By the pitter-patter of their tiny feet and their nose
Deep in the sewers lies discarded human waste. Hydrogen sulphide, which is produced by bacteria decomposing organic matter found in an oxygen-deprived environment, is the most toxic component of sewer gas.
Hydrogen sulphide is deadly to humans at concentrations as low as 300 parts per million. Since it is heavier than air, it settles at the lower levels of the sewer.