It's well-known that hydrogen sulfide, H2S, is one of the worst-smelling common gases. It's responsible for the rotten-egg smell of decomposing albumen (and any other rotting organic stuff with sulfur in it) and it's also quite poisonous, comparable to hydrogen cyanide in fact.
They used to generate this stuff in routine laboratory chemistry for selective precipitation of metals, as in the famous (or infamous) qualitative-analysis scheme for common metals devised by A. A. Noyes and adapted for chemistry students. Imagine a bunch of schoolkids all merrily bubbling H2S into test tubes and somehow not killing everyone in the room. (That we know of.)
There's an especially insidious problem with H2s as well, true for bad smells in general, but I've heard that the problem is especially pronounced with hydrogen sulfide: it only reeks at first. You catch your first whiff and think oh my gawd what is that stench but the nose gets dulled to it and even begins to perceive the odor as pleasant. Hence smell alone is a flawed warning.
In general terms this is called saturation of a signal or a perception. Your internal H2S meter has redlined, certain neurochemicals are getting slowly depleted from trying to maintain the redlined signal, and so your perception weakens. This is a survival feature in part, enabling an animal to endure unpleasant and prolonged stimuli, such as chronic pain. But plainly it's also dangerous.
It has occurred to me that the marketing people, and right-wing propagandists in particular, have learned from experience that such a process of saturation and progressive desensitization is found to occur in mass media and the news. Blare any one story loud enough and long enough and everyone tires of it, even if the problem hasn't gone away. Consider the issue of Donald Trump's obvious unfitness for office. It's been an issue for years and that's just the problem: everyone in the media seems to have tuned it out, written it off as sore-loser talk from Democrats and leftists, and now the GOP have an ungodly amount of latitude to run around in public with a blithering incoherent Trump. It's all gotten flattened down to a harmless "he's just so zany" attitude.
~Chara of Pnictogen
