Did you know that when you yawn you help yourself breathe? This is because yawning ventilates the lungs more completely. With regular breathing, obviously not all air cells called alveoli in the lungs are ventilated equally. Some of these cells can sometimes be closed. The blood that passes through them enters the arteries without being fully oxygenated. This dilutes the average oxygen content of the blood.
The way these closed air cells in the lungs can be opened is with a long, deep yawn. And since most of the muscles in the body are involved in this action, yawning can also be used to force stagnant blood out of the vessels where it was collected. When you yawn, your throat also opens wide and is completely relaxed for a while. It's good for your voice when you speak.
What can you do if you hear a yawn approaching? Maybe you can just put your hand lightly over your mouth and reap the benefits of a full yawn.
Good one