Houston residents face a less obvious threat from the air as they deal with historic flooding: The Hurricane Harvey-caused shutdown of the region’s massive petrochemical industry released more than 2 million pounds of harmful pollutants into the air as of Aug. 29, according to initial reports to Texas regulators.

“I think it’s one of those unseen events that perhaps doesn’t get recognized in all of the chaos and devastation of the storm,” said Elena Craft, a senior scientist with...