As she brought the light to her cigarette it set fire to the oxygen emitting from the nearby medical equipment, causing the gas to explode in her face.
We are surrounded by idiots. I'll lay odds the fecking author has a degree too . .
Added to which (as someone who uses medical oxygen at home) there are abundant warnings about naked lights, smoking etc. and it would have been explained to her by the local oxygen delivery service (Dolby Vivisol in my instance - good opportunity for some high fidelity jokes in that name!).