Thanks for the links, I have started perusing them. As someone pointed out in a reply to another comment in this thread, oxygen is especially toxic 33 feet bel

Thanks for the links, I have started perusing them. As someone pointed out in a reply to another comment in this thread, oxygen is especially toxic 33 feet below the surface of the water (because of higher pressure), but can have toxic effects at the surface too. I don't grok respiration yet, but my understanding is that altitude/pressure is essential for putting the relative N2/O2/CO2 partial pressures into context.