Derbyshire Natural Minerogenic Dust and Human Health

Atmospheric aerosols include gases and liquids, as well as solid particles. They include material derived from both oceans and landmass, as well as particles that form within the atmosphere, such as sulfates. Solid particles entrained in the atmosphere by detachment from poorly vegetated surfaces (the process of deflation) include, for example, rock and mineral dust, fine mineral aggregates, fibrous minerals, fibrous organic materials, and sea salt. The burning of vegetation (biomass) yields black carbon, which adds to the opacity of the atmosphere. Smoke plumes from fires, both natural and anthropogenic, are often carried thousands of kilometers from their sources, so increasing the air pollution hazard. In addition to the strongly seasonal inputs from the world’s drylands, mineral-dust loading of the atmosphere is also enhanced by injection of fine volcanic mineral particles (tephra) containing variously toxic minerals. This paper considers only those dusts that are derived naturally from the land surface, especially in and around the world’s drylands. Anthropogenically generated dusts (such as those derived from industries, vehicles, etc.) are not considered here, although mention is made that both inorganic and organic toxic substances may become attached to natural atmospheric dusts, including toxins of human origin. Atmospheric aerosols also influence the chemistry of the troposphere, including the proportion of ozone. Particle size and chemistry of the dust load affect air temperatures (by varying absorption and scattering of solar radiation). Given the present global warming trend, progressive desertification, and human actions that continue to increase the atmospheric dust loading, the intimate relations between aerosols and the global environment have obvious implications for future climatic change (1), and yet further indirect effects on human health around the world. Detachment of mineral dust from the ground surface (‘‘deflation’’), and its entrainment and transport by the wind is a function of several variables, including wind speed (notably the critical wind speed or threshold velocity required to dislodge particles), the degree of atmospheric instability, the size and shape of the particles, the roughness and moisture content of the land surface, and the degree of particle exposure. The claysize (,2 lm: Fig. 1) component of soils and sediments is not readily detached from a land surface by the wind as individual particles because of the high interparticle cohesive forces typical of such colloidal materials. Entrainment of these finer particles usually occurs in association with the coarser (silt-sized) grains, as well as in the form of coarse or medium silt-sized aggregates made up of variable mixtures of fine silt and clay-grade particles. Once entrained, however, fine dust particles may travel a thousand or more kilometers before being deposited, some of the finest particles being transported as much as 20 000 km from their source (2). Travel distance is largely a function of particle size and atmospheric conditions, the coarser fractions being deposited much closer to their source. The medium and coarser fractions of such dust (mainly in the silt range: 2–63 lm) may accumulate to form the sediment type known as loess (3, 4). Loess is made up of wind-lain geogenic dust, which is subject to varying degrees of postdepositional alteration, particularly by the processes of weathering and soil formation. Loess has been accumulating on the continents, especially Eurasia and the Americas, for millions of years. Its susceptibility to erosion by both wind and water, especially along desert margins and on degraded dryland surfaces, make it a secondary source of windblown mineral dust, as mentioned further on. Dominant dust sources around the world are almost wholly in or adjacent to the great drylands of the northern hemisphere. The greatest of these includes a broad swathe of land across North Africa, the Middle East, northwest India, and central and eastern Asia—from the western Sahara to the Yellow Sea. Other notable sources are found in the Great Basin of the United States and, in the southern hemisphere, east-central Australia, central and northern Argentina, and parts of southern Africa. In both North Africa and China, dried-out former lake beds are a major source of fine, readily deflated mineral dust (e.g., 5, 6). The Bodélé depression [Chad, North Africa (7)] and the numerous lake depressions in northern China and central Asia are major dust sources of global significance (Figs. 1 and 2). Saharan dust, driven by the northeast trade winds, takes about a week to cross the Atlantic Ocean, reaching northeastern South America in the (northern) late winter and spring, and the Caribbean, Central America, and the southeastern United States in summer and early autumn (8) ( Fig. 2 ). The midlatitude deserts of Asia are a source of substantial airborne dust, especially during spring and early summer. The two major Chinese dust sources (Mongolia and the Tarim Basin–Taklamakan Desert) are of global importance, fine dust from both these regions having been traced to North America, Greenland, and Europe. Driven in the winter half year by large ‘‘Siberian High’’ pressure cells, locally easterly winds flow around the southern flank of the seasonal high pressure cell that