He noticed the doors suddenly slam and bolt shut, although he made no specific move as he took a slight breath. He had a bad feeling, but he kept his cool. He sat there, noticing officers putting on gas masks he expected the worst. After a moment, he took a deep breath and would hold it for as long as he possibly could. Maybe a show of strength to help him in his last moments, or just a futile attempt to keep his life for as long as he could.
However, despite this, his stoic expression did not change. There was no panic, no distress. He only kept himself in the position, his hands flattened against his knees and waited.