Here is a factfile on health-damaging atmospheric pollutants, following a determination on October 17 by the World Health Organisation (WHO) that outdoor air pollution is a leading cause of cancer:
TOLL
According to WHO estimates, urban outdoor air pollution causes 1.3 million deaths globally every year. Nearly a quarter of a million of these deaths are caused by lung cancer.
PARTICULAR MATTER (PM)
These are microscopic particles generated by coal, oil and forest fires, but also by natural sources, including volcanoes and dust storms. PM is so light that it can float on air. Some particles are so small that they can penetrate deep in the lungs and even cross into the bloodstream.
The components include sulphate, nitrates, ammonia, sodium chloride, carbon and mineral dust. They may also carry traces of heavy or toxic metals such as arsenic, cadmium, nickel and mercury. Chronic exposure contributes to cardiovascular and respiratory diseases as well as lung cancer.
PM is divided into two categories: PM10, which comprises particles between 2.5 and 10 thousandths of a millimetre, or micrometres; and PM2.5, which is smaller than 2.5 micrometres - about a 30th the width of a human hair. PM2.5 is estimated to reduce life expectancy in the European Union by more than eight months, according to the European Environmental Agency (EEA).
Exposure to even low levels of PM2.5 during pregnancy increases the risk of a baby with a low birthweight, The Lancet Respiratory Journal reported this week.
OZONE (O3)
In the stratosphere, this triple-atom molecule of oxygen is naturally occurring and protects us from the Sun's ultra-violet radiation. At ground level, where it is formed in a chemical reaction between sunlight and exhaust gases, ozone is a component of photochemical smog.
High ozone levels can cause breathing problems, trigger asthmatic attacks and worsen respiratory disease. The EEA says half the EU population in urban areas may have been exposed to levels exceeding EU targets.
OTHERS
Several other pollutants, also the residues of combustion or the result of reactions between exhaust and atmospheric gases, are a major concern:
They include the smog ingredient nitrogen dioxide (NO2); sulphur dioxide (SO2); carbon monoxide (CO); and a variety of heavy metals.
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