what will happen if we start exhaling hydrogen gas? if everything around will get burned, why it is so?
The consequences of exhaling hydrogen gas are minimal.
Hydrogen may be notorious for being highly flammable, but it is actually quite stable. Like most flammable materials, hydrogen needs an actual ignition source, and human body heat isn't enough to do it. Even if you wore braces and chewed Aluminum, it would be unlikely that you would ignite exhaled hydrogen.
Hyrdogen has the lowest density of all gases at atmospheric pressure. That is why the German Zeppelin company used hydrogen to fill their dirigibles (including the infamous Hindenburgh). If you exhaled hydrogen, the gas would quickly disperse and rise through the surrounding atmosphere.
The final effect, would be similar to the effect of similarly low density gas, Helium. You've probably tried inhaling the gas from a balloon, and giggled at how it turned your voice to "Alvin and the Chipmunks." This was caused by the Helium's low density enabling your vocal chords to vibrate faster than normal, producing that high squeaky voice. If you exhaled hydrogen, you'd get the same effect.