Dust and Human Health - Mineral Dust: A Key Player in the Earth System - page 381

including the Air Pollution Control Act (1955) that provided funds for air pollution
research and the Clean Air Act of 1963 that focused on air pollution control.
The term particulate matter (PM) is widely used and in the United States, for
example, such usage was the result of the legislated requirements under the Clean
Air Act (McClellan 2000 ). Regulations to limit emissions from mobile and station-
ary (industrial) sources were established with the Clean Air Act of 1970 through
several programs such as the National Air Quality Standards (NAAQS) program.
Standards for six principal pollutants (carbon monoxide, lead, nitrogen dioxide,
ozone, particle pollution, and sulfur dioxide) were established, thus providing
enforceable criteria for the protection of sensitive populations, decreased visibility,
and damage to animals and crops. Many other nations have also established
regulatory criteria for PM and other pollutants.
According to the World Health Organization (WHO), particulate matter affects
more people than any other pollutant and is primarily composed of sulfates,
nitrates, ammonia, sodium chloride, carbon, mineral dust, and water (WHO 2012a ).
Particulate matter sources may be natural (e.g., volcanic ash, forest fires, and dusts)
or anthropogenic (e.g., fossil fuel combustion).
The classification used by WHO and the US EPA defines coarse particles as those
less than or equal to 10 but greater than 2.5 micrometers (m) in diameter (PM10);
and fine particles are those less than 2.5 m (PM2.5) (WHO 2011 ;USEPA 2012a ).
The definition of fine particles, in some schemes, is less than 2.5-0.1 manda
category is added for ultrafine particles (those less than 0.1 m). Coarse (PM10)
particles may also be described as inhalable particles, or the fraction that enters
the body but is generally trapped within the nose, mouth, and upper respiratory
tract, whereas fine (PM2.5) particles may penetrate into the lower respiratory tract.
It should be noted that particles contained in aerosols, sometimes also referred
to as far-traveled dusts, fall within these guidelines in terms of size distribution
(see Scheuvens and Kandler, Chap. 1—this volume). In the United States, PM
monitoring is only required in areas with populations of 100,000 or more (USEPA
2012b ). Thus, in areas where regional inorganic minerals dusts may provide
greater exposures, such as close to rural dryland dust sources, PM concentrations
are largely unavailable. Hence, large urban areas capable of providing sufficient
hospitalization and PM data necessary for statistical analysis have long been the
focus for epidemiological studies and data collection.

15.3 Human Exposure Pathways

A toxicant may enter the body by one of several exposure pathways: inhalation,
passive ingestion (larger particles are swallowed), ingestion, and dermal or skin
contact. Inhalation and ingestion are the primary routes of exposure for particulate
matter. Particles are transported with inhaled air through the nose, or in some cases
such as during exercise, the mouth.