Particularly nasty because it dissociates into
ammonia and
water. The ammonia is a very lightweight molecule (about 1/2 the molecular weight of oxygen) and diffuses very quickly through the air. This is especially problematic because the
dissociation reaction is reversible, so the ammonia forms ammonium hydroxide when it reaches more water, which there happens to be plenty of in your
eyes,
sinuses, and
lungs.
A few years back when I was an intern my supervisor broke a bottle and spilled about a gallon of concentrated ammonium hydroxide on the floor. Needless to say, the ammonia instantly filled the room and proceded to diffuse into the room next door. I was in the lab at the time, and the ammonia caused a most painful experience which it why I never wear contact lenses in the lab.