RENTON - Criminal charges against two building contractors for possible Clean Air Act violations were dropped by the King County prosecutor's office yesterday.

The office said it could not prove the firms' workers knew asbestos insulation existed at a renovation project at Valley Medical Center.

The prosecutor had charged that asbestos may have blown through a ventilation system into the hospital and its maternity ward for six days in 1991. The Puget Sound Air Pollution Control Agency alleged employees of Nuprecon Inc. of Redmond and Gall Landau Young Construction Co. of Bellevue knew they were breaking the law.

But the prosecutor's office said the charges had been based on a letter sent by Valley Medical Center to Gall Landau, addressing the possibility that asbestos was present. Dan Satterberg, the prosecutor's chief of staff, said the letter was not a clear statement asbestos was there.

Because this was the first time criminal charges were filed in a Clean Air Act case in the state, clear evidence was needed that workers knew there was asbestos, he said.

The Air Pollution Control Agency was unhappy with the prosecutor's decision, said general counsel Laurie Halvorson.

Nuprecon's President John Hennessy was elated. He has claimed his innocence all along and blamed the agency and an assistant prosecutor for "zealotry."

Hennessy said his company has lost a half-million dollars in revenues and legal fees. He said he would pursue a civil case against the air-pollution control agency.