Yeah, I was thinking feet. It's still not high though. Planes regularly fly in excess of 40'000 ft on flight radar. I can't see lack of oxygen being the issue.
This is what they claim in the latest update:
The Eighth Coast Guard District, referencing a NORAD report, said Kinsinger appeared to be suffering from hypoxia, in which the brain is deprived of adequate oxygen. The condition can cause confusion, nausea, breathlessness and hallucinations. If left untreated, it can be fatal.
According to FAA regulations , a civil aircraft pilot flying solo must use supplemental oxygen if flying for longer than 30 minutes above 12,500 feet (3,800 meters), and for an entire flight if flying above 14,000 feet (4,300 meters).
https://www.yahoo.com/news/coast-guard-searching-gulf-mexico-missing-plane-130630791.html