Ten US states have called on the Federal Trade Commission (FTC) to tighten up the rules governing the sale of carbon offset credits, citing concerns over the transparency and regulation of the credits.

The letter, sent from Californian Attorney General Edmund G Brown's office, was a response to a call for comment from the FTC, which had solicited opinion on the topic during its 2007 Carbon Offset and Renewable Energy Workshop.

The states called for clarity in several areas, including a clear definition of 'additionality' or how much new carbon lowering activity an offset credit should support, and a standard way to measure baseline carbon emissions and reductions from offset projects.

The letter also voiced concerns that carbon offsets could be sold more than once, and also called for a decision on whether renewable energy credits – certificates for energy produced using renewable resources – could be counted as carbon offset credits. "Some regard offsets as limited to actions that directly reduce emissions from an existing practice (e.g., capturing emissions from an existing landfill)," said the letter.

California, Vermont, Arkansas, Delaware, Maine, Mississippi, Oklahoma, Illinois, Connecticut and New Hampshire all endorsed the letter, which recommended research into consumer perceptions, and into the best ways to communicate the facts about carbon offsets.

The carbon trading market in the US operates on a voluntary basis today, said Tiffany McCormick Potter, senior analyst at Point Carbon, which advises on greenhouse gas trading markets. She explained that the voluntary market is very influenced by the standards used under the UN's compliance-based clean development mechanism, but currently lacks formal enforcement and auditing mechanisms to check on carbon reduction claims.

The letter further cranks up pressure on the FTC to set out clear best practice guidelines for US carbon offsets. The Commission recently began work on updating its so-called green guides, which outline the rules governing how firms market environmental initiatives to customers, and it is expected that guidelines for the emerging offsetting industry will be included in the new versions of these guides.

Meanwhile, separate US carbon trading and offsetting regimes are fast emerging. A multi-state cap and trade programme called the Regional Greenhouse Gas Initiative has already created criteria which will form the basis for regulating carbon offset requirements across North East and Mid-Atlantic States, while Midwestern states are also planning a similar cap-and-trade scheme.

Additionally, the UK government is also working on finalising a code of best practice for carbon offsets that it hopes will become a gold standard for credible offset projects and providers.