Properties of H2S Colour: Colourless (Invisible.) Odour commonly referred to as odour of rotten eggs Vapour density 1.189 (Air=1.0) H2S is heavier than air Explosive limits Highly explosive. (4.3 to 46.) Percent by volume in air Auto ignition temp. 260 degrees C Flammability Forms explosive mixture with air or oxygen Water solubility 2.9 percent (2.9g/100 ml water at 20 degrees C Can destroy steel and rubber seals very quickly. Effects of H2S 10,000 parts per million = 1 percent 1 ppm: Can be smelled. or to put it another way, 1 second in 13 days. Not a lot of room for error. 10 ppm: Occupational Exposure Limit, for 8 hours: At very low concentrations of less than 10-100 ppm, it gives off unpleasant odour - like to rotten eggs. "the threshold limit". 15 ppm: Occupational Exposure Limit, allowable for 15 minutes of exposure. 20 ppm: Occupational Exposure Limit, At this level workers must wear appropriate breathing apparatus. The limits here are set in Canada and conform very much to many other countries 100 ppm: the gas kills the sense of smell in 3-15 minutes and will cause you to cough or your eyes to water, possible headache, nausea, throat irritation. Note~  Smell is only a temporary warning. Do not rely on your nose. 200 ppm: your eyes and throat will begin to burn. and you will get Headaches. Sense of smell lost rapidly. 300 ppm: Immediately Dangerous to Life and Health level. Positive pressure breathing apparatus required. 500 ppm: Loss of reasoning and balance. Respiratory disturbances in 2 to 15 minutes. 700 ppm: Immediate unconsciousness. Death will result if not rescued ASP. 1000 ppm: Immediate unconsciousness. Causes seizures, loss of control of bowel and bladder.  Breathing will stop and death will result if not rescued promptly. Immediate resuscitation needed. Comparison of H2S with other lethal gasses will help you to understand the importance why (a you can not work alone (b the reason for prompt and swift action. H2S has a threshold limit of 20 ppm and a lethal concentration of 600 ppm. Hydrogen cyanide (used in gas chambers) has a threshold limit of 10 ppm and a lethal concentration of 300 ppm. Carbon monoxide (CO) has a threshold limit of 50 ppm and a lethal concentration of 1,000 ppm. You should by now be getting the picture and an understanding of why so much preparation and precautions need to be taken before any attempt is made to drill or workover a well that expects to encounter H2s. Over the following pages we will put together some useful information that will show you that it can be drilled and drilled safely providing you respect and follow the rules. When using the term  "the threshold limit". This is the concentration at which it is believed you may be repeatedly exposed, day after day, without adverse affects.