Abdominal, or stomach, breathing seems to increase relaxation. It works best when it is slow and deep. Concerning exhaling, since most people have a tendency to breathe shallowly, it may be a good idea to completely empty the lungs occasionally. Shallow breathing causes air to sit around in the lungs for an extended period of time and it can become stale. Blowing out this air will allow it to be replaced by a batch of fresh air.
Smoking tobacco limits the quantity and quality of oxygen being inhaled.
The result can be a lack in endurance and difficulty with breathing. Some people may feel that they are not getting enough air during exercise. One reason is that the tar from tobacco smoke can clog the passages within the lung's bronchioles. This is where the gas exchange takes place between the lungs and the bloodstream. If this process is limited, the body will not get enough oxygen in its bloodstream to function at peak efficiency.