There is a variable response to exposures lasting between 30 minutes and 2 h. However, it now seems likely that exposures of more than 2 h are likely to cause intereference with hepatic methionine synthase activity. The paucity of human data makes it more difficult to say how long an exposure is required to cause significant intereference with DNA synthesis. It is likely that there will be considerable individual variation and results obtained in healthy patients cannot be extrapolated to the patient who is seriously ill. Nevertheless, it seems likely that, once methionine synthase activity is inhibited, it will remain so for days.
Nitrous oxide is an example.
A gas of low anesthetic potency, it is incapable of inducing deep levels of anesthesia if an adequate oxygen concentration is maintained. Nitrous oxide induces a state of behavioral disinhibition, analgesia, and euphoria. But in order to achieve high enough concentrations of nitrous oxide to get a good behavioral effect, concentrations of 50 percent or greater must be inhaled. If such concentrations are mixed with room air, inhaled oxygen concentartions drop to low levels and the hypoxia may result in irreversible brain damage. Nevertheless, there is some evidence that excessive or prolonged use of it can damage the bone marrow and nervous system by interfering with the action of vitamin B-12.