Please cover your mouth when you yawn

We've all seen it. People yawning on BART, letting out a huge breath of air from the mouth that hits the passengers next to them on BART. Maybe it's just me but I really don't like it when someone's massive exhale blows on me so hard that it makes my hair move. I usually hold my breath for at least 10 seconds when that happens. I really don't feel like smelling someone else's breath. Besides, I personally ALWAYS cover my mouth when I yawn, or minimize my yawn within my mouth. Why don't other people do that?? Instead, they must announce to the train that they are tired by letting out a heavy breath of air. Just another pet peeve of mine. I think it's basic etiquette.