Lisha Burnett, a spokeswoman with Suncor, says a processing unit malfunctioned and that created the cloud around 1:30 p.m. The refinery is on Brighton Boulevard.
The plant's alarm was sounded as a precaution, but employees were able to return within 15 minutes.
Suncor sent in a safety team to test the air and it found no unsafe readings.
No injuries were reported.
(KUSA-TV © 2011 Multimedia Holdings Corporation)