Look up: Entonox

1. Entonox
   A gas, made up of 50% oxygen and 50% nitrous oxide. The vast majority of maternity hospitals pump gas and air to all of their delivery rooms from a central supply, so it's always available when you want it.
   Found on http://www.encyclo.co.uk/local/20415
   
   
2. Entonox
   A mixture of 50% oxygen and 50% nitrous oxygen that is used by some laboring women during childbirth. There is no color or odor to the gas and most hospitals have the mixture readily available. The gas is breathed in through a mask that is placed over the face and the additional oxygen can be beneficial for the fetus. Etonox is only a mild pain ...
   Found on http://www.pregnology.com/AZ/E/3
   
   
3. Entonox
   Also known as gas and air. It's a mixture of 50% oxygen and 50% nitrous oxygen, breathed in through a mask during labour. It gives pain relief as contractions peak.
   Found on http://www.thebabyshow.co.uk/family-glos