High carbon dioxide levels are currently reported in some Mendip caves. Manor Farm Swallet, Swildons Hole and G.B. Cavern are three popular examples. Other caves where this is a regular problem are White Pit, Cuckoo Cleaves and Tynings Barrow Swallet. On a visit to Swildons Hole and G.B. Cavern on the weekend of 24th and 25th September, the author experienced symptoms suggesting approximately 1 percent concentration of the gas. In Swildons the effects are noticeable below the Upper Series, and in G.B. Cavern most of the cave appears to be affected. It is probably a good idea for anybody planning to visit caves in the area to be aware of the signs and symptoms associated with high carbon dioxide levels, and be prepared to change your plans should you find yourself or anyone in your group affected by the current conditions.
Dr. Tony Boycott has published some useful information on how carbon dioxide will affect you if you encounter concentrations of it underground. The information can be found here.