Normal blood pressure readings are usually 120/80 but for some people a normal blood pressure reading can be as high as 135 over 85.
However once your blood pressure reading starts getting higher than 135 over 85 then you're getting into hypertension and should start lowering your blood pressure.
A dangerous high blood pressure reading is anything over 180 over 20 or 180/20 and if it does get to 180 over 20 and won't go down you need to call 911 or get to the emergency room right away.
Now a really, really dangerous high blood pressure reading is anything 200 or over which can be deadly quickly if you do not get medical treatment.
High blood pressure is not healthy and can cause heart attacks, strokes, etc and death if not lowered in time.