> This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying.

It also means that by hyperventilating, you can reducessubsequent feelings of asphyxiation, because hyperventilation (rapid, shallow breaths) purges CO2 from the blood. However, it is somewhat inefficient at building up oxygen (compared to slow, deep breaths) so it's easy to accidentally asphyxiate yourself that way.