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20 December 2009
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Whereas we deplore the depletion of ozone in the stratosphere, the
gas is dangerous for health in the air we breathe and the quantities can
be alarming. A concentration of 120 µg/m3,
of ozone is considered the acceptable limit by many authorities.
In nature, ozone is generated by ionisation of oxygen. A lightning
strike, for example, will cause ozone to be formed. Anyone who has been
close to a strike can smell its very distinctive odour. However, this is
concentrated in an area of a few metres around the strike and is soon
dissipated to negligible levels. More insidious, ozone is also formed when
the sun shines on a mixture of hydrocarbons (volatile organic compounds or
VOCs) and an oxidising gas, such as
nitrous oxide in what is called a photochemical reaction. In nature, the
hydrocarbons may be terpenes from pine and citrus trees or terpenoids from
aromatic plants, such as thyme, lavender etc. The nitrous oxide is a gas
that is produced by forest fires, lightning strikes etc.
Unfortunately, human activities also produce large emissions of
hydrocarbons from fossil fuels and nitrous oxide from the combustion of the
same fuels. The levels of ozone from these sources is far greater than from
natural sources, especially in sunny climates. At the same time, the
decomposed hydrocarbons in these photochemical reactions form particulate
matter and the combination of particles and ozone forms "summer smog", which
is particularly dangerous to health.
The first city where the problem became apparent was Los Angeles, because
the topography made it difficult for the smog to dissipate. I have seen a
pall of smog in a valley below the Mojave Desert tens of kilometres from LA.
With the increase in automobile usage, many other cities became famous, or
rather infamous, for their smog: Athens, Bangkok, Beijing, Cairo, Hong Kong,
Mexico City, New Delhi, Sao Paulo etc. In these places, ozone levels have
frequently exceeded the recommended limit, sometimes by a factor of 4 or
more.
What effect does smog have on the human body? The ozone destroys mucous
membranes, including alveolar lung tissue, literally eating it away. The
particles, which are very fine, also clog the alveoli, thus reducing their
ability to transport oxygen into the blood stream. It is therefore a
double-whammy that has caused - and is causing - literally millions of
premature deaths per year, not to mention incapacitating illnesses that are
a burden on society. The cost of traffic accidents in human lives is almost
nothing in comparison, yet the deaths from ozone poisoning also have most of
their origin in the same motor vehicles.
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