Has anyone encountered low oxygen environments in their explorations? I know to keep this in mind while in the mines but how much of an issue is it in buildings, confined spaces and tunnels. I assume asbestos, mold, rodent and bird droppings are a far bigger concern than bad air?

Low oxygen levels below 19.5% is an immediate danger to human life.
H2S levels above 15 PPM are dangerous for prolonged exposures.

Physiological responses to various airborne concentrations of H2S
Air Concentration: Physiological Response
1,000-2,000 ppm: Loss of consciousness and possible death
100-1,000 ppm: Serious respiratory, central nervous, and cardiovascular system effects
150-200 ppm: Olfactory fatigue (sense of smell is significantly impaired)
100 ppm: Immediately Dangerous to Life and Health (IDLH concentration)
5-30 ppm: Moderate irritation of the eyes
5-10 ppm: Relatively minor metabolic changes in exercising individuals during short-term exposures
Less than 5 ppm: Metabolic changes observed in exercising individuals, but not clinically significant
5 ppm: Increase in anxiety symptoms (single exposure)
5 ppm: Start of the dose-response curve (short-term exposure)
0.032-0.02 ppm: Olfactory threshold (begin to smell)
Adapted from ACGIH and NIOSH sources

Never enter a confined space unless you are certain there is adequate oxygen and levels of H2S are in the safe range. H2S is an insidious poison. I suggest you read more about it.

Mold, asbestos,etc can be a long term health hazard, poisonous or low level O2 atmospheres can kill you in minutes or less.

I've ran into H2s mostly in poorly draining caves and cisterns. Given the amount of rot and stagnation that can build up in a tunnel or floors extending underground, I'm starting to wonder if it's worthwhile to carry a multi-gas air meter. If there's anything positive about H2s is you can smell it; unlike co2, natural gas and methane.

Posted by OscarZulu I've ran into H2s mostly in poorly draining caves and cisterns. Given the amount of rot and stagnation that can build up in a tunnel or floors extending underground, I'm starting to wonder if it's worthwhile to carry a multi-gas air meter. If there's anything positive about H2s is you can smell it; unlike co2, natural gas and methane.

Unfortunately you need to learn a lot more about H2S. Your sense of smell is useless after exposure;it's one of the first things to go. If you walk into >500 PPM you will most likely be unconscious with your first breathe. Even if you try to hold your breathe, the gas can trigger an involuntary cough or gasp. At concentrations of 700 PPM or higher you will instantly go into respiratory arrest. Without immediate CPR you will be dead in minutes. Even if you survive a high dose exposure chemical pneumonia is likely and can be fatal. Individual tolerances can vary drastically. Drinking the night before increases sensitivity to it.

This is a -insidious- poison. I know people who barely got away with their lives and only because someone literally risked their lives without SCBA to save them. Do not underestimate it!

With no moving ventilation the hazards increase exponentially if it is present. Rust scale, mud, and decaying matter can trap large amounts unbeknownst to you until you disturb them. Many people have been killed like this.

I'm aware of this, but I also recall eye and respiratory irritation at around 65 ppm on my meter. Yes I still have much to learn about H2s and urbex in general; Which is why I came here seeking input from the experienced.

not only CPR but without defibrillation (shock) the chances of survival narrow with every minute that passes. CPR prolongs death, defib restarts the heart

The 1st time as an onshore oilfield roustabout we were doing well surveys for Shell in Denver City, TX. The process consisted of me jumping into a hole dug around the wellhead and just below the casing, typically about 3'-4' below grade then doing stuff that's not important here. Everyone wore H2S monitor's.

Twice I jumped in and completely blacked out immediately. I woke up with an oxygen mask over my face and people hovering over me. I was lucky the first time. The second time was a frickin miracle.

Buy all the detection equipment you want. If the concentrations are high enough you won't even hear the buzzer go off...

Posted by Peptic Ulcer The 1st time as an onshore oilfield roustabout we were doing well surveys for Shell in Denver City, TX. The process consisted of me jumping into a hole dug around the wellhead and just below the casing, typically about 3'-4' below grade then doing stuff that's not important here. Everyone wore H2S monitor's. Twice I jumped in and completely blacked out immediately. I woke up with an oxygen mask over my face and people hovering over me. I was lucky the first time. The second time was a frickin miracle. Buy all the detection equipment you want. If the concentrations are high enough you won't even hear the buzzer go off...

The idea was to test before entry. With oil wells being notorious for producing that very gas, what the hell were they thinking just letting you jump in?

Posted by RescueMe1060 not only CPR but without defibrillation (shock) the chances of survival narrow with every minute that passes. CPR prolongs death, defib restarts the heart

This is assuming they've gone into arrhythmia or fibrillation correct?

Posted by RescueMe1060 not only CPR but without defibrillation (shock) the chances of survival narrow with every minute that passes. CPR prolongs death, defib restarts the heart

Depending on the concentration even with defib and pure O2 revival may be impossible. Nothing short of a hyperbaric chamber with pure O2 could save you in that event. That's not going to happen...

Like carbon monoxide there is a point of no return. Like CO, H2S displaces O2 from the hemoglobin. One of the reasons H2S is so lethal.

Posted by Peptic Ulcer The 1st time as an onshore oilfield roustabout we were doing well surveys for Shell in Denver City, TX. The process consisted of me jumping into a hole dug around the wellhead and just below the casing, typically about 3'-4' below grade then doing stuff that's not important here. Everyone wore H2S monitor's. Twice I jumped in and completely blacked out immediately. I woke up with an oxygen mask over my face and people hovering over me. I was lucky the first time. The second time was a frickin miracle. Buy all the detection equipment you want. If the concentrations are high enough you won't even hear the buzzer go off...

People who never worked around H2S have no idea how treacherous this gas is. Glad you survived. The oilfields can be crazy dangerous.

Posted by OscarZulu The idea was to test before entry. With oil wells being notorious for producing that very gas, what the hell were they thinking just letting you jump in?

To safely test it you would need to be wearing SCBA or lower a probe into the area, and that test point(s) would valid for that one area. Assuming you didn't disturb the substrate and cause a release.
If walking through a tunnel, concentrations can change.