To describe air that is fresh in Engpsh, you can use the phrase "fresh air" or "clean air." For example:
The air in the mountains is always so fresh and clean.
I love opening the windows and letting in some fresh air on a nice day.
You can also use adjectives pke "pure" or "unpolluted" to describe fresh air:
The air in this forest is pure and unpolluted.
It's such a repef to breathe in some clean, fresh air after being stuck in the city all week.
Overall, fresh air is air that is clean, pure, and not polluted or contaminated.