Hydrogen Sulfide (Different health effects on different concentrations)

Hydrogen sulfide is the chemical compound with the formula H₂S. It is a color-less gas with the characteristic foul odor of rotten eggs; it is heavier than air, very poisonous, corrosive, flammable, and explosive. (Wikipedia)

Concentration in parts per million (ppm)*	Observations and health effects
Less than 1	Most people smell “rotten eggs.”
3 to 5	Odour is strong.
20 to 150	Nose and throat feel dry and irritated. Eyes sting, itch, or water; and “gas eye” symptoms may occur. Prolonged exposure may cause coughing,  hoarseness, shortness of breath, and runny nose.
150 to 200	Sense of smell is blocked.
200 to 250	Major irritation of the nose, throat, and lungs occurs, along with headache, nausea, vomiting, and dizziness. Prolonged exposure can cause fluid build-up in the lungs (pulmonary edema), which can be fatal.
300 to 500	Symptoms are the same as above, but more severe. Death can occur within 1 to 4 hours of exposure.
Above 500	Immediate loss of consciousness. Death is rapid, sometimes immediate.

* 1 ppm = 1 part of gas per million parts of air by volume