Nitric oxide

Colorless gas, relatively insoluble in water, practically of the same density as air (NO-air density ratio is 1,03:1). Pure NO is odorless, but in the air it oxidizes to NO2 which has a pungent odor.
Irritates respiratory tract and eyes, at high concentrations may cause lung edema and death.

Health exposure limits:

8 hours TWA = 25 ppm (Directive 91/322/EEC)
IDLH = 100 ppm (NIOSH)