how low does the oxygen level have to go before you need a ventilator?

This is a tricky question because it is not necessarily only based on the oxygen (O2) level. There are several reasons why a patient would need to go on a ventillator. It is often a judgement call from the physician backed up by other findings. If the decision is based on oxygen, generally the answer would be when we are unable to provide enough external oxygen to keep a patient’s O2 saturation up. For healthy individuals a normal O2 sat is close to 100%. We start to worry if it drops to 90% or below. Room air is about 21% oxygen. We can increase that at most to about 40-50% by using a mask to administer O2. If we put in a breathing tube and use a respirator we can get that to 100%.Other factors (CO2 retention, airway protection, etc) can lead to mechanical ventillation which I will not get into because the answer would get too long.

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A far more important indicator is whether or not the patient is able to produce effective ventilations on his or her own to keep the O2 levels adequate.