The verdict came after 1 1/2 days of deliberations following a three-week trial.
In a brief statement, BP denied it had harmed any of the workers.
"We are shocked and outraged by today's verdict, and we will appeal," BP said. "The verdict, and punitive damages award in particular, is utterly unjustified, improper and unsupportable."
Tony Buzbee, the workers' attorney, said his clients were pleased with the jury's decision, adding he had approached BP before the trial and had offered to settle for $10,000 for each worker. BP rejected the proposal, he said.
"They tried to minimize it, attack the credibility of the workers," Buzbee said. "They simply would not admit that they had released something."
The refinery, about 30 miles southeast of Houston, was the site of a 2005 explosion that killed 15 people and injured 170 -- the worst U.S. industrial accident since 1990.
The refinery has a history of fires, chemical releases and worker deaths.
The U.S. Chemical Safety and Hazard Investigation Board, a group that investigated the 2005 blast, found BP fostered bad management at the plant and that cost-cutting moves by BP were factors in the explosion.
The jury's award also comes after the Occupational Safety and Health Administration in October imposed a record $87 million fine against BP for failing to correct safety hazards after the 2005 blast.
The workers' lawsuit claimed that in April 2007, more than 100 contract employees at the plant were sent to hospitals after claiming they were exposed to a toxic substance released at the refinery.
The workers said their injuries included dizziness and sore throats, with one employee passing out, after inhaling the substance. Buzbee said the workers did not have any long-term damage to their lungs.
Through the trial, BP claimed no toxic substance was released at the refinery on the date in question.
Buzbee said he tested one of the worker's gas masks and it revealed they had been exposed to carbon disulfide, a toxic colorless liquid.
"Jurors were angry about the (safety) record I put forth that showed BP has a long history of leaks and releases," he said. "My point was BP has not changed, not turned the corner, and if you don't do something about this, no one will."
The 10 workers are part of a group of 143 that Buzbee represents in this lawsuit. Buzbee said he plans on taking the other workers' claims to trial as well.