dust

dust, in general, any microscopic, powderlike particle or collection of powderlike particles that is made up of sand, soil, fragments of organic tissue or manufactured materials, or some combination of these sources and is both light enough to be carried by the wind and heavy enough to be deposited on a surface. Individual dust particles can be large enough to be observed by the unaided eye, and large concentrations of dust driven by wind are often conspicuous as dust devils and dust storms, which can impair normal visibility and make breathing difficult. Inorganic components of dust include small bits of rock, salts, and pollutants, such as insecticides and fumes given off by heavy metals such as copper, iron, and mercury. Organic components include pet dander, hair or fur, skin fragments, textile fibers, and ash—as well as tiny living things such as pollen, mites, and mold spores.

Dust is ubiquitous in natural and human settings. It is suspended in the air and collects on virtually all surfaces, including the surfaces of individual plants, between hairlike structures of animals, and on the ground in cultivated and uncultivated landscapes, as well as on furniture and other elements of the indoor environment. Suspended dust that floats gently in the air may be almost imperceptible unless light from a sunbeam reflects off individual particles. In contrast, volcanic dust (which is ejected as part of a plume of larger particles during an eruption) and walls of dust (such those originating in the Sahara that are transported hundreds of kilometers over the Atlantic Ocean) are large enough to be seen from space (see also harmattan). At interstellar scales, large concentrations of dust particles make up planetary rings, such as those encircling Saturn, and the clouds that form nebulae and galaxies.

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Particle size

Dust particles range in size from 1 to 400 micrometers (μm; 1 μm = 0.001 mm [about 0.00004 inch]). Particles larger than about 100 μm (0.1 mm [0.004 inch]), which is about the width of a human hair, are visible to the unaided eye. Human lungs are capable of filtering out particles as small as 1 μm. However, smaller particles—which include allergens, some types of smoke and ash particles, soot, and dust that remains suspended in the air—can lodge in the lungs and enter the circulatory system.

The inhalation of dust is responsible for a number of lung and respiratory disorders, whose symptoms and severity depend on the particles’ composition and size, the amount of dust inhaled, and the length of exposure. Larger allergens, such as pollen (which ranges in size from 20 to 100 μm), skin fragments (which range in size from 1 to 40 μm), and mold spores (which range in size from 3 to 10 μm), can trigger allergic reactions and asthma. In addition, lung diseases known as the pneumoconioses result when certain inhaled mineral dusts are deposited in the lungs, where they cause a chronic fibrotic reaction that leads to decreasing capacity for exercise and increasing breathlessness, cough, and respiratory difficulty. Such larger particles can deliver into the lungs smaller agents, such as heavy metals, viruses, and bacteria, which can later enter the bloodstream. Soot and tobacco smoke particles (which range in size from 0.01 to 0.15 μm and from 0.01 to 1 μm, respectively), known for their ability to cause cancer and raise the risk of heart attack and stroke in human beings, are small enough to easily pass the body’s blood-gas barrier.

Indoor dust

Within the household, opening the door and entering from the outdoors can ferry dirt and pollen to join the dust accumulating inside. Some research suggests that about two-thirds of household dust comes from outdoor sources such as outdoor dust and air pollution, while the remaining one-third derives from indoor inorganic sources. Along with these main elements of dust, all manner of other chemical compounds tend to show up in the mix. Other components of dust include colonies of mold, shreds of paint, and traces of soot. Dust also contains small fibers of cloth pulled from clothes and furniture. Dangerous chemical pollutants, such as the toxic insecticide DDT, have persisted in U.S. household dust as recently as 2022. Though DDT was banned in most countries in the 1970s due to its dangerous properties, the insecticide has persisted in nature and in households, where it may endanger children and other vulnerable groups.

While human skin cells and hair are not the only component of indoor dust, these materials do make up a significant portion of the mixture. Each human sheds about 500 million skin cells per day. Estimates of the percentage of dust composed of flakes of human skin vary, one study placing the figure as high as 75 to 90 percent. However, the portion of dust derived from living things does not necessarily come from humans alone. Traces of fur, dander, and feathers from pets add to the mix. Such cast-off cells are not the only contribution pets make to dust in the house. Research has indicated that households with dogs have more types of bacteria in their dust than pet-free households do. Other biotic substances, such as small living and dead insects, can also appear in dust. Dust mites, household pests that live in dust, can cause allergies.