<unk> (N2O) is itself active (does not require any changes in the body to become active), and so has an onset in roughly the lung–brain circulation time. This gives it a peak action 30 seconds after the start of administration; Entonox should thus be used accordingly, i.e. inhalation should start 30 seconds before a contraction becomes painful in labour. It is removed from the body unchanged via the lungs, and does not accumulate under normal conditions, explaining the rapid offset of around 60 seconds. It is effective in managing pain during labor and delivery.<unk> is more soluble than oxygen and nitrogen, so will tend to diffuse into any air spaces within the body. This makes it dangerous to use in patients with pneumothorax or those who have recently been scuba diving, and there are cautions over its use with any bowel obstruction. Its analgesic effect is strong (equivalent to 15 mg of subcutaneous route morphine) and characterised by rapid onset and offset, i.e. it is very fast-acting and wears off very quickly.
Nitrous oxide 
Nitrosamine
Benzene
Nitrobenzene
Nicosan
Vinyl chloride
Carbon monoxide
Alkylphenol