Regional environmental officials found asbestos in three of eight randomly chosen piles of debris from repair and demolition projects under way in Northridge, according to a statement issued on Monday.
The samples were identified in field tests conducted by the Southern California Air Quality Management District last week, said Bill Kelly, an AQMD spokesman.
The results of those tests prompted city and regional environmental officials to issue a cautionary warning to property owners who are making post-earthquake repairs.
"It appears to be from do-it-yourselfers or contractors who don't know about the presence of asbestos," Kelly said, "and they're just ripping the stuff out and putting it on the curb. I'm not saying we know the complete scope of this problem, but we are trying to head it off from getting worse."
The AQMD statement urged property owners to have a licensed laboratory test materials suspected of containing asbestos. If asbestos is found, it should be removed only by a contractor licensed to do such work, it said.
"If there are existing demolition and remodeling debris piles with materials suspected of containing asbestos, keep the material damp until it can be properly removed," the statement said.