The ability to trace toxic particles was carefully developed. A whole generation of genetically modified humans with a heightened sense of smell prevented the end of civilization for a century. As their lives expired, one by one, society trusted toxic levels to be so low that there were no replacements. At the turn of the century, when the flow of time shifted unexplainably, humans became lost in a world of mutating units, as they were called, the ones who went outside. They had a vague recollection that there was a solution for the problem but they just couldn’t remember it.
100 Word Stories (prompt: smell)