Hydrogen sulphide is exceedingly poisonous; according to Dr. J. S. Haldane it is five times as deadly as carbon monoxide, but it is seldom found in quantities dangerous to life, whilst its characteristic pungent odour gives warning of smaller quantities than would prove to be poisonous. Traces of the gas are associated with gob fires, but it seldom does any harm beyond causing discomfort to the eyes, nose, and throat. Hydrogen sulphide is somewhat heavier than air, its specific gravity being 1.18. Small warm-blooded animals like birds or mice may be used to estimate small quantities, but its smell serves to indicate its presence even in traces.

Physiological Effects of Hydrogen Sulphide

0.07 % in air, if breathed for a long period, may prove fatal. 

1.0% will cause death in a very short time.