If memory serves:
1) The most dangerous aspect of Hypoxia can be that you don't notice it coming. Most dangerous symptom, can you nail it down....death? I guess bad decision making is a good one. Symptoms: Euphoria, lack of good judgement, lack of concentration, fatigue, blurred vision (lack of night vision a first symptom), tunnel vision, unconsciousness, death.
2) 1-100Hz
3)18,000 ft.