WASHINGTON, July 31 (Reuters) - The National Highway

Traffic Safety Administration said Wednesday it was standing by

its initial decision that 51 million air bag inflators in 49

million U.S. vehicles built by 13 automakers pose serious safety

risks.

NHTSA argued at a hearing in October that inflators

produced by the two air bag manufacturers, ARC Automotive and

Delphi Automotive, should be recalled because they may rupture

and send metal fragments flying. After automakers raised

objections in December, the agency did not immediately finalize

its decision. The agency said Wednesday it was supplementing its

initial determination and giving automakers another 30 days to

respond before it could finalize the determination and formally

demand recalls.