Intel told Oregon employees this week that an outbreak of colds -- not toxic exposure -- is responsible for some of the breathing problems that sent nearly two dozen to hospitals and prompted the company to close a major facility for nine days.

Two incidents in four days in late November and early December sent 21 Hillsboro workers to local hospitals. Though emergency responders said none of the cases was serious, Intel closed the 400,000-square-foot manufacturing support building at its Ronler Acres campus as a precaution.

The building reopened Wednesday after Intel said there was no threat to worker safety. In a brief public statement, the company said it will change its air intake system to address an unspecified odor it believed was responsible for the issue.

Intel issued a fuller explanation to workers and invited them to briefings to describe the findings of its investigation into the incidents.

“We have methodically examined the potential root cause, including exhaustive evaluation of our tools, facilities, materials and potential external sources such as wood-burning stoves, fires and burn piles,” Intel told employees in a message obtained by The Oregonian/OregonLive.

The company said it believes its air handlers were responsible for an odor similar to burning wood and that it added filters to air handlers to minimize smells “due to external smoke, possibly caused by the ongoing burn season and wildfires.”

While Intel said it does not have medical information on individual workers, the company said “the site physician saw a number of them and found their symptoms were consistent with viral upper respiratory infections (colds) rather than a problem related to the” manufacturing support building.

“We ask individuals to remain at home if they are sick,” the company said.

In response to an inquiry from The Oregonian/OregonLive, Intel confirmed its doctor attributed symptoms to an upper respiratory infection. The company did not respond to a question about whether there was any reason to believe its Ronler Acres campus had suffered a large outbreak of colds, or whether people outside the support building reported similar symptoms.

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