Soil-Borne Particles and Their Impact on Environment and Human Health

Abstract

Dust particles can consist of either natural soil-borne particles or of particulate matter from human activities, or both of them. Particulate matter is a complex mixture of extremely small particles and liquid droplets consisting of soil or dust particles, metals, organic chemicals, and acids. Naturally generated particles consist of weathered rock materials, dryland soil and sediment materials, biogenic fibres and residues from forest fires, and ash developed during volcanic eruptions. World dust emissions from drylands amount to about 5 billion Mg per year. 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 large belt from the western Sahara to the Yellow Sea, across North Africa, the Middle East, northwest India, and central and eastern Asia. Saharan dust, driven by the northeast trade winds, takes about a week to cross the Atlantic Ocean, reaching northeastern South America the Caribbean, Central America, and the southeastern USA. The mid latitude deserts of Asia are a source of substantial airborne dust, especially during spring and early summer. Mongolia and the Tarim Basin-Taklamakan Desert are the two major dust sources of China. They are also of worldwide importance, as fine dusts from these regions have been traced to North America, Greenland and Europe. Other notable sources of dusts include the Great Basin of the USA and, in the southern hemisphere, central and northern Argentina, parts of southern Africa and East-central Australia. Former lake basins are major sources of fine, readily wind-entrained mineral dusts, which may include salts and elevated levels of toxic elements. For example, the Bodélé depression in Chad (North Africa) and the numerous lake depressions in central Asia (e.g. Aral Sea region) and northern China are major dust sources of global significance. Sea spray produces aerosols containing particles that are commonly of salt, but can also contain radionuclides.

About 9% of the global population, more than 500 million people, lives within potential exposure range of a volcano that has been active within recorded history. There are at present an estimated 550 active volcanoes, many of which are in locations experiencing rapid population growth. Major urban centers are commonly found within close proximity to volcanoes, such as Naples in Italy and the capital cities of Mexico, Japan, and the Philippines. Population density generally decreases with increasing distance from the volcano, with the highest population densities in close proximity to volcanoes in Southeast Asia and Central America. Of all eruptive hazards, ash-fall can affect most people because of the extent of areas that can be covered by fallout. Although eruptions are often short-lived, ash-fall deposits can remain in the local environment for years to decades, being remobilized by human activity or simply re-suspended by wind.

Potentially toxic natural particulate dusts include asbestos minerals and several species of crystalline silica and fibrous silicates, and dusts containing toxic trace elements such as volcanic ash particles, which hold transition metals and other toxic elements on their surfaces. The impact of high concentrations of naturally occurring silica-rich dust on human and animal health received little attention until recently, although the so-called desert lung syndrome (non-occupational silicosis with asthmatic symptoms) has been known for more than a century. Large quantities of silica and silicates, together with a range of chemicals including potentially toxic trace elements, are released during some volcanic eruptions. Inhaled ash can exacerbate symptoms in people who are susceptible to asthma and respiratory disease.

Anthropogenic sources of particulate matter include dusts from mining and quarrying, agricultural soils, and combustion of fossil fuel for energy generation and heating. Petrol and diesel-powered vehicles are an important source of particulate and gaseous atmospheric pollution. Residual ash from liquid fossil fuels has been categorized to be more harmful to human health than coal fly ash. Particulate matter released by biomass burning from forest clearance and agricultural practices continues to be important. The burning of biomass both natural and anthropogenic yields black carbon which adds to the opacity of the atmosphere. Smoke plumes from fires, are often carried thousands of kilometres from their sources. Potentially toxic particulate dust arising from anthropogenic activities includes quartz and other silicates from quarrying and mining, agricultural biomass burning and wild fires and higher-rank coal dust from coal extraction and processing. Workers employed in industries such as mining, quarrying, sand blasting, silica milling and stone masonry are particularly exposed to fine, crystalline quartz dust and can develop inflammation and fibrosis of the lung (silicosis), which is one of the most studied occupational lung diseases. Crystalline silica is also classed as a human carcinogen. Asbestosis is a progressive, incurable chronic lung disease which is attributable to prolonged exposure to asbestos. Unfortunately, the important insulation and fire proof properties of asbestos promoted its widespread use in construction, ship building and industrial refrigeration plants, despite the known link to serious lung disease. However, up to now it is not clear which components in coal cause pneumoconiosis. Coal fly ash can contain a component of unburnt organic matter and is widespread in industrial, urban and some natural environments. Human-health implications of fly ash in some regions of the world are still a subject of very high concern. For example, in northern China the domestic burning of local Permian coals has resulted in clusters of lung cancer. The relatively few studies of coal fly ash toxicity have yet to provide evidence of human lung inflammation and there is a continuing discussion about the importance of toxic trace elements being part of this material. There is evidence that oil fly ash (including diesel) is still more important to human health compared to coal fly ash because the former is smaller in diameter, chemically complex and rich in metals. In this chapter, main emphasis is given to the source, release, transportation, and deposition of mineral particulate aerosols derived from volcanoes, soils, sediments, and weathered rock surfaces and their impact on human health when in suspension in the atmosphere.