Understanding H2S in Drilling Operations Hydrogen Sulfide (H₂S) is a highly toxic and flammable gas commonly encountered in oil and gas drilling—especially in sour fields like those in parts of Iraq and Kurdistan. Colorless but has a rotten egg smell at low concentrations. Heavier than air, so it tends to accumulate in low-lying areas. Exposure at even low ppm levels is hazardous; concentrations over 100 ppm can be immediately life-threatening. Normal Response to H2S Detection Standard protocol