Emergency responders were back at Intel's Ronler Acres campus in Hillsboro on Monday, treating employees for mysterious respiratory issues for the second time in less than a week.

Metro West Ambulance said it transported six people to nearby hospitals for what Intel described as "treatment of minor breathing difficulty." Public safety officials described their conditions as "non-critical."

Metro West public information officer Jan Lee said emergency responders examined 35 people altogether, all with "very minor symptoms." Initial reports from Intel said three people went to the hospital -- the company later confirmed Monday's total was six.

Fifteen people from the Intel site went to Washington County hospitals late Thursday afternoon complaining of breathing problems. The workers reported "respiratory irritation" and "minor breathing difficulty," according to Intel.

Intel said its Manufacturing Support Building, a four-story, 429,000-square-foot facility, will remain offline "until further notice." The company said the incident didn't affect other facilities on the campus and a Washington County official said there is no indication of a hazardous material release outside the building.

"Our previous investigation as well as our current, ongoing investigation show no hazardous materials were released," Intel said in a written statement Monday. "However, as a precaution, we have now evacuated all individuals from the affected building."

Intel said it has bolstered its health services personnel at Ronler Acres to accommodate requests for health screening. The company said it evaluated its manufacturing tools for chemical or heat issues and found nothing unusual.

Intel said its heating and ventilation systems also checked out and there is no indication that external air pulled into Ronler Acres' intake systems created the recent issues. Intel said it is using temporary monitoring systems in the manufacturing building and checking them hourly while expanding air filtration on the campus.

In a tweet Monday, the Hillsboro Fire Department said it was treating "multiple patients" with five engines on site -- plus five ambulances and a hazardous materials unit.

HFD response to Intel to treat multiple patients. 5 engines, 5 MWA ambulances, and 1 Hazmat unit on scene working. pic.twitter.com/i1e6jTEtES — Hillsboro Fire Dept (@HillsboroFire) December 3, 2018

All the workers in Thursday's incident were home from the hospital by late Friday.

Intel's Ronler Acres campus is the company's most advanced manufacturing facility, where the company develops each new generation of microprocessor. It's a massive, multibillion-dollar complex of several interconnected factories and support buildings.

Chip production involves many hazardous chemicals and manufacturers across the Portland area have suffered a number of serious incidents in the four decades since semiconductor companies began arriving in the city's suburbs. It's exceedingly rare, though, to have two incidents at the same site so close together.

Intel did not say which building it evacuated or how long it will be closed. Semiconductor manufacturing is a carefully choreographed process, timed to meet demand just as it arises.

There is no indication Intel is facing a significant disruption from the Hillsboro incidents, but the company is already suffering from a shortage of its current generation of 14-nanometer technology.

This article has been updated with additional detail about Monday's incident and comment from Intel.

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