What are the dangers of low blood pressure?

Dizziness , fainting: If it is low enough , it can cause dizziness , and fainting.

How low?: It's normal for healthy 30 year olds to have "low bp" and this is actually a sign of above average health. For instance, 80/60 if you're feeling fine and otherwise healthy would be very good. But if your normal BP is 140/80, you become ill, and your BP drops to 80/60, it's quite serious! so, it's relative. There are limits: i've never seen a healthy adult with a BP of <80 systolic (top number).