An Agilent Technologies engineer was burned badly Tuesday in a chemical explosion that forced the evacuation of a building at the company's Fountaingrove Parkway campus in Santa Rosa.

Patrick Colbus, 45, a Sonoma State University graduate was in critical condition late Tuesday, said staff at the University of California Davis Medical Center in Sacramento. Colbus, who lives in Santa Rosa, worked in high-tech materials engineering and coating.

"I'm very concerned about him but was comforted hearing that when they transported him he was conscious," said Saleem Odeh, vice president of sales for the company's electronic measurement group housed in the building. "I'm just praying he's OK."

Another employee was hurt by a falling object and about 20 others complained of respiratory problems and underwent decontamination procedures after the quick but powerful explosion spewed plumes of smoke that carried a faint chemical smell.

More than 80 people had been under observation as emergency personnel evaluated them for respiratory symptoms. By late afternoon, most had been released to their homes or referred to doctors at Kaiser Medical Center.

Agilent officials late Tuesday said their chemical emergency experts were trying to determine what caused the blast.

"At this point, I have no idea," Odeh said.

Colbus was alone in the ground-floor lab in the two-story Building 1 at about 10:30 a.m. when the explosion rocked the structure. He had been cleaning a molecular beam epitaxy device, a large piece of equipment used for producing coatings on integrated circuits, officials said.

A combination of substancess, including red and white phosphorous, gallium, aluminum powder and arsenic were involved in the flash explosion that severely burned Colbus' face, head and torso, said Mark Basque, a battalion chief for the Santa Rosa Fire Department. The chemicals are integral to the production of the advanced circuits.

"Something caused them to ignite and explode, but it's unknown if it was a result of a chemical interaction or a mechanical failure," Basque said.

The explosion blasted holes in at least one wall, cracked glass, overturned equipment and left a burn mark where the flash occurred, Basque said.

Firefighters discovered light smoke, which carried a chemical odor described as resembling the smell of a road flare.

When firefighters arrived, Colbus was in a lobby area near an outer door, Basque said. He was taken by ambulance to Memorial Hospital and later flown to the burn unit in Sacramento.

Some employees said the explosion was similar to that of a sonic boom.

Shayne Bowen was in a second-floor office when he felt the building jolt. "My first thought was it was an earthquake," said Bowen, an executive assistant.

"It was like a crane dropped a giant shipping container," said John Dang, a software engineer who was in a bathroom when the explosion hit.

The boom triggered an alarm system within seconds, the men said. An on-site chemical emergency response team rushed toward the explosion, passing people filing outside.

"First responders were pouring down the stairs," Bowen said.

As the building was evacuated, some people passed directly through the smoke. The building is one of four on the Agilent complex.

Agilent's safety response protocols prompted the evacuation, health monitoring of employees and activation of a chemical emergency response team, Agilent spokesman Jeff Weber said.

Because the explosion involved a variety of chemicals, Santa Rosa fire officials relied on the expertise of Agilent chemists. A two-member Agilent team donned blue protective suits, pink respirators and green gloves to enter the lab. They inspected the damage, sealed off the area and made plans for decontamination and repairs.

The risks from the chemicals and metals were not immediately known, prompting Agilent and fire officials to sequester 82 employees in the cafeteria to monitor their vital signs and breathing.

As many as nine emergency responders underwent decontamination after treating the burn victim and taking him to the hospital. That involved removing their clothes and being completely scrubbed down, fire officials said.

Included in the nine were a "limited number" of hospital medical staff who underwent decontamination just outside the emergency department as a precautionary measure, a hospital spokeswoman said.

At Agilent, at least 20 employees underwent a voluntary decontamination process, Odeh said. People took very thorough showers at the facility, threw out their clothes and put on clothes provided by the company. A small group who were still concerned about their symptoms were referred Kaiser Medical Center in Santa Rosa, he said.

As a precaution, firefighters also decontaminated the ambulance used to transport Colbus, said Deputy Fire Chief Mike Jones.

He said it was unclear whether the chemicals were destroyed in the flash explosion or were dispersed in the air. However, "scrubbers" in the building's air ducts are intended to clean air before it escapes through vents, fire officials said.

Employees were not allowed back into the building and taxis were called for those whose car keys remained inside.

A woman working on the ground floor suffered a minor head injury when the building shook and caused an item to fall on her head. She was walking around outside afterward with an ice pack held to her head and went on her own to Kaiser Permanente Medical Center for treatment, officials said. She works as a contract employee at the Agilent campus.

"Some of our employees are kind of traumatized. It's upsetting," Weber said.

An investigator for CalOSHA arrived at Agilent shortly before 2 p.m. to begin an occupational safety investigation in connection with the on-the-job injury.