What kind of long term damage can be done when a baby's oxygen level is (for an extended period of time) 30%? (on into adulthood)

Much depends on. The gestational age at birth, the reason for the hypoxemia, the duration, & if/when episodes of decreased oxygen flow to the fetal brain occurred prior to labor & delivery.The best prognostic measures are MRI findings at the time of discharge from the nicu, the presence & severity of hypoxic ischemic encephalopathy in the newborn period & response to early intervention developmental therapies.