Many haunted houses have been investigated and found to contain high levels of carbon monoxide or other poisons, which can cause hallucinations. The carbon monoxide theory explains why haunted houses are mostly older houses, which are more likely to contain aging and defective appliances, and why more hauntings are reported in the colder months. Carbon monoxide poisoning explains many of the occurrences in haunted houses, such as feelings of being watched, hearing footsteps or voices, seeing "ghosts", headaches, dizziness, and sudden death or illness of people or pets, and also strange behavior in pets such as excessive barking or meowing. The carbon monoxide theory also explains why some ghosts don't show up on photographs or videos (photographs that do show "ghosts" are usually caused by dust, insects, fingers or camera strap in front of the lens, and multiple exposures).