What is dust in the air

Various kinds of materials, such as rocks, automobile tires, street and highway surfaces, the skin and hair of animals, soil, and clothing, are constantly being reduced to dust. Some of this dust is fine enough to be carried by the air even when the air is scarcely moving.

Some of it can be carried only when strong winds are blowing. Dust discharged from volcanoes is sometimes so fine that it may remain in the air for many months or even years, and may circulate several times round the earth before falling.

Dust consists partly of small living things that have become dry. Some of this living dust has been collected from more than six miles above the earth. It has been found high above the oceans, as well as above the land.

When living dust settles to the ground in favorable places, it proceeds to grow, Thus the air may be said to carry both living and nonliving dust.