0.00047ppm is the level 50% of the population can detect the odour of hydrogen sulphide, workers in the petrochemical industry carry alarms set to go off at between 5 - 15ppm
less that 10ppm has an exposure limit of 8 hrs a day
10–20 ppm is the borderline concentration for eye irritation.
50–100 ppm leads to eye damage.
At 100–150 ppm the
olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
[13][14]
320–530 ppm leads to
pulmonary edema with the possibility of death.
530–1000 ppm causes strong stimulation of the
central nervous system and rapid breathing, leading to loss of breathing.
800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (
lc50).
Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
I dont think you have a h2s problem unless you have a petro/chem plant under your stairs.
But i could be wrong