<strong>Characteristics:</strong> Not all air pollutants are
gases. <em>Particulate matter</em> is a collective term used for
very small solid and/or liquid particles found in the atmosphere.
While individual particles cannot be seen with the naked eye,
collectively they can appear as black soot, dust clouds or grey
hazes. Particulate matter may be generated by natural processes
(e.g., pollen, bacteria, viruses, fungi, mold, yeast, salt spray,
soil from erosion) or through human activities, including diesel
trucks, power plants, wood stoves and industrial processes.
Individual particles vary considerably in size, geometry,
chemical composition and physical properties. The effect of
particulates on human health and the environment varies with the
physical and chemical makeup of the particulates.
<p>
One of the major characteristics of particulate matter is
particle size. Particles can range in size from 0.005 - 500
micrometers or microns (um), which is equal to one-millionth (10
<sub>-6</sub>) of a meter. Particles less than 2.5 microns in
diameter are known as "fine" particles; those larger than 2.5
microns are known as "coarse" particles. Fine particles with
diameters of less than 1 um, move like gases. Because of their
low settling velocities, fine particles may be transported 1,000
kilometers or more from their source. Under the influence of
gravity, larger particles do not remain suspended and tend to
settle out of the air, sometimes creating localized areas of high
particle disposition.
<p>
<em>Total suspended particulate matter</em> (TSP) refers to all
particles in the atmosphere. TSP was the first indicator used to
represent suspended particles in the ambient air.
<p>
In July 1987, EPA began using a new indicator, PM-10, which
includes only those particles with aerodynamic diameter smaller
than 10 um. Ten microns is approximately one seventh the
diameter of a human hair. This fraction of TSP is responsible
for most of the adverse human health effects of particulate
matter because of the particles' ability to reach the lower
regions of the respiratory tract. Recent data suggests that
particles 2.5 um or smaller may pose the greatest threat to human
health because, for the same mass, they absorb more toxic and
carcinogenic compounds than larger particles and penetrate more
easily deep into the lungs. EPA is considering adopting a new
standard for PM-2.5 to better address the potential health
problems associated with these tiny particles.
<p>
<strong>Sources</strong>: Particles are either emitted directly
into the atmosphere or produced in the atmosphere from the
physical and chemical transformation of other vaporous or gaseous
pollutants.
<p>
The major sources of atmospheric particulates are fossil-fuel
combustion (which produces ash and soot), industrial processes
(involving metals, fibers, etc.), transportation, wind and soil
erosion (producing fugitive dust), and photochemical reactions
(complex chain reactions between sunlight and gaseous
pollutants). Fugitive dust and particles from industrial
processes tend to be larger in size ( > 1 um). Particles from
combustion and photochemical reactions are usually smaller in
size ( < 1 um).
<p>
Because of the large number of sources, particulate matter may
contain hundreds of different chemical elements. Fine particles
(PM-10) may contain substantial quantities of sulfate, ammonium,
nitrate, elemental carbon and condensed organic compounds.
Carcinogenic compounds and heavy metals such as arsenic,
selenium, cadmium and zinc are also concentrated in these
particles. Larger particles, such as soil particles, fly ash,
road aggregate, wood ash, soot and pollen are composed primarily
of minerals, including silicon, aluminum, potassium, iron,
calcium and other alkaline elements.
<p>
<strong>Concentration</strong>: Particle concentrations are
expressed as ug/m<sub>3</sub> (micrograms of particle per cubic
meter of ambient air). Under very clean atmospheric conditions,
the TSP level can be as low as 0-10 ug/m<sub>3</sub>. In a very
dirty environment, TSP concentration can be as high as 1,500
ug/m<sub>3</sub>. In monitoring studies of more than 1,400
sampling sites across the United States, annual arithmetic mean
TSP values ranged from nine to 288 ug/m<sub>3</sub>. The lowest
values were reported at remote sites, while high values were
associated with industrial settings.
<p>
<strong>Health Effects</strong>: Studies of air pollution on
health have linked particulate matter (alone or in combination
with other air pollutants) with a number of significant health
effects. These include increased mortality and aggravation of
existing respiratory and cardiovascular disease, as evidenced by
increased hospitalization, school absences and lost work days.
<p>
The health effects of atmospheric particulate matter are related
to its ability to penetrate the respiratory system. In general,
respiratory defense mechanisms are able to remove 99 percent of
particles larger than 10 um from the inhaled air stream. Smaller
particles ( > 2.5 um), called "inhalable," can cling to
protective mucous and removed in the upper respiratory system.
Fine particles ( < 2.5 um), also called "respirable," can enter
the lungs and end up in lung capillaries and air sacs (alveoli).
<p>
In the lungs, particulates slow the exchange of oxygen and carbon
dioxide in the blood, causing shortness of breath. The heart may
be strained because it must work harder to compensate for oxygen
loss. Laboratory studies show that high concentrations of
components of particulate matter cause persistent cough, phlegm,
wheezing and physical discomfort. Particulate matter can also
alter the immune system and affect removal of foreign material
from the lung (i.e., bacteria and pollen).
<p>
Absorbed substances of particular concern include sulfur oxides
(SO<sub>x</sub>), chlorinated hydrocarbons, polycyclic aromatic
hydrocarbons (PAHs), and heavy metals such as lead, cadmium, zinc
and mercury. Soot, fly ash and particles from automobile and
diesel exhaust contain many of these compounds. PAHs and heavy
metals are predominantly found in very small particles, which is
of concern because some PAHs, such as benzo(a)pyrene, are known
carcinogens. Chronic exposure to these particles causes cancers
in laboratory animals.
<p>
As of 1994, there were 31 million people in the United States in
areas that do not meet EPA's current standard for PM-10.
However, because PM-10 standards may not be a reliable indicator
of the problem presented by fine particles (< 2.5 microns), the
31 million number may be much lower than the real number of
people affected by fine particles.
<p>
<strong>Ecological Effects</strong>: Particulate matter causes
damage and soiling to materials, and is a major cause of
visibility impairment in many parts of the U.S. The same fine
particles linked to serious health effects also affects our
ability to see by scattering and/or absorbing light. Certain
particles, such as sulfates and nitrates, grow in size as
humidity increases, making them more effective as impairing
visibility.
<p>
Visibility conditions vary across the country. Much of the rural
eastern U.S. has poorer visibility than rural western sites due
to higher levels of particulates from both anthropogenic and
natural sources, as well as higher average humidity.
<p>
Natural visual ranges in the eastern U.S. is about 90 miles, but
current conditions range only from 14-24 miles. In the western
U.S., natural visual range is about 140 miles; current conditions
range from 33-90 miles.
<p>
The CAS established special protection for visibility in certain
national parks and wilderness areas. In response, EPA is
developing a "regional haze" program intended to ensure continued
progress toward the national visibility goal of "no man-made
impairment."
<p>
Particulate matter can damage vegetation both directly and
indirectly. When exposed to particulates, plants may suffer
increased disease, leaf cells may be damaged, yield and growth
rates may be reduced and plants may even die.
<p>
Dust on the leaves of crops, trees and shrubs inhibits
photosynthesis and plant growth. Particles carrying heavy metals
can contaminate soil and vegetation in urban areas, along
highways and near smelters. Once in the soil, heavy metals can
accumulate to phytotoxic levels in vegetation and suppress
growth. In addition, particulate matter can scatter sunlight and
cause a reduction in solar radiation, thereby affecting crop
productivity.
<p>
One of the most visible effects of particulate matter is the
soiling or staining of buildings and textiles. Particulates can
also cause chemical deterioration of paint, resulting in
corrosion of metal objects.