FDA, are needlessly exposing millions of Americans to a chemical that causes cancer," says Michael Jacobson, CSPI's head. "If companies can make brown food coloring that is carcinogen-free, the industry should use that." It's currently illegal to sell products that contain enough 4-MI to increase the risk of cancer by more than 1 person per 100,000 — and CSPI claims that the level of 4-MI in Coke and other sodas presents a risk equivalent to 4.8 people in 100,000.

What does the government say?
The FDA says it's investigating CSPI's claims, but appears to be siding with the soda makers. "A consumer would have to consume well over a thousand cans of soda a day to reach the doses administered in the studies that have shown links to cancers in rodents," says Doug Karas, an FDA spokesman.

What does Coca Cola say?
The company says it has begun modifying its manufacturing process so that the levels of 4-MI in its formula will comply with California law. However, Coke points out that "outside of California, no regulatory agency concerned with protecting the public's health has stated that 4-MI is a human carcinogen."

So Coke fans are safe?
Not exactly. Even CSPI acknowledges that the main health threat posed by sodas is their high levels of fructose corn syrup, which has been linked to obesity, diabetes, and other serious health problems.

Sources: The Huffington PostThe Los Angeles Times, NPR, ReutersUSA Today