What you can't see is worse than what you can

Remember this phrase: What you can't see, is worse than what you can see. What you can't smell is worse than what you can smell.

Dig this. H2S, as we all know, smells like "rotten eggs", which is a good thing, because H2S is a very deadly gas. Here is the problem, at the levels we can smell it, it is not going to kill anyone. But, as the levels of H2S rise in parts per million, it reaches a level where is numbs, deactivates the olfactory nerves, preventing humans from smelling it. As levels continue to rise, the higher levels are undetected by the impaired olfactory nerves, and can reach a level, where the gas shuts down the human physiological systems (combo of lungs, heart) and causes loss of consciousness and can cause death.

NOTE: Natural gas is odorless. Gas companies ADD odor to it in the industrial process, in order to make it detectable by human olfactory glands. Natural gas coming out of the BP blowout has no perfume added.