Specifies minimum performance and safety requirements for humidifiers commonly used in anaesthesia and respiratory care. Two different classes of humidifiers are specified: those intended for use with intubated patients and those for use with patients with intact airways. Performance, general design, internal compliance, breathing connections, requirements for the humidifier chamber and/or water reservoir, temperature of the gas, pressure relief, operator controls, electrical and alarm requirements are laid down. Appendices include methods for determining container overflow, output temperature, water output and oxygen concentration.