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Smog, which consists of a mix of pollutants, is the yellowish fog that sometimes envelops urban centres. Two main ingredients of smog are ground-level ozone and particles less than 10 microns wide (particulate matter).

Ground-level ozone, a mix of volatile organic compounds and nitrogen oxides, causes serious health problems, such as respiratory problems, premature death and increased risk of cardiac problems in children and the elderly.

Particulates are airborne particles that can cause decreased visibility, as well as aggravate asthma and other respiratory illnesses. These particles are very small, and often travel thousands of kilometers from their source.

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Increasing hospital admissions and mortality associated with acute respiratory disease has been linked to heightened levels of airborne particulate matter, especially small particulate.