Great discussion, guys.
One note about flying at 12,500...There's nothing in the regs that say you can't fly above this altitude for 30 min, then sink below, then climb back up with a new 30 min clock. Not safe, but not illegal, either.
If anyone was curious about the types, they are hypoxic, histotoxic, hypemic, and stagnant. Hypoxic is due to altitude; histotoxic is caused by toxins interfering with the cells' ability to use oxygen (alcohol, drugs, etc); hypemic is due to smoking (CO bonds with hemoglobin better than oxygen); and stagnant is from blood pooling like when your arm falls asleep.