A chemical reaction that began yesterday afternoon continued to cause problems in downtown Vancouver on Saturday, forcing emergency crews to evacuate a building for the second time.

Fire and Rescue personnel were called to the building, located in the 900-block of Nelson Street, three times on Friday after a urethane foam product unexpectedly reacted during a renovation project.

Crews initially determined the fumes coming from the foam were not harmful, but returned to the building on Saturday to reassess.

"We now believe that we have to do a more thorough examination to determine just what is coming off in the form of toxicity in the gasses," battalion chief Rod MacDonald said.

"We don't know if it's harmful, that's what we're trying to determine. We're dealing with a bit of the unknown."

MacDonald said crews also have to determine if the fumes absorbed into the buildings walls. If they have, it's possible they could escape in the future. 

The foam was being used to raise the buildings concrete floor.

Witnesses said fumes filled the building with an "overpowering solvent-like stench" on Friday that progressively grew worse over the course of the day.

Smoke then started billowing out of the building into the street. "It looked like it was a really foggy day," Ed Lee, who works in the building, told CTV News. "That's obviously when we all started becoming really concerned.

Sections of Nelson and Hornby streets were blocked off two days in a row to facilitate the evacuation, causing traffic disruption in the area.

Authorities have not said whether residents will be allowed to return home on Saturday night.

WorkSafe B.C. will be investigating the incident.