http://chemsee-foods.com/hydrogen-sulfide-monitoring/H2S-KT1

Hydrogen sulfide is a flammable, poisonous gas with a characteristic odor of "rotten eggs." H2S can be found in natural gas, petroleum deposits and hot springs, as well as naturally in the breakdown of organic matter by specific bacteria. Often elevated levels of H2S are found near areas with high quantities of animal waste, such as farms. Many industrial facilities produce hydrogen sulfide including paper mills, food processing plants and petroleum refineries.

Hydrogen sulfide may be detected by smell in concentrations as low as 0.30 ppm. At 0.50 ppm slight depression begins. In the range of 20 to 150 ppm eye irritation begins, and at 500 ppm severe headaches occur. At 800 ppm, paralysis begins. The sense of smell is not the best detector for hydrogen sulfide; when the hydrogen sulfide concentration reaches approximately 15 ppm, the human nose becomes desensitized and the ability to smell H2S declines. Moreover, our ability to smell other gases also declines, which can have tragic consequences due to the fact that synergistic toxicity effects of H2S and other gases often exist. The cumulative effect of hydrogen sulfide results in the depletion of iron in the blood and interference with the breathing mechanism. Sufficiently large doses of H2S can cause immediate death. Therefore it is essential to have a trustworthy device to indicate the presence of hydrogen sulfide at any concentration.