Dust consists of dry, solid particles less than 0.0625 millimeter (1/400 inch) in diameter–smaller than the tiniest grains of sand. Most dust is mineral material that comes from bare soil, plowed fields, river flood plains, and floors of desert basins. Mineral dust is one type of atmospheric aerosol, tiny particles that float in air. Other atmospheric aerosols include salt from ocean spray, fragments of meteoroids, decaying organic materials, and smoke and ash produced by fires, power plants, and volcanic eruptions. The wind lifts such particles and often carries them for long distances. Winds in the stratosphere can carry fine volcanic dust around the world several times.
Dust deposits.
The wind picks up and deposits fine particles of dust repeatedly. The coarsest grains, however, tend to settle rapidly to the surface and remain there. Dust washed into streams may reach the oceans and gather as mud.
Thick deposits of coarse dust called loess have formed on the land in central Asia and east-central China, central and eastern Europe, central and northwestern United States, and Argentina. Deposits in China reach a thickness of 1,300 feet (400 meters). One major source of loess is the floodplains of meltwater streams that drained the major ice sheets during the glacial ages. Desert basins are another major source of loess.
Influence on climate.
Dust particles in the atmosphere scatter sunlight, reflecting some of it back into space. Volcanic dust can thus lower surface temperatures, as seen after the eruption of Mount Pinatubo in the Philippines in 1991. Also, particles of dust can cause water in the atmosphere to condense around them, influencing the formation of clouds and precipitation.
Influence on human health.
Large amounts of dust are in the air at many quarries, mines, and factories. Workers who breathe in much of the dust can develop such lung diseases as black lung, brown lung, and silicosis (see Black lung ; Brown lung ; Silicosis ). Dust can also carry bacteria that cause diseases. The spore stages of some disease bacteria can be thought of as dust particles. Organic materials that are in house dust may also cause allergies.
