When your rib box shrinks to smaller than it should be ,your heart and lungs struggles to do it job ,no matter how much you eat or exercise . your heart and lungs needs room to do their job.This is achievable only by practising yoga and, more over SMOKERS you would have felt the difficulty to breath sometimes,some pain in your lungs or near your stomach ,while sleeping you would have felt some pain on your lungs,you can overcome this by practising the following asana every day....
How to do :
1.sit comfortably in a straight chair,hands resting palms down on your thighs.
2.start breathing slowly,way down deep.
3.Expand the breathing till your rib box is popping from front to back and from side to side