Air pollutants

When fuels are burned, a number of atmospheric pollutants are produced. Here are some of the pollutants from burning fuels, and where they come from:

Carbon dioxide, CO2Complete combustion of any fuel containing carbon atoms
Carbon monoxide, COIncomplete combustion of any fuel containing carbon atoms
Particulate carbon, C (soot)Incomplete combustion of any fuel containing carbon atoms
Unburned hydrocarbonsHydrocarbon fuel molecules which have not been oxidised at all
Sulfur dioxide, SO2Combustion of a fossil fuel which contains sulfur impurities
Nitrogen oxides, NOxOxidation of nitrogen from the air inside the engine of a car, lorry, etc

Carbon monoxide as a pollutant

Incomplete combustion of a hydrocarbon fuel occurs when there is not enough oxygen for complete combustion, caused by a poor supply of air. Less energy is released. Instead of carbon dioxide, you might get carbon monoxide or particulate carbon, known commonly as soot, or a mixture of both.

Carbon monoxide is a poisonous gas which binds to haemoglobin in red blood cells, preventing them from carrying oxygen to the cells in the body. Carbon monoxide is colourless and has no smell, so it is very difficult to tell if it's being inhaled. At low doses it reduces the ability of the blood to carry oxygen, which puts a strain on the heart. At high doses, it can be fatal.

Particulates as a pollutant

Particulates (eg particulate carbon from diesel engines, dust from roads and industry) can cause health problems for humans because it irritates the lining of the lungs, eg it can make asthma worse. The smaller particulates are able to travel deeper into the lung tissue and cause damage that could lead to cancer and heart disease.

Sulfur dioxide as a pollutant

Some fossil fuels contain sulfur impurities. These fuels include:

  • coal burnt in some power stations
  • some diesel fuel burnt in ships and heavy vehicles

Sulfur dioxide is caused when the sulfur atoms are oxidised:

sulfur + oxygen → sulfur dioxide

S + O2 → SO2

Sulfur dioxide then reacts with oxygen and water in moist air to make acid rain, which is a dilute solution of sulfuric acid, H2SO4.

Acid rain harms and kills plants and animals, especially those that live in water. It can also damage man-made objects like statues and buildings.

NOx gases as pollutants

Nitrogen is not present in fuels, but the high temperatures and pressures inside a car engine can cause the nitrogen and oxygen in the air to react together to make oxides of nitrogen.

There are several compounds formed when nitrogen bonds with oxygen, but the two which are made inside engines are NO and NO2. These two pollutants are grouped together with the general formula NOx.

NOx gases can cause acid rain, and they also react in the atmosphere with other pollutants to make photochemical smog. Smog can have major health effects, causing asthma attacks and even death.


Diesel sometimes contains sulfur impurities. List the pollutants which could be produced when it is burned incompletely inside a car engine.

Carbon monoxide, particulate carbon, sulfur dioxide and nitrogen oxides.

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