Is it safe to breathe in air through mouth?

It is safe to breathe through the mouth, but it is a tiny, tiny bit better to breathe through the nose.

If you breathe through the nose, the air has to travel longer before it reaches the lungs. And that longer joúrney means that any dust in the air has had a better chance of getting caught on the inner surface of the airways, the air will be closer to body temperature, and if the air is dry, it'll have picked up some moisture along the way as well too. Not actually required, but things that'll make life easier on the lungs.