Particulate matter, SO2 and NOx

Particulate matter enters the air in many ways. It has two forms: solid particulate matter and liquid particulate matter.

Sulphur dioxide (SO₂)

Sulphur dioxide, a gas with a pungent smell, entering the atmosphere is a cause of the environmental crisis called acid rain. It also causes breathing problems. It is released by heavy use of coal as fuel, burning petroleum fuels, burning vulcanised rubber products, bacteria acting on some organic matter, and volcanic eruptions.

Oxides of nitrogen (NO_x)

Oxides of nitrogen (NO, NO₂) entering the atmosphere affect its composition. They also cause acid rain and breathing diseases. They form when lightning makes atmospheric nitrogen react with oxygen, and when nitrogen reacts with oxygen inside the internal combustion engines of some vehicles.

Why small particles are the dangerous ones

Large dust grains are caught by the hairs and mucus of the nose. Very fine particles slip past and reach deep into the lungs, where they irritate the airways and, in the case of asbestos fibres and heavy-metal dust, can stay for years and cause disease. SO₂ and NO_x dissolve in the moisture of the airways to form acids, which is why smoky, traffic-heavy air worsens asthma.