The exposure happened when the prevailing wind didn't carry the gas away from the plant like usual, Ysebaert added.
"It blew the fumes, the gases, back into the plant," he said.
The state agency also says Sinclair failed to fix an emergency eye wash station despite three previous citations or develop safe operating procedures for two systems at the refinery.
Sinclair didn't immediately respond to messages seeking clarification on whether it intends to pay the fines.
In a statement, Sinclair Vice President Clint Ensign said the company is working in good faith with state safety officials.
"Many of these concerns have been corrected or are in the process of being mitigated," he said.
Ysebaert said the state OSHA office recognizes that effort.
"There's so much for them to do at this time, and they are trying to really address everything," he said. "But it's quite a task."
Wyoming refineries, including Sinclair, have been meeting regularly since last year to discuss ways to improve safety. Representatives of Gov. Matt Mead's office have taken part.
"Officials from the company assured me they are making strides and will continue to do so," Mead said in a statement through his spokesman.
Recent safety and environmental problems at the Sinclair refinery have been numerous.
Last year, three fires at the refinery injured seven workers. The state OSHA office assessed more than $200,000 in fines for those accidents.
In 2011, the state reached a $5.4 million settlement with Sinclair for spills into a pond on the refinery site that killed nearly 150 birds. More than 200 birds had to be rescued.
In 2009, the refinery was the scene of one of the biggest spills in Wyoming history when the floating roof of a storage tank sank and punctured the tank's bottom. More than 2.7 million gallons of gasoline-grade petrochemicals leaked onto the refinery grounds.
The Sinclair website says the refinery in 2005 received the Gold Award for Achievement in Safety from the American Fuel and Petrochemical Manufacturers, then called the National Petrochemical and Refiners Association.