The only safe Inhalant (that I know of).
Acetone Inhalation Side Effects:
"concentrations of approximately 300-500 (ppm?) were reported to cause slight irritation of the nose and throat."
"Exposure to 250 ppm for 4 hours has caused mild effects on performance in some behavioural tests (auditory tone discrimination and a mood test)."
"As concentrations approach 1000 ppm, noticeable irritation has occurred and some people have reported headaches, light-headedness and tiredness."
"Inhalation of concentrations higher than 2000 ppm can cause dizziness, a feeling of drunkenness, drowsiness, nausea and vomiting."
"Unconsciousness may result if exposure is extremely high (greater than 10000 ppm). Intolerable nose and throat irritation would also occur at these concentrations. Even higher concentrations can cause collapse, coma and death"
Although it is generally non-lethal, and low concentrations (even over extended durations) have not been proven to be detramental to one's health, I would advise avoiding it.