A group of seven cavers were visiting McClure's Cave in Pennsylvania in the summer of 1977. After tiring exploration the group made to exit. All were anxious to leave and proceeded into the New Extension Crawl, a very low, tight passage, all together. When a novice became stuck there was a delay. All soon began to experience symptoms of "hypoxia and carbon dioxide poisioning," rapid panting, extreme headaches and rapidly advancing exhaustion. Before anyone succumbed, they were able to proceed.
NSS News, May 1981 (Part 2) V39N5, Page AA27.
Editorial "Cave Pearls" The Dripstone Spring 1979 9:2 p 6.
An interesting situation. Some caves are low on oxygen or high in carbon dioxide but not the extent one would normally notice.