Yes, very dangerous... Low bp is called hypotension.
Hypotension usually means blood pressure that is lower than 90/60 mmhg (millimeters of mercury) or is low enough to cause symptoms. When the blood pressure is too low there is inadequate blood flow to the brain and other vital organs, light-headedness or fainting may occur.
A common type of hypotension is postural (also known as orthostatic) hypotension, in which suddenly standing leads to light-headedness and fainting.