HOUSTON – The strange odor that had Houstonians holding their noses Monday morning remains a mystery.

Harris County Emergency Management officials are still investigating the burning plastic smell reported by residents across the Houston area.

At about 11:30 a.m. Monday the Houston Fire Department tweeted, "@cohoustonfire is experiencing an increase in 911 calls b/c of suspicious odor. Air-quality-monitoring shows no cause for concern."

Some KHOU 11 viewers on the southeast side first reported the smell around 10 a.m.

By 11 a.m. the smell and an apparent haze covered most of downtown Houston and the west side, with some reports from as far north as Bush Airport.

Air Alliance Houston says the CAER phone line, which alerts the community to plant and refinery operations, reported there were flare-offs at the Lyondell Basell facility in Channelview. The plant there indicated there was "no risk to community." It's not yet confirmed that the smell indeed came from Lyondell Basell.

KHOU 11’s tower camera and Houston TranStar freeway cameras showed a white haze across the area.

By noon, the haze over downtown appeared to be clearing.

Lyondell confirmed it had a controlled release Monday morning because of a steam and oil leak, but they say there's no confirmation the smell was connected to that release.

"We initiated air monitoring shortly after the event took place. The monitoring shows no issue of concern for the community and that air quality is within all regulatory and safety standards," Lyondell said in a statement.

At least one school district, Katy ISD, temporarily restricted outdoor activities Monday due to concerns about the air quality. The restriction was lifted Monday afternoon.