Recently on Reddit, a post claimed that many "haunted" houses have been investigated, and the culprit found not to be supernatural but carbon monoxide poisoning.
Is that the case? Well, it certainly has been the explanation for a number of cases we can find, and according to one toxicologist carbon monoxide and other poisonings could explain a surge in hauntings and ghost stories during the Victorian era.
One incredibly detailed report published in the American Journal of Ophthalmology in 1921 documents the case of a family who moved into a house nine years earlier, and began to experience ghostly apparitions. The father, who liked to eat fruit alone in the kitchen before bed, started to feel like someone was watching him at all times from behind, and turned to face the hallway to watch out for other presences. The children became sick and pale, and lost all interest in playing, while the father hallucinated a "strange woman" dressed in black walking towards him from the dining room.
One night after the theater, both husband and wife had a strange old time.
"I had vague and strange dreams, which appeared to last for hours," the mother told the ophthalmologist years later. "When the morning came, I felt too tired and ill to get up. G. told me that in the middle of the night he woke up, feeling as if someone had grabbed him by the throat and was trying to strangle him."
It was the father's brother who came up with the logical explanation, which was later confirmed by the ophthalmologist who found telltale damage in the children's eyes. Two days into their stay, the house's furnace had broke down, the likely source of the carbon monoxide poisoning that had caused their headaches, ill health, and hauntings.
In another far more recent case, writer Carrie Poppy began to feel like she was being followed by a "bad spiritual presence". Poppy felt like she was being watched, before she developed auditory hallucinations and a strange feeling of pressure on her chest.
"I would hear this like whoosh (ph) like as if something was passing by me - maybe the sound of the ocean, you might say," she told NPR's TED Radio Hour. "And then just this this disquieting feeling that something was there."
Rather than a haunting, Poppy felt like there was a demon in her house.
"Every day, I'd come home. And, you guys, this feeling got so bad. I would sit there in bed at night. I would cry every night. And the feeling on my chest got worse and worse. It was physically painful."
After some Googling, she got in touch with a forum of "ghost hunters". Fortunately for Poppy, they were ghost hunters of the skeptical variety, and soon they were asking her if she had heard of carbon monoxide poisoning. Sure enough...
"I looked it up, and the symptoms of carbon monoxide poisoning include a pressure on your chest, auditory hallucinations, whoosh (ph), and an unexplained feeling of dread."
Sure enough, the gas company confirmed a leak, and told her that had it been left unchecked she could have been dead soon.
Carbon monoxide poisoning of course does not explain all reports of ghosts, which can also be explained by hoaxes, hallucinations during sleep paralysis, poor sleep, and hotel managers down on their luck trying to drum up business if it wasn't for this pesky team of teenagers and their cowardly talking dog.
However, toxicologist Albert Donnay believes that carbon monoxide and other poisonings can cause hallucinations of hauntings, and that the theory would explain why Victorians – with their gasoline lamps and toxic wallpaper – experienced so many hauntings, and became fascinated by them.
In short, you haven't seen a ghost, Andrew, but you might want to service your boiler.