ฝุ่นละออง (Particulate Matter)

(Particulate Matter)

Particulates – also known as particulate matter (PM), suspended particulate matter (SPM), fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be man made or natural. Air pollution and water pollution can take the form of solid particulate matter, or be dissolved. Salt is an example of a dissolved contaminant in water, while sand is generally a solid particulate.

To improve water quality, solid particulates can be removed by water filters or settling and is referred to as insoluble particulate matter. Dissolved contaminants in water are often collected by distilling, allowing the water to evaporate and the contaminants to return to particle form and precipitate.

Some particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes also generate significant amounts of particulates. Coal combustion in developing countries is the primary method for heating homes and supplying energy. Because salt spray over the oceans is the overwhelmingly most common form of particulate in the atmosphere, anthropogenic aerosols—those made by human activities—currently account for about 10 percent of the total amount of aerosols in our atmosphere. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer.

Particle Pollution: PM₁₀

Particulate matter 10 microns or less in diameter (PM₁₀) has been U.S. EPA's indicator for the short and long-term health related particulate standards. In establishing this particulate standard, U.S. EPA considered the following:

• Biological responses to particles that penetrate and are deposited in the various regions of the respiratory tract influence health risks posed by inhalation.
• Particles deposited in the thorax (trachea, bronchial and alveolar regions) pose the greatest risk.
• Particles deposited elsewhere in the body pose relatively low risks.

At the time , U.S. EPA chose the 10 micrometer "cut point" in setting the size-specific indicator for the standard. This indicator ensured that the standard accounted for the full range of particles capable of penetrating to sensitive lung areas.

Sources

Most small particles are "secondary" particulates, i.e., they are particles or aerosols formed in the atmosphere from other vaporous or gaseous pollutants. For example, sulfates are small particles formed when sulfur dioxide oxidation products combine with water vapor or ammonia. Fine carbon particles are formed when organic compounds, in the presence of catalytic particles, are reduced to elemental carbon.

In addition, certain materials processing operations such as coal mining, mineral refining and quarrying activities may suspend PM₁₀ in the air.

Health Effects

Particulates under 10 microns in diameter may lodge deep in the lungs. PM₁₀ can affect health in many ways. For example:

• Short-term exposure can irritate the lungs and perhaps cause immune responses; lung constriction, producing shortness of breath and cough may result.
• Materials dissolving from the particles can damage cells.
• Long-term, lower level exposure may cause cancer and premature deaths. (A series of studies has related daily mortality in some U.S. cities with PM₁₀concentrations.)

Those with a known history of asthma or chronic lung disease are especially sensitive to these effects. The elderly or those with pre-existing heart conditions may also have severe reactions, since the resulting lack of oxygen may strain the heart.

Particulates which cause mechanical irritation include soot, fly ash, pollen, fungi, yeast and salt.

Particles may also be composed of compounds which form acids when mixed with moisture in the lung. Zinc ammonium sulfate is often reported as a constituent of smog, and particles of this compound form sulfuric acid in the lungs. Animals exposed to approximately 1 micrometer particles of this compound were more susceptible to respiratory infection than animals exposed to other particulates of similar sizes. Particles of this compound also cause bronchial tubes to constrict more easily than particles of other compounds, especially in asthmatics and those who have chronic lung diseases.

Soot, fly ash, automobile and diesel exhaust particles contain many compounds such as benzo(a)pyrene, which the U.S. EPA classifies as probable human carcinogens. (Chronic inhalation exposure to these types of particles induces a number of different cancer types in laboratory animals.) Diesel exhaust particles are particularly potent tumor inducers.