Gas and air(Entonox): This is a mixture of nitrous oxide(laughing gas) and oxygen. As you feel a contraction starting, you breathe in the mixture through a mouthpiece or a mask placed over your nose. It's a mild painkiller and will make your less aware of your pain although not all women find it effective. Some women find that Entonox can make them feel sick and light-headed.
Attribution: BUPA