Honestly, I'm not sure. I'd try to look in the PPeH or look for the Exit Bag Megathread
Sooo, looking at the PPeH, it cites Nitrogen and Carbon Monoxide as being two good candidates. However, using Nitrogen would require the exclusion of oxygen whereas exposure to large amounts of CO may have side effects. Exposure to enough CO to cause CO poisoning can cause side effects such as headaches, blurry or double vision, shortness of breath, nausea / vomiting and chest pain.
Sooo, looking at the PPeH, it cites Nitrogen and Carbon Monoxide as being two good candidates. However, using Nitrogen would require the exclusion of oxygen whereas exposure to large amounts of CO may have side effects. Exposure to enough CO to cause CO poisoning can cause side effects such as headaches, blurry or double vision, shortness of breath, nausea / vomiting and chest pain.