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Particulate matter (PM) is defined in many ways but is essentially
particles that are floating or are suspended in the atmosphere. They are
categorised according to the effective maximum particle size in
micrometres as follows:
| PM10 |
These are large particles that will often settle out or be
washed out by rain. If breathed in, they will mostly be trapped in
the nose and throat and few will reach the lungs |
| PM2.5 |
These are smaller particles which have a longer lifetime. They
will reach the lungs and can cause cumulative alveolar damage. |
| PM1.0 |
These are still smaller and will readily reach the lungs. |
| PM0.1 |
These are classed as nanoparticles. There is research being
conducted on nanoparticles which are known to pass into the blood
stream and, from there, through body membranes into organs. |
The great majority of particles is classified as coarse, between 10 and
2.5 µm. Fine particles are in the PM2.5 and PM1.0 categories, while the
PM0.1 category is ultrafine.
The effect of particles on human tissue depends on their physical and
chemical characteristics. Sharp-edged particles, such as silica, can cut
lung tissue, causing such diseases as pneumosilicosis and
pneumoconiosis. The needle-shaped particles of some grades of asbestos
can cause mesothelioma and asbestosis. Chemically neutral rounded
particles would be less dangerous. Most organic particles, such as from
cigarette smoke, diesel exhausts etc. may bbe potentially dangerous,
causing emphysema, asthma, arteriosclerosis, lowered immunity and
cancer.
Particles may come from many different sources, the great majority
being natural, if averaged over the earth's surface. These include
volcanoes, forest fires, wind-blown desert dust, sea salt, desquamated
particles from living organisms (including human skin), pollen etc. As a
general rule, these have little effect on human health (exception:
allergic reactions), provided they are not present in excessive
quantities. On the other hand, some of the man-made particles can be
very harmful, even in small quantities, such as those resulting
from unfiltered diesel exhausts, tyre and road bitumen particles, smog
particles resulting from the photochemical reactions between volatile
organic compounds and nitrogen oxides, power station stacks, domestic
chimneys, heavy metal compounds etc. The chemicals in these pollutants
often have a cumulative effect; for example, it is known that mercury
compounds will slowly cause lesions of the central nervous system,
especially in children.
The recommended maximum particulate matter PM10 concentration recommended
by some bodies is 50 µg/m3 with a
2005 EU limit of 40 µg/m3 and a proposed 20 µg/m3 for 2010.
These are annual mean values, with a number of daily events exceeding
these values permitted.
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