California’s Proposition 65, officially called the Safe Drinking Water and Toxic Enforcement Act of 1986, requires businesses to warn customers if they could be exposed to any of the more than 900 confirmed or suspected carcinogens.

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LOS ANGELES — The California court ruling that a cancer-warning label should be required on coffee has left the scientific community puzzled.

There is plenty of research showing coffee doesn’t cause cancer, and can prevent liver and endometrial cancer. The World Health Organization announced two years ago that there was “no conclusive evidence for a carcinogenic effect of drinking coffee.”

The California decision has put public-health experts at odds with a state law aimed at safeguarding the health of Californians.

“I can understand the logic of the judge, by going by the book. But I can also understand the science,” said Mariana Carla Stern, a University of Southern California professor who studies diet and cancer. “From the science standpoint, there’s no reason the public should worry about drinking coffee.”

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California’s Proposition 65, officially known as the Safe Drinking Water and Toxic Enforcement Act of 1986, requires that businesses warn customers if they could be exposed to any of the more than 900 confirmed or suspected carcinogens. One of compounds on the list is acrylamide, which is found in roasted coffee beans and French fries, burnt toast, crackers, cookies, cereal and other high-carbohydrate foods as a byproduct of roasting, baking, toasting or frying.

Some studies in animals have found that exposure to high levels of acrylamide causes cancer, but there’s little evidence of that in humans, said Kathryn Wilson, a Harvard senior research scientist who studies links between diet and cancer. Researchers also warn that it’s unwise to extrapolate acrylamide studies in animals to humans because the species metabolize the compound differently.

“I think it’s crazy,” Wilson said of the court’s decision. “Reducing coffee or French fries to their acrylamide content isn’t how we study diet and nutrition.”

“At the minimum, coffee is neutral. If anything, there is fairly good evidence of the benefit of coffee on cancer,” said Dr. Edward Giovannucci, a nutrition expert at the Harvard School of Public Health.

Coffee companies, led by Starbucks, acknowledge the presence of the chemical, but said it’s found in trace levels that are harmless. They argue any risks are outweighed by other health benefits from drinking the beverage.

Anyone living or visiting California may have noticed signs warning about cancer risks at gas stations, hardware stores, cafes and even Disneyland. The law was passed by voters more than 30 years ago, and the signs are so ubiquitous and often vaguely worded that most people pay little heed. For example, parking garages caution: “This area contains chemicals known to the state of California to cause cancer, birth defects and other reproductive harm.”

What about positives?

Many scientists pointed out that Proposition 65 doesn’t account for the positive benefits of coffee. “This is an unfortunate ruling that demonizes coffee as a carcinogen when the overwhelming evidence in humans is for benefit or at least no detrimental effect,” Dr. Nigel Brockton, director of research at the American Institute of Cancer Research, said in a statement.

However, Nina Fujii, who was picking up a cup of coffee at Starbucks in the Los Feliz neighborhood on Friday, said a label would make her rethink her coffee consumption.

Fujii, 24, an actress, said she remembered seeing a cancer warning at an El Pollo Loco two years ago. The sign is part of the reason she stopped going there, she said.

More than 90 coffee roasters, retailers and distributors, including Starbucks, Whole Foods, Kraft and Green Mountain Coffee Roasters, stand to be affected by the decision.

William Murray, president of the National Coffee Association, said the industry is considering legal action. The decision “does nothing to improve public health,” he said in a statement.

The Council for Education and Research on Toxics sued ready-to-drink coffee companies in 2010 for not providing cancer-hazard warnings due to the acrylamide content.

In a tentative decision, Los Angeles County Superior Court Judge Elihu M. Berle said the coffee companies failed to prove there was a safe level of acrylamide in their products. He wrote that while the plaintiff showed that coffee can harm people, “defendants’ medical and epidemiology experts testified that they had no opinion on causation.”

Berle will issue a final decision after giving each side an opportunity to object. The next phase of the trial will determine the civil penalties to be levied on the defendants.

The law allows for as little as a cent and up to $2,500 for each time a consumer was exposed to the chemical without being warned, said Raphael Metzger, the plaintiff’s attorney.

The law applies only to California, but the state is such a massive market that tailoring packaging with warnings specifically to stores in the state could be a tall order, experts said.

Potential for confusion

Public-health experts said they worried the ruling would confuse the public, who already often feel that nutrition science is constantly shifting.

Wilson, the Harvard researcher, said she feared it would detract attention from things that are clearly linked to cancer, such as obesity and lack of exercise.

“It’s too bad for public health,” she said.

Dr. Frank Hu, chair of the nutrition department at the Harvard T.H. Chan School of Public Health, pointed out that coffee has been found to reduce the risk of diabetes and heart disease. The latest American dietary guidelines say the beverage is OK to drink (up to five cups a day). He said the ruling seemed meaningless given the “minuscule amount” of acrylamide in coffee.

“If the concentration level is so low, then what’s the meaning of labeling those foods?” he said.

Will Kiker, 37, said a warning wouldn’t keep him from his daily venti coffee with an extra shot.

He doesn’t eat processed food or high-fructose corn syrup, but he smokes cigarettes and walks into friends’ apartment buildings even if they have posted signs about carcinogens. He said he could understand a need for labels on a new product, but he found it pointless for coffee.

“You could put a warning label on L.A. You’re breathing in toxic fumes if you put your windows down on the freeway,” said Kiker, a music manager. “It’s a little ridiculous to put a warning on coffee.”