Accumulation
Part 1: Background:
Airborne Particulates
The following photographs
show four views of the same scene on days with different levels of smog.
(Photo credits: The
Nicholas School of the Environment, Duke University)
Air pollution is a serious
problem, and not just because it spoils our view of nature -- it also has
serious health consequences. For example, it was recently estimated that
particulate air pollution kills three times as many people in Houston,
Texas, as die by homicides.
Particulate matter (PM)
is one of six "criteria" air pollutants regulated by the Environmental
Protection Agency (EPA) under the Clean Air Act and various amendments.
EPA is required to review the scientific research every five years to determine
if stricter air quality standards are justified to protect the environment
or the public health. Before 1987, the standard for PM made no distinctions
in particle sizes, even though larger particles are filtered out by the
nose, whereas smaller ones (10 microns in diameter or less -- a micron
is one-millionth of a meter) can penetrate deep into the lungs. This "respirable
particulate matter" (abbreviated PM10) includes liquids, hydrocarbons,
soots, dusts, acids from aerosols, and smoke particles, sometimes accompanied
by toxic chemical compounds that attach themselves to the particles. The
current 24-hour National Ambient Air Quality Standard (NAAQS) for PM10,
established by EPA in 1987, is 150 micrograms per cubic meter, with an
annual average not exceeding 50 micrograms per cubic meter.
More recent research shows
that even smaller particulates, those of 2.5 micron diameter or less, are
even more dangerous than those between 2.5 and 10 microns. In 1997 EPA
is attempting to implement a PM2.5 standard, against aggressive, widespread,
and well-financed opposition from industry. The EPA proposal is for an
annual standard in the 12.5 to 20 microgram per cubic meter range. The
National Resources Defense Council (NRDC) and American Lung Association
(ALA) have recommended an annual PM2.5 standard of 10 ug/m3.
Here are some sources for
the preceding paragraphs and related information, with emphasis on the
Houston problem:
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