Low oxygen levels in the blood, known as hypoxemia, occur when the oxygen saturation falls below 90%, a dangerous threshold that can severely impact organ function. Causes of low oxygen levels include lung conditions, heart failure, and anemia. Normal oxygen levels for adults typically range between 95% and 100%, ensuring proper oxygenation to vital organs. Monitoring oxygen saturation with a pulse oximeter is critical for those with chronic health conditions to avoid complications like confusion, shortness of breath, or fainting.