N2O should not be used in patients with bowel obstruction, pneumothorax, or middle ear or sinus disease, and should also not be used on any patient who has been scuba diving within the preceding 24 hours or in violently disturbed psychiatric patients. There are also clinical cautions in place for the first two trimesters of pregnancy and in patients with decreased levels of consciousness.
Nitrous oxide 
Nitrosamine
Benzene
Nitrobenzene
Nicosan
Vinyl chloride
Carbon monoxide
Alkylphenol