Workers have begun misting debris from a three-alarm warehouse fire in a round-the-clock effort to prevent the release of more asbestos fibers into the air, officials said Friday.

The misting began Friday morning and is part of a response to the blaze that includes air sampling and debris cleanup. Officials have said the building's roofing paper contained high levels of asbestos and that some debris contains the fibrous mineral, which is known to cause cancer and other diseases.

The fire started Sunday night at 1300 N. River St. Fire officials said earlier this week there had been reports of cooking and warming fires in the area, but they didn't specify what caused the blaze. The warehouse was previously cited for a host of violations, according to city records.

Air sampling has returned "encouraging" initial results — all of which showed no asbestos fibers or amounts far under action levels, the state Department of Environmental Quality and federal Environmental Protection Agency said in a news release.

Crews on Friday scoured places including parts of downtown Portland and the Pearl District for debris that contains asbestos, removing it as necessary, officials said.

Officials have warned people to avoid debris that looks like ash or paper. Anyone who finds such debris should call 503-229-6488 and leave their name and phone number, where they found the debris and what it looks like.

People can view air sampling results online.

— Jim Ryan

jryan@oregonian.com

503-221-8005; @Jimryan015

Tony Hernandez of The Oregonian/OregonLive staff contributed to this report