By Anna Yukhananov

WASHINGTON, March 30 (Reuters) - U.S. health regulators said on Friday that tobacco companies must report how much formaldehyde, nicotine or any of 18 other harmful chemicals are in their products.

The regulators also proposed a rule that places limits on advertising that claims tobacco products are less harmful than others - such as "tar-free" or "light" - without providing evidence they actually are safer.

The measures are part of the U.S. Food and Drug Administration's enforcement of a 2009 law that gives it broad authority to oversee the manufacturing and marketing of cigarettes and other tobacco products.

"Tobacco products, in this country at least, are the only mass-consumed products that consumers don't know what's in them," Dr. Lawrence Deyton, director of the FDA's Center for Tobacco Products, told reporters. "Today, we are ending that era."

Cigarette makers said they were reviewing the measures.

Reynolds American Inc spokesman Richard Smith said the company was committed to working with the regulatory agency to reach "a sound, science-based regulatory structure."

David Sylvia, spokesman for Altria Group Inc, the parent of Philip Morris USA, had no immediate comment on the ingredient disclosure rule, but said standards for evaluating lower-risk tobacco products need to be "rigorous, yet feasible."

Some 8 million Americans have smoking-related illnesses and as many as 443,000 Americans die each year from smoking-related causes such as lung cancer. Smoking is the No. 1 preventable cause of illness and death in the United States and contributes about $96 billion each year to health care costs.

The rule would force companies to tell the FDA whether their products contain any of 20 harmful or potentially harmful ingredients found in tobacco or tobacco smoke and the amount of each.

The ingredients will not be listed on the packaging. Rather, the FDA will compile information about each product and provide it to the public by April 2013. The FDA said it has not decided how it will present the information.

The 20 chemicals are the easiest to test for immediately, but the FDA will later make companies provide information for a full list of 93 chemicals.

Deyton, head of the FDA's tobacco center, said most people are aware of the dangers of smoking in general, but may not know which chemicals in tobacco are harmful and why.

For example, ingesting carbon monoxide - produced any time something is burned and present in tobacco smoke - is known to increase the risk of heart and lung disease, the FDA said.

Besides informing the public, regulators said they hope the rules will encourage tobacco companies such as Lorillard Inc and Altria to make products safer and less addictive.

Representatives from the companies could not be immediately reached for comment.

The announcement comes only a month after the government suffered a blow in court trying to enforce another tobacco law that required large graphic warnings on cigarette packaging. A U.S. District Court judge sided with the companies and ruled the labels were unconstitutional. The United States is appealing the decision.

The second rule would require FDA approval to sell tobacco merchandise claimed to be less harmful, known as modified risk products. The companies would have to submit scientific studies and analyses to the FDA proving their tobacco products actually benefit public health, or reduce harm.

To counter a decline in smoking in the United States, cigarette makers have focused on smokeless tobacco and other "modified risk" products.

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