(Bloomberg News) The U.S. Federal Trade Commission proposed new "green" advertising guidelines that warn against making unsupported or misleading environmental claims.

The proposed changes to the agency's Green Guides, which were issued in 1992 and last updated in 1998, add advice on claims involving renewable energy and materials, carbon- pollution offsets and carbon-neutral products, according to a statement issued by the FTC today.

Advertisers should avoid making blanket claims that a product is "environmentally friendly" or "eco-friendly" because such assertions are almost impossible to verify, the FTC said. If a product is labeled "degradable," it should decompose in no more than one year, according to the new guidelines.

"What companies think green claims mean and what consumers really understand are sometimes two different things," said FTC Chairman Jon Leibowitz. "The proposed updates to the Green Guides will help businesses better align their product claims with consumer expectations."

Products shouldn't be described as made with renewable energy if any part of the product was derived from fossil fuels, the guidelines said. When renewable-energy claims are made, the energy source should be specified.

Carbon-offset claims, involving supporting projects that reduce greenhouse gases to counterbalance emissions elsewhere, should state whether the emission reductions will take longer than two years to achieve, the FTC said. Marketers shouldn't advertise offset activities if they are already required by law, the guidelines said.

The FTC is seeking public comment on the new guidelines by Dec. 10. Responses can be mailed to the FTC or registered online at: https://ftcpublic.commentworks.com/ftc/revisedgreenguides.