In Refence to the Transit Museum Post
From Eric L., in reference to subway workers having badges specifying which hospital to be taken to in case of an accident:
Often when working below the water table, the area is sealed off with airlocks and high pressure air is pumped in to keep the water out. I normally read about this in the cassions around bridge footings on the riverbed, but wouldn't surprise me for some tunneling work too.
Just like in scuba diving, breathing compressed air adds more nitrogen to your system. One way to look at it is at 2 atmospheres of pressure you're effectively breathing 40% oxygen and 160% nitrogen. Each 33' of water is 1 atmosphere. At 100', you're at 4atm and breathing 80% oxygen and 320% nitrogen. (above 160% at 8atm, oxygen starts to become toxic, so deeper divers will breather oxygen mixtures that have lower than surface %s. At that same depth the 400% nitrogen becomes narcotic, making you drunk, so deeper divers often replace some of the nitrogen with helium to offset that. Lastly, for shallow (less than 100') recreational scuba dives, people often use "nitrox" mixtures with higher-than-normal O2 levels, with 32% and 36% being most common. Using this means you have less nitrogen in your air, so at say 66' with 32% you would have 96% o2 and 204% N2 instead of the 320% with compressed air)
If you do this for long enough you get super saturated with nitrogen in your body, and if you quickly go up to surface pressure of 1atm the nitrogen will come bubbling out of solution like the co2 bubbles when you open a can of soda. Those nitrogen bubbles hurt and can kill you, it's called the bends. To prevent this if you're down working for a full day, you exit into an air chamber that is very slowly reduced in pressure so you can offgas the nitrogen slowly not have it form gas bubbles.
To treat someone with the bends, you need to slap them into a hyberbaric chamber, seal it, and pressurize them back to the pressure they were working at, then slowly over many hours reduce it until you are at 1atm again.
Not every hospital has a hyperbaric chamber, so I am sure those badges were to tell the saturation workers where to go, don't just go to the nearest ER that won't be able to treat the bends.
I did some quick research and confirmed that Eric is right. Whatever limitations there might have been on hospital access for different races/religions back then, those badges were indeed to send the workers to a specific hospital for decompression sickness.
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