UPDATE 1-Two groups oppose increasing US ethanol blend rate

(Adds Growth Energy says foes rely on myths; paragraph 6 new)

WASHINGTON, May 18 (Reuters) - Blending more than 10 percent ethanol into U.S. gasoline will result in more air pollution and more damage to engines, said an environmental group and a boating industry trade group on Monday.

The Environmental Working Group (EWG) and the National Marine Manufacturers Association (NMMA) said evidence weighed against allowing a blend of up to 15 percent ethanol in gasoline.

The groups will forward their findings to the U.S. Environmental Protection Agency, which is accepting comments until July 20 and has until Dec. 1 to make a decision.

Ethanol is now approved to make up 10 percent of gasoline in cars and trucks. Producers are pushing the government to allow higher ethanol blend levels, as more ethanol will be required each year under federal law.

Congress required 10.5 billion gallons of ethanol to be blended into gasoline this year. The amount will rise annually toward 36 billion gallons a year in 2022.

"The science simply does not support Growth Energy's position," said EWG's Craig Cox, referring to the ethanol group that requested a higher blend rate. "The available science argues against it."

In response, Growth Energy said "there has been more testing of E15 than there has been of any other fuel additive in the history of the EPA waiver process" and the studies show the blend does not affect emissions and does not harm engines. The tests included one of small engines, it said.

EWG said studies "do not demonstrate" that engines will meet emissions limits if the blend rate is raised. The group also said, "E15 could damage vehicle emission control systems, decrease fuel economy, pose fire risks during transportation and retail, degrade water quality, worsen emissions of some air pollutants and escalate health risks for children and other vulnerable people."