Entonox, referred to colloquially as gas and air, is an anaesthetic used to ease pain during childbirth and in other situations when other anaesthetics would not be appropriate. It is a mixture of fifty percent nitrous oxide (N2O or laughing gas) and fifty percent air. It is administered by having the patient inhale it. It is very fast-acting, typically taking effect between twenty seconds to one minute after inhalation.
It should be stored away from sources of heat or flame, and should under no circumstances come into contact with oil or grease as this can form a mixture with a room-temperature combustion point. Similarly, no-one should smoke in the vicinity of an Entonox cylinder which is in use.
The nitrous oxide constituent of Entonox causes inactivation of vitamin B12 which is a co-factor of methionine synthase. Folate metabolism is consequently interfered with and DNA synthesis is impaired following prolonged nitrous oxide administration. These disturbances result in megaloblastic bone marrow changes.
Exceptionally heavy occupational exposure and addiction have resulted in myeloneuropathy and subacute combined degeneration. Theoretically similar adverse results could occur from heavy and prolonged Entonox exposure.
All these effects are well documented, extremely rare and may follow prolonged exposure to levels of nitrous oxide over 5000ppm or to frequent (more than once every two days) exposure to analgesic concentrations. It has been suggested that prolonged occupational exposure to high levels of nitrous oxide may affect a woman's ability to become pregnant, may result in bowel distension, middle ear damage and rupture of ear drums. Addiction to Entonox has been reported.
Occupational exposure standard (OES)
There is no OES for medical oxygen in the U.K. The OES for medical nitrous oxide is proposed as follows: Long-term exposure limit (8 hour TWA reference period) -100 ppm (ref. HSE EH40).
Entonox may in some circumstances affect the health and safety of midwives at work, please see RCM guidance notes on how it can be used and stored.