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Washington — Americans throw away 40 percent of the food they buy, often because of misleading expiration dates that have nothing to do with safety, said a study released Wednesday by Harvard University Law School and the Natural Resources Defense Council, an environmental group.

“The date labeling system is not a system at all,” said NRDC staff scientist Dana Gunders, co-author of the report, the first to assess date labeling laws nationwide.

The report said 90 percent of Americans toss good food into the garbage because they mistakenly think that “sell by,” “best before,” “use by” or “packed on” dates on food containers indicate safety. One-fifth of consumers, the report said, “always” throw away food based on package dates.

In fact, “sell by” dates are used by retailers for inventory control. “Best before” or “use by” dates usually reflect manufacturer estimates of peak quality. While some labels are intended to indicate freshness, none of them reflects edibility or safety, said Ted Labuza, a food science professor at the University of Minnesota who collaborated with the authors.

“If food looks rotten and smells bad, throw it away, but just because it reaches a certain date does not mean the food is unsafe,” Labuza said. “I don’t know of any food poisoning outbreak that came from people eating food that was past its shelf-life date.”

The report estimated the value of food tossed away at $165 billion a year.