atmospheric dust

dust, atmospheric, minute particles slowly settling or suspended by slight currents and existing in varying amounts in all air. There is least dust at high levels over the ocean and most at low levels over cities; dust from smoke is a serious urban problem (see air pollution). Sources of atmospheric dust are winds blowing over dry earth (plowed fields, deserts, and roads), the various products of combustion, volcanic eruptions, salt spray from the oceans, pollen and other material from plants, and meteoric particles. The detonation of nuclear devices in the atmosphere creates radioactive dust (fallout), a serious hazard to all forms of life.

Dust sometimes settles quickly on surfaces, but vast quantities are carried to the upper layers of the air and suspended there for long periods of time. The effects of a volcanic eruption such as that of Krakatoa in Indonesia have been observed three years after its occurrence. Large seasonal dust storms occur in the Sahara and neighboring W Africa and in the Taklimakan and Gobi deserts and neighboring NE Asia; Beijing is annually affected by such storms. Dust from large storms in Africa often travels as far as the S United States and the Caribbean, where it can affect air quality, and dust from the Gobi Desert in Asia has been carried as far east as Minnesota. Such dust storms, which are aggravated by desertification, can have negative health and economic effects; in addition to potentially harmful mineral particles, the dust may include bacteria, fungi, viruses, and various pollutants.

Hygroscopic dust particles (those to which water adheres) are the nuclei of condensation in free air; the nucleus of each droplet in a fog or cloud and of each raindrop and snowflake is one of these invisible particles of inorganic or organic dust. John Aitken, a Scottish physicist who in 1880 invented a device for counting particles in air, first correlated dust particles and condensation. Dust is also chiefly responsible, through its scattering effect upon light (diffusion), for one type of haze and for sunrise and sunset colors.

Domestic dust

Composition and types

The precise composition of domestic dust can vary widely:

The quantity and composition of house dust varies greatly with seasonal and environmental factors such as the surroundings, exchange of outside air, age of the house, building materials and their condition, and the quantity of furniture and carpets, as well as their state of preservation. It varies further with ventilation and heating systems, cleaning habits, activities of the occupants or users of a room, etc. House dust consists of inorganic and organic matter, yet the relative proportions of these components may vary considerably. "House dust" from kindergartens often consists almost completely of inorganic materials such as sand, loam, and clay from sand pits. On the other hand, house dust from residences of animal owners with worn out carpets may consist nearly completely of organic material. The proportion of organic matter in 318 house dust samples was found to vary between <5% and >95% (Butte and Walker, 1994). Fergusson et al. (1986) reported the organic content of house dust from 11 homes in Christchurch, New Zealand, to be within the range from 25.7% to 56.5%. Floor dust from seven Danish offices had a mean organic fraction of 33% (Mølhave et al., 2000).

According to the German Environmental Survey, approximately 6 mg/m²/day of house dust is formed in private households, depending primarily on the amount of time spent at home. Nearly 1000 dust particles per square centimeter settle on domestic surfaces every hour. Some dust consists of human skin; it is estimated that the entire outer layer of skin is shed every day or two at a rate of 7 million skin flakes per minute, which corresponds to a mass emission rate of about 20 mg/minute. "Dust bunnies" are little clumps of fluff that form when sufficient dust accumulates.

Domestic dust and humans

Insects and other small fauna found in houses have their own subtle interactions with dust that may have adverse impact on the health of its regular occupants. Dust may worsen hay fever. Thus, in many climates it is wise to keep a modicum of airflow going through a house, by keeping doors and windows open or at least slightly ajar. In colder climates, it is essential to manage dust and airflow, since the climate encourages occupants to seal even the smallest air gaps, and thus eliminate any possibility of fresh air entering.

House dust mites are on all surfaces and even suspended in air. Dust mites feed on minute particles of organic matter, the main constituent of house dust. They excrete enzymes to digest dust particles; these enzymes and their feces, in turn, become part of house dust and can provoke allergic reactions in humans. Dust mites flourish in the fibres of bedding, furniture, and carpets.

On the other hand the hygiene hypothesis suggests that the modern obsession with cleanliness may be counterproductive; that it may in fact encourage the development of health conditions including hay fever and asthma.

Atmospheric dust

Airborne dust is considered an aerosol and can have a strong local radiative forcing on the atmosphere and significant effects on climate. In addition, if enough of the minute particles are dispersed within the air in a given area (such as flour or coal dust), under certain circumstances this can be an explosion hazard .

Coal dust is responsible for the lung disease known as Pneumoconiosis, including black lung disease, which occurs among coal miners. This danger has resulted in a number of laws (only some being passed) regulating environmental standards for working conditions.

Road dust

Dust kicked up by vehicles travelling on roads may make up 33% of air pollution. Road dust consists of deposition of vehicle exhausts and industrial exhausts, tire and brake wears, dust from paved roads or potholes, and dust from construction sites. Road dust represents a significant source contributing to the generation and release of particulate matter into the atmoshpere . Control of road dust is a significant challenge in urban areas, and also in other spheres with high levels of vehicular traffic upon unsealed roads such as mines and garbage dumps. Road dust may be suppressed by mechanical methods like sweeping vehicles, with vegetable oils, or with water sprayers.

Dust control

Dust control is the Suppression of solid particles with diameters less than 500 micrometers. This is because dust in the air is a serious health threat to children, older people, and those with respiratory illnesses.

Control of domestic dust

House dust can be controlled by many methods and devices, including:

wiping with a wet cloth or sponge
dusting with a feather duster
sweeping with a broom
mopping
use of a vacuum cleaner
an air filter within an air conditioning system.

The particles that make up house dust can easily become airborne, so care must be exercised when removing dust, as the activity intended to sanitize or remove dust may make it airborne. The device being used traps the dust; however, some may become airborne and come to settle in the cleaner's lungs, thus making the activity somewhat hazardous. Another way to repel dust is with some kind of electrical charge .

Control of atmospheric dust

The U. S. Environmental Protection Agency (EPA) mandates facilities that generate dust must work to minimize it in their operations. The most frequent dust control violations occurred at new residential housing developments in urban areas. Federal law in the United States requires permits for earth moving at construction sites, include plans to control dust emissions. Control measures include such simple practices as watering down construction and demolition sites, as well as preventing dust from being tracked out onto adjacent roads. US federal laws require dust controls on sources such as vacant lots, unpaved parking lots, and unpaved roads. Dust in such places may be suppressed by mechanical methods include paving or gravel, or by stabilizing the surface with water, vegetable oils or other dust suppressants, or with water misters for dust which is already airborne .

Dust in other contexts

Dust in outer space

Cosmic dust is widely present in space, where gas and dust clouds are primary precursors for planetary systems. The zodiacal light, seen in the sky on a dark night, is produced by sunlight reflected from particles of dust in orbit around the Sun. The tails of comets are produced by emissions of dust and ionized gas from the body of the comet. Dust also covers solid planetary bodies, and vast dust storms can occur on Mars that can cover almost the entire planet. Interstellar dust is found between the stars, and high concentrations can produce diffuse nebulae and reflection nebulae.

Dust samples returned from outer space could provide information about conditions in the early solar system. Several spacecraft have been launched in an attempt to gather samples of dust and other materials. Among these was Stardust, which flew past Comet Wild 2 in 2004 and returned a capsule of the comet's remains to Earth in January 2006. The Japanese Hayabusa spacecraft is currently on a mission to collect samples of dust from the surface of an asteroid.

Dust in fiction

In JM Barrie's children's novel Peter Pan (1911), "pixie dust" is a substance used to help make people fly who can't already.
In Arthur C. Clarke's A Fall of Moondust (1961), 21st century tourists "cruise" across the Moon's "Seas" that have filled over eons with very fine dust, which is so fine that it almost behaves like water.
"Dust" is an important plot point in His Dark Materials series by Phillip Pullman.

Dust in religion

In ancient Sumerian mythology the afterlife consists of the dreary "House of Dust and Darkness".
In Genesis 3:19, God — following The Fall, Adam and Eve's transgression — states to the couple: By the sweat of your brow you will eat your food until you return to the ground, since from it you were taken; for dust you are and to dust you will return. [Emphasis added] This latter clause is used in the Ash Wednesday service in some churches for the administering of ashes, and is adapted in funeral services to the common prayer "Dust to Dust".
In Genesis 13:16, God states to Abram (later Abraham): I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted.

Note however that in both of these Biblical passages, the Hebrew word is עפר (`âfâr), which can also mean earth or dirt.

In John 8:1-11, Jesus writes on the ground, or as it is often interpreted, dust.

References

Holmes, Hannah; (2001)The Secret Life of Dust. Wiley. ISBN 0-471-37743-0
Steedman, Carolyn; (2002) Dust. Manchester University Press. ISBN-13: 978-0719060151