The fire broke out around 10am at the Coker unit, which is part of the refining process.
PHOTOS: Chemical plant fire rages in southeast Houston
There was briefly a shelter-in-place was briefly in effect for residents near the plant, but was quickly lifted.
"The latest reports show the air quality has never really hit a dangerous level. So even though we did a shelter in place our air readings are showing positive right now," said Sr. Capt Ruy Lozano of the Houston Fire Department.
All employees have been accounted for and no injuries were reported. LyondellBasell says air monitoring shows no impact.
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