The two volatile agents within the scope of this session are isoflurane and sevoflurane (Fig 1).

Any agent that can exist as a liquid at room temperature is correctly called a vapour. Any agent that cannot be liquefied at room temperature, whatever the pressure, is correctly called a gas. Volatile anaesthetic agents are all liquids at room temperature, so strictly speaking are vapours and not gases. However, it is quite usual for anaesthetists to refer to them colloquially as 'gases'.

As all volatile agents are liquids at room temperature they are administered via vaporizers that are calibrated for a single agent (Fig 2). Note that the vaporizers are calibrated in 'percent'. An output of 1% indicates that the vapour occupies 1% of the outflow from the vaporizer