Hot weather and sunlight cause ground-level ozone
to form in harmful concentrations in the air. Carbon
monoxide (CO) is an odourless, colourless gas
formed by incomplete carbon combustion. It is
mainly emitted from motor vehicle exhaust,
followed by non-road engines such as construction
equipment, industrial processes and wood burning.
The increasing number of cars has an important
role in the increase in CO emission worldwide.
Sulfur dioxide (SO2) is a gas formed when fuel
containing sulfur, such as coal and oil, is burned,
and when gasoline is extracted from oil or metals
are extracted from ore.