Yet humans are rarely attacked by a shark, while millions of sharks are killed by humans. Some populations of shark species, such as the shortspine spurdog, may have dropped by 95 percent. The sharksâ population decline has a ripple effect â throwing entire marine ecosystems out of balance.
Quick Answer. A normal blood oxygen level during sleep is approximately 94 to 98 percent, according to Sleep Apnea Guide. Levels below 90 percent oxygen saturation are dangerous to the human body. The human brain can only survive four minutes without oxygen before cell death occurs.
Oxygen saturation refers to the level of oxygen carried by red blood cells through the arteries and delivered to internal organs. As red blood cells travel through the lungs, they are saturated with oxygen. A low saturation level could indicate a respiratory illness or other medical condition.