Incineration of anything, including garbage and hazardous chemical
wastes, produces a kind of pollution that is uniquely dangerous to
humans: fine particles.
In this series, we will first discuss the characteristics of fine
particles, and later we will discuss health studies showing the
consequences of breathing fine particles.
The process of incineration turns solids and liquids partly into gases
and partly into tiny particles of soot or ash. As the gases rise in the
smoke stack, they cool and some of the gas molecules come together to
form additional fine particles. The resulting particles are exceedingly
small when they are emitted into the environment. Scientists who study
particles make a distinction between coarse (large) particles and fine
(small) particles. Fine particles behave entirely differently from
coarse particles and, as we will see, are much more dangerous to
humans. Fine particles are also much more difficult and expensive to
control. They are also invisible, so when they are not controlled,
there is no way to know it except by monitoring with the proper
Coarse particles are those with a diameter larger than 2 micrometers
(um); fine particles are those with a diameter less than 2 micrometers.
A micrometer (um) is a millionth of a meter and a meter is about a
yard. (An older term for micrometer is micron.)
Incinerators emit large numbers of particles, despite the best
available control technology. Half of all the particles emitted will
have a diameter less that 2 um, and the majority of these will have a
diameter of 0.3 um.
It is difficult to imagine how small these particles are. To help
understand what we're talking about, look at the dot over the letter i
in this newsletter; that dot measures about 400 micrometers in
diameter. You can fit 40,000 particles with a diameter of 2 um on the
dot. When the particles have a diameter of 0.3 um, you can fit 1.7
million particles on the dot over the i.
Unfortunately, U.S. EPA [Environmental Protection Agency] regulations
do not take into consideration the sizes of the particles emitted by an
incinerator. For regulatory purposes, coarse particles are considered
to be the same as fine particles, as if they were all equivalent. The
regulations issued as part of the Resource Conservation and Recovery
Act (RCRA) allow the emission of 0.08 grains per dry standard cubic
foot of stack gas (or 180 milligrams per dry standard cubic meter).
There are 437.5 grains in an ounce. Measurements show that half these
particles will have diameters ranging from 2.5 down to 0.1 micrometers;
of that half, a majority will have a diameter of 0.3 micrometers. If we
assume that 25% are 2 um, 25% are 1 um, 35% are 0.3 um and 15% are 0.1
um in diameter, we can generalize about the fine particle emissions
from an incinerator.
Each pound of fine particles emitted from an incinerator will consist
of 140 quadrillion (1.4 x 10 17) individual particles. A quadrillion is
1000 trillion. Over a year's time, an incinerator meeting the federal
standards will legally emit anywhere from 10 to 1000 TONS of fine
particles, depending upon the size of the incinerator.
Breaking things into fine particles has the effect of vastly increasing
their surface area. A single lump of waste weighing a pound (and having
the same density as water) would have a surface area of about 44 square
inches (a square 6.5" on a side), about the size of a large post card.
But when that same pound is broken into fine particles, its combined
surface area grows to 9900 square yards (approximately two football
This is important for several reasons: as these fine particles move
upward in the smoke stack, they are immersed in a bath of gaseous
chemicals that are cooling and are "looking" for a place to turn from a
gaseous to a solid state. Fine particles, with their large surface
area, provide an inviting place, and so the surfaces of fine particles
become covered with pollutants ("enriched" is the technical term for
this) before they are released into the local air. In particular, fine
particles become coated with toxic metals (lead, cadmium, arsenic,
chromium, and zinc, and with sulfur and polycyclic aromatic
hydrocarbons--or with whatever else is in the smoke stack).
As the human body evolved throughout its long history, it adapted to
the environment. One factor in the environment has always been dust,
principally from dust storms. Dust from storms is larger than 5 um in
diameter and the human body evolved mechanisms for protection against
such large particles. The hairs inside the nose, the mucous membranes
lining the nose, throat and lungs, and even the shape of the throat,
help to trap dust. As air is inhaled, the shape of the throat causes
the air to swirl, so heavy dust particles are thrown outward by
centrifugal force, where they strike the mucous-lined walls. As the
tubes and passageways leading to the lungs twist and branch, they
provide many opportunities for particles to collide with sticky walls
and become trapped before they enter the lungs. Once trapped by mucous,
coarse particles are coughed up and excreted.
Nature has gone to great lengths to protect the lungs because the
deepest regions of our lungs provide places (called alveolar sacs, or
alveoli) where oxygen passes into the blood and carbon dioxide passes
out of the blood. The lungs provide a large surface area for contact
with air, and thus with fine particles; the surface area of the alveoli
is 65 square yards, which is larger than two tennis courts.
Thus, the deep regions of the lung provide direct access to the blood
stream and, by this means, to every part of the body.
Unfortunately, humans now produce huge numbers of fine particles that
elude the body's protective mechanisms entirely. Fine particles pass
easily into the deepest regions of the lungs, the alveoli, or alveolar
sacs. There they remain indefinitely because no clearance mechanisms
effectively remove them. Nature did not protect us against such
particles, because none existed until very recently.
Once lodged in the deep regions of the lung, fine particles, with their
enormous surface area enriched with toxics, provide a particularly
efficient means for delivering metals and organic pollutants directly
into the blood stream. Their large surface area provides effective
contact with moist tissue and the opportunity for dissolving or for
other chemical reactions, putting pollutants directly into the victim's
blood. Once in the circulatory system, toxics are then distributed
throughout the body.
[To be continued.]
The best books on fine particles are those of the National Research
Council, National Academy of Sciences: AIRBORNE PARTICLES (Baltimore,
MD: University Park Press, 1979) and CONTROLLING AIRBORNE PARTICLES
(Washington, DC: National Academy of Sciences, 1980); a short summary
appears in FINE PARTICULATE POLLUTION, A REPORT OF THE UNITED NATIONS
ECONOMIC COMMISSION FOR EUROPE (London and NY: Pergamon Press, 1979). A
good, though very technical, introduction is William Hinds, AEROSOL
TECHNOLOGY; PROPERTIES, BEHAVIOR AND MEASUREMENT OF AIRBORNE PARTICLES
(NY: John Wiley and Sons, 1982). Incineration of anything, including
garbage and hazardous chemical wastes, produces a kind of pollution
that is uniquely dangerous to humans: fine particles.
Descriptor terms: particulates; air pollution; air quality;
incineration; epa policies; rcra; emissions;