|
Size is the main determinant of the
behaviour of an atmospheric particle. The size is usually
expressed in terms of the ‘aerodynamic diameter’ which refers to
unit density of spherical particles with the same aerodynamic
properties, such as the falling speed.
Larger particles (greater than 50µm)
usually only remain in the air for a few minutes and settle near
the source. A µm is one millionth of a metre, or 0.000001m).
Smaller particles (less than 10µm, known as PM10) can remain in
the air for several days and can be spread by winds over wide
areas or long distances from the original source.
Fine particles (between 0.1-2.5µm)
may remain in the atmosphere indefinitely. Fine particles are
capable of scattering light, causing a reduction in visibility.
Particles are generally removed from
the atmosphere by rain or when they come into contact with
surfaces. Some particles may have other pollutants attached to
them, which may react with those surfaces.
Windblown dusts, pollens from plants
and sea salts are natural sources of particles in the
atmosphere. Bushfires, agricultural and forest hazard-reduction
burning release smoke particles into the air.
Combustion processes using coal and
other fossil fuels, such as power generation, industrial
operations and motor vehicle fuels, emit most of the particulate
matter in urban areas. Other noticeable sources of particles
include agricultural burning practices (e.g. burning of sugar
cane prior to harvesting) and emissions from domestic solid fuel
heaters and woodstoves
Environmental effects of particulate
matter Particulate air pollution can cause a wide range of
damage to surfaces and materials. Merely by requiring more
frequent cleaning, particulates can accelerate deterioration. If
the particle is corrosive or has other pollutants, for example
sulfur dioxide, attached to it then it may also react with or
corrode the surface or material.
Health effects of particulate matter
Under normal conditions a human respiratory tract in good health
is able to deal with inhaled particles without undue stress or
long-term effects. In sensitive individuals, or when high levels
of particles are present, particulate matter may contribute to
increased rates of respiratory illnesses and symptoms.
Studies indicate that such adverse
effects are dependent on a number of factors, including:
particle size (whether particles can
penetrate the lower airways), the intensity of the exposure, the
chemical nature of the particles and their interaction with
human tissue, the presence or absence of pre-existing conditions
(especially diseases of the respiratory tract), and
meteorological factors such as winds, humidity, a temperature
inversion, rain or thunderstorms. Air quality goal Inhalable
particles (those with diameter less than 10µm) are commonly
understood to pose the greatest risk to human health. There have
been extensive studies into the health effects of different
levels of particles and pollution mixes. However, no studies
have yet determined a threshold value for long-term or
short-term exposure below which no adverse health effects are
observed.
Some facts on respirable suspended
particulate (aka RSP) |
