The smell of nothing is the most dangerous smell of all. Methane may be dangerous in potent quantities, helium is damaging (but fun as hell), but they don't even begin to compair to the peril caused by the smell of nothing. The reasoning behind this is actually simple and was thought out in a locker room while some students were changing for gym class.
The smell of nothing is dangrous because there is only one place to smell nothing, and that just happens to be deep space. Deep space is the only place that is not cluttered with smelly things like nitrogen, oxygen, or any of the other gasses humans need to survive. In order to smell nothing, we must first travel into space, exit the spacecraft we ventured in, and inhale deeply.
The guide states that if you take a deep breath, you can survive for 30 seconds in the vaccume of space. It does not tell you that if you let your breath out or try to inhale, the vaccume of space will suck your insides out and turn you into a free floating extra for an Evil Dead movie.
It, therefor, can be concluded that while some scents may be potentially lethal, the smell of nothing is 100% deadly (and quite messy for passing ships), but if you choose to dissagree, you are invited to try and prove this fact wrong.