If an anaesthetist at high altitude uses plenum vaporizers. It will deliver the vapour concentration –

Plenum vaporizer is typically set to deliver a 1-2 kPa partial pressure of anaesthetic agent. At sea level, where atmospheric pressure is around 100 kPa, this equates conveniently to a concentration of 1-2% of anaesthetic agent (1-2kPa out of 100 kPa). At higher altitude, the atmospheric pressure is low (e.g. 75 kPa). The plenum vaporizer deliver 1-2 kPa partial pressure of anaesthetic agent. So, the concentration of anaesthetic vapours will 1.25 - 2.5% → Greater than the concentration of anaesthetic agent at same partial pressure at sea level.