The 2003 UBSS expedition to Northern Thailand tested the practical use of closed-circuit oxygen rebreathers as tools to explore, survey and analyse cave atmospheres in bad air passages of Tham Tab Tao, Chiang Mai Province. 80 m of new cave passage was explored and surveyed and 22 paired readings of CO2/O2 were obtained; the first such paired measurements made in Thailand caves. The ratio of CO2 enrichment:O2 depletion (1:1.2), combined with the maximum calculated CO2 concentrations (5.1%) indicate that the microbial breakdown of organic matter is the source of bad air within the cave. The source of organic matter is largely from bat guano. It was found that although rebreathers were ideal for collecting uncontaminated cave atmosphere samples, on future expeditions lightweight carbon composite air cylinders with SCUBA regulators and nose-clamps may provide the safest and most practical way to extend future exploration.