All 15 Intel employees taken to Washington County hospitals after suffering breathing problems Thursday afternoon have been released, according to the company. Intel said authorities determined “there was no hazardous materials release” in the incident.

Emergency responders reacted to reports Thursday of a possible “toxic exposure” at Intel’s massive Ronler Acres campus in Hillsboro. Intel said the employees reported “respiratory irritation” and “minor breathing difficulty.”

Initial reports said ambulances took 11 employees to the hospital. Emergency responders said Friday that they transported four more later in the afternoon.

The cause remains a mystery. Bruce Montgomery, public information officer for the Hillsboro Fire Department, said his department and Intel worked together to monitor the air quality in the facility and found nothing amiss.

“There was no sign of anything leaking or anything like that in the areas where they were checking it out,” Montgomery said.

Intel said it has launched an internal investigation to identify the root cause of the incident. Oregon’s Occupational Safety and Health Administration said it is looking into the case but would not comment while its inquiry is active.

Though semiconductor production takes place in “clean rooms,” designed to prevent even microscopic contamination of computer chips, the manufacturing process involves many hazardous chemicals that can cause severe injuries if they escape containment. There have been several accidents in the regional chip industry since the 1980s.

Most seriously, an Intel contractor servicing a manufacturing tool died last year when he was crushed inside the machine.

-- Mike Rogoway | twitter: @rogoway | 503-294-7699