Why does Montag vomit at the smell of kerosene?

The smell of kerosene immediately reminds Montag of the old woman who was burned that day. It makes him sick.

"Yes, the bunch, the bunch, the bunch." He pressed at the pain in his eyes and suddenly the odour of kerosene made him vomit. Mildred came in, humming.

She was surprised. "Why'd you do that?"

He looked with dismay at the floor. "We burned an old woman with her books."