Well ... yes and no.

The symptoms of short-term carbon monoxide poisoning are headache, dizziness, nausea, and at higher concentrations, convulsions, unconsciousness, and death. If you're exposed to low levels over a long period, that's when you can start to have heart problems, delirium, confusion, and hallucinations. Nancy was only there a short time when she started to see ghosts, so I don't know about her sightings. Clara and Harper and perhaps Jess might certainly have reached the point where they were having hallucinations, I suppose.

Could it account for ghost sightings in the Victorian era, as Nancy says in her wrap-up? I think so, yes. In Victorian America and England, people heated their houses with open fires, often burning soft polluting coal (which also accounts for the thick poisonous yellow fog they often had in London). It would be more amazing if there weren't some level of CO poisoning in those days.