What causes blood oxygen levels to become low?

Blood oxygen levels can get low due to any of the problems that include:

• Low air oxygen levels: Atmospheric oxygen becomes extremely low at high altitudes such as mountainous regions.
• Decreased capacity of the body to take in oxygen:
  • This can be caused by lung conditions that include:
    • Asthma
    • Emphysema (damage of the air sacs in the lung)
    • Bronchitis
    • Pneumonia
    • Pneumothorax (leaking of air in the space between the lung and chest wall)
    • Acute respiratory distress syndrome (ARDS)
    • Pulmonary edema (the lung swells due to build-up of fluid)
    • Pulmonary fibrosis (scarring of the lungs)
    • Interstitial lung disease (a large group of lung disorders that generally cause progressive scarring of the lungs)
    • Viral infections such as COVID-19
• Other conditions include:
  • Anemia
  • Sleep apnea (temporary cessation of breathing during sleep)
  • Smoking
• Decreased capacity of the heart to supply the oxygenated blood back to the lungs: The most common causes are congenital heart diseases (heart defects present by birth).