Particulate Matter. Why does it matter?
Particulate matter is the generic term
used for a type of air pollution that consists of complex and varying mixtures
of particles suspended in the air we breathe. Particulate matter is a
combination of fine solids such as dirt, soil dust, pollens, moulds, ashes, and
soot; and aerosols (smog) that are formed in the atmosphere from gaseous
combustion by-products such as volatile organic compounds, sulfur dioxide and
nitrogen oxides. Major sources of particulate matter (Sydney 1998) are motor
vehicles 23%, other mobile sources 25%, domestic wood heating 25%, industrial
18%, open burning 6% (on a gravimetric basis). (2)
Particles of special concern to the
protection of lung health are those known as fine particles, less than 2.5
microns in diameter. (For comparison, a human hair is about 75 microns in
diameter.) Fine particles are easily inhaled deeply into the lungs where they
can be absorbed into the bloodstream or remain embedded for long periods of
time. There is no demonstrable 'safe' limit of exposure.
Particulate matter air pollution is
especially harmful to people with lung disease such as asthma, chronic
bronchitis and emphysema or to those with heart disease. It has been shown to
be a cause of lung cancer. Recent research has also linked exposure to
relatively low concentrations of particulate matter with premature death.(1).
Measurement.
The most common measure of particulate
matter is PM10, specifically the weight of all particles with an 'aerodynamic'
diameter of less than 10 microns (millionths of a meter) contained in one cubic
metre of air. The Australian standard is 50 µg/m3 , averaged over a
24 hour period. Currently there is no standard for PM 2.5.
|
PARTICLE size |
PM10 |
PM 2.5 |
PM1.0 |
PM0.1 |
PM0.01 |
|
|
Compare with |
grapefruit |
golf ball |
cherry |
wheat seed |
sand grain |
|
|
Number for equal mass |
1 |
64 |
1000 |
1000,000 |
1,000,000,000 |
|
|
Surface area for equal mass |
1 |
4 |
10 |
100 |
1000 |
|
|
Functional classification |
Coarse mode |
Accumulation mode |
Nuclei mode |
|||
When particles become smaller
and smaller, for an equal mass of particles, the surface area exposed becomes
greater and greater. Almost all vehicle exhaust particles by both number and
weight are less than 1 micron in diameter. For this reason PM10 is not a
suitable measure for the impact of vehicle emissions as in a strongly traffic
influenced environment they make up only 3% by weight of the measurement(3)
but would provide in excess of 95% of the surface area. Health impacts are
thought to be best correlated to surface area - the area available to carry
toxins into the lungs. PM 2.5 measurements give more information about vehicle
emissions but are still dominated by non vehicle factors in all locations and
in suburban background, the location of most monitoring stations, 99% of the measurement
is of coarse mode, non vehicle particles.(3).
The only appropriate measure
for both vehicle emissions and products of combustion is PM1 and monitoring for
this component is not carried out anywhere on a regular basis. (3)
Implication for roadside and
tunnel regulation.
Inside a tunnel particle
levels can be above 1000 µg/m3. Almost all of these will be less
than 1 micron (PM1) with a median size between 0.15 and 0.18 µm (4).
If PM10 is used as the basis measurement to regulate the emissions from the
tunnel, the actual impact of the emissions on the health of those outside will
be underestimated by a factor between 10 and 50 because all of the extra impact
is from the most harmful components. This is one reason (of many) why the NEPM
goals are unsuitable the regulation of point sources such as tunnel stacks and
vents.
". the air quality of some
localised areas within major airsheds are dominated by local activities such as
that experienced in a road tunnel or a heavily trafficked canyon street. Air
quality management in these areas is complex and needs a different approach to
that directed at meeting ambient standards intended to reflect the general air
quality in the airshed". p 13 NEPM 1998
The reported health impacts of
particulate matter, with predictable increases in mortality and morbidity with
each 10 µg/m3 rise in PM 10, refer to 'ambient' air, averaged over
large, city sized areas. These increases are largely the end result of the
impacts on individuals who are exposed to the sources which cause the increase
in pollution. The clusters of illness observed near to the ends of early
tunnels and the adverse health outcomes observed near busy roadsides are
examples of this.
In a suburban environment,
an apparently insignificant increase in measured PM10 of 0.5 µg/m3
caused by tunnel exhaust could actually represent a doubling or more of the
health impact. Tunnel exhaust and other concentrated traffic emission
sources like toll plazas and busy highways cause a fundamental change in the
local air quality and that air is much more harmful than the general 'ambient'
air.
This is the fundamental flaw
in the control approach referring to ambient air quality goals adopted by some
planners and regulators. It is essential to define and control the actual
components of concern. Dilution and dispersal of pollution should never be
regarded as a solution. This applies as much to air pollution as it does to
sewerage. It is essential that another approach is adopted, preferably that of
MACT - maximum achievable control technology, to achieve the greatest possible
reduction in the universally harmful impacts of air pollution.
This information is based
on the publications of the American lung association (1), the NSW EPA (2) and
published papers by:
(3)Morawska,L.,&
Thomas,S. (2000) Modality of Ambient Particle Distributions as a Basis for
Developing Air Quality Regulations. Proc. 15th Clean Air & Environment
Conference Sydney.2000
(4)Kittelson,D
B, Watts, Jr.W. F (1998) Review of Diesel Particulate Matter Sampling Methods.
Supplemental Report # 2 EPA Grant Review Of Diesel Particulate Matter.
http://www.me.umn.edu/centers/cdr/reports/EPAreport2.pdf
MC #2 24.04.02