If the low pressure lies to the north of us, the air rushes northward over us to fill it, and we say the wind is from the south; if the air is flowing to the south of us, we say the wind is from the north.
How Air is Purified.

In these winds certain small amounts of dust, or dirt, or leaf mould are whirled up into the air, but these are promptly washed down again whenever it rains; and the same is true of the smoke impurities in the air of our great cities.

Air is also constantly being purified by the heat and light of the sunbeams, burned clean in streaks by the jagged bolt of the lightning in summer, and frozen sweet and pure by the frosts every winter.

So that air in the open, or connected with the open, and free to move as it will, is always pure and wholesome.

But to be sure of this, it must be "eaten alive"-- that is, in motion.
Stagnant air is always dead and, like all dead things, has begun to decay.
The Carbon Dioxid in the Air.