Normally a human being breathes mainly through one nostril and the other comparatively idle or with lesser effect. This is where Pranayama helps, by breathing through one nostril, exhale through the other, inhale through the same and exhale through the other. This process balances the breathing muscle activity, and increases the O-2 levels in blood to saturation.-95-99%. I have seen blood Oxygen levels go as low as 53% and still patients do not show the symptoms of hypoxia, unless the blood Oxygen level is measured. The finger-tip Oxypulse meter is available and the readings are instant and accurate. Inadequate gas exchange in lungs due to infection or inner coating/ blockade by phlegm are main causes and this might cause increase in PAP- Pulmonary Artery Pressure, that might be a heavy load on heart and at times dangerous. Homoeopathy has excellent remedies to clear the lungs and bring back the O-2 levels to normal. I know of a case in Sundaram Medical Centre Annanagar, where the patient could not be revived for 5 days and was to be shifted to ICU for fixing up ventilator, but was brought back to 83% O-2 within 48 hours by Homoe remedies, given as "Theertham" from temple in the ICU. Banyan Tree- both Peral or Arasamaram have the highest rate of produciing Oxygen during daytime by photosynthses absorbing CO-2 effectively. Greenery like Thulasi at the entrance of the house augurs well for Oxygenating the air that enters the house.