Yes, H2S is suppose to be detectable by your nose at low or high levels, but the nose can't tell if it is a dangerous level or not. But in any event, we have never detected any levels of H2S in any mine. Methane, we have detected in only VERY low levels. I think the highest we ever detected is about 2 or 3% of the LEL of Methane. This would be 2 - 3% of the lower explosive limit of Methane. Methane would need to be 100% of the Lower Explosive Limit before it would be explosive.
For low levels of O2, like Chris said, it is generally very predicable by the mine. Certain mines will have consistent problems based on the temperature outside, and where you are in the mine. This is only predicable in the sense that we know which mines have issues with lower oxygen levels and where the problems are in the various mines. Some anthracite mines you can't go in safely in the winter, other anthracite mines will have issues in the summer. Some mines have perfect oxygen levels anytime of the year. Needless to say, you should always be sure to carry the air monitoring equipment with you in the event you hit a mine with air troubles. A mine which is fine in the winter may have bad oxygen in the summer.
Greg
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