Hydrogen sulfide irritates the mucous membranes and respiratory tract. Poisoning can be noticed by headaches, nausea, cramps, eye or skin irritation. Dangerous damage to the lungs is often not noticed right away.
Even if the smell actually warns against it, even small doses of the gas can switch off the sense of smell, so that the poisoning is not noticed at first. In no case is the smell a reliable indicator.
The dangers depend on the H2S concentration and duration of exposure:
< 100 ppm: life-threatening after several hours.
above 100 ppm the sense of smell becomes numb
> 100 ppm: life-threatening after more than one hour, irritation of eyes and respiratory tract
from approx. 1000 ppm: life-threatening in a few minutes