Cryotherapy gas--to use nitrous oxide or carbon dioxide?

Cryotherapy is regularly used in our clinic for treating genital warts. Nitrous oxide was used as the cryogenic gas. Following a health and safety review it was decided to monitor the nitrous oxide levels in the treatment room under different conditions. The Occupational Exposure Standard for nitrous oxide is 100 parts per million (PPM) (8-h time weighted average) and an indicative short-term exposure limit of 300 PPM (15-min reference period). High levels of gas were detected, especially when the exhaust was not vented to the outside. Venting of the gas to the outside could also present a hazard to adjacent areas. The situation was considered to be unacceptable and carbon dioxide was proposed as an alternative. The Occupational Exposure Standard for carbon dioxide is 5000 PPM (8-h time weighted average) and a short-term limit of 15,000 PPM (15-min reference period). Carbon dioxide levels were found to be within the Occupational Exposure Standard. There is no noticeable difference in the cryogenic efficacy of the 2 gases. Carbon dioxide is, therefore, a safer alternative. It also offers significant savings when compared with nitrous oxide.