I recall hearing in a high school chemistry class that as H2S gets stronger, people lose their ability to smell it. So someone exposed to an increasing concentration will often be unaware that they are about to be knocked out.
That’s true - H2S saturates the olfactory sensors and you can’t smell any ‘new’ gas.
But that’s true for a lot of smells.
0.00047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic odor of hydrogen sulfide,[12] normally described as resembling "a rotten egg".
Less than 10 ppm has an exposure limit of 8 hours per day. 10–20 ppm is the borderline concentration for eye irritation.
50–100 ppm leads to eye damage.
At 100–150 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.[13][14]
320–530 ppm leads to pulmonary edema with the possibility of death.
530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (LC50).
Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
Nasty stuff, not to mess with, but seldom found in high enough concentrations to kill you outside of enclosed spaces, gas plants, and (sour) oil production locations. In an airspace as large as the Gulf, it is highly unlikely that there is sufficient H2S in any offshore well to present a hazard to anyone on land. The H2S if present will dissipate before the oil gets to shore.