Air Pollution part 1: Grade 9 understanding for IGCSE Biology 4.12

The extended topic on “Human Influences on the environment” is one that is well worth revising thoroughly.  I haven’t done any analysis of past papers (life is too short) but my hunch is that questions on these topics have been over-represented in the past few years.   This first blog post on air pollution is just going to summarise the consequences of pollution of the atmosphere by one gas – sulphur dioxide. I apologise but I cannot bring myself to spell it sulfur dioxide as it is in the specification – old man syndrome I’m afraid…..

Sulphur Dioxide

99% of the sulphur dioxide in the air comes from human sources.  It is an acidic gas that acts as a pollutant in two ways.  Firstly direct on human lungs and airways where it is an irritant and can cause wheezing, tightness in the chest and lead to lung disease.  This can be a particular problem for people already suffering with asthma and other diseases of the lung.  Sulphur dioxide (along with various oxides of nitrogen) is also a major contributor to the environmental problem of acid rain.

Sulphur dioxide is produced when any fossil fuel containing sulphur is burned.  The biggest contributor of this pollutant gas (~70% of the total emissions) is the industrial combustion of coal and natural gas for electricity production.  ~20% of total emission comes from other industrial processes and the remainder from burning petrol and other domestic fossil fuels.


The major environmental problem with sulphur dioxide is acid rain.


Sulphur dioxide and various nitrogen oxides are released from coal-fired power stations when coal is burned.  These gases can react in the atmosphere under the influence of radiation from the sun and then dissolve in water to form sulphuric and nitric acids.  These then cause the rainfall to be excessively acidic and this often occurs long distances from where the pollutants were released.

NB All rainfall is slightly acidic due to carbon dioxide in the atmosphere forming carbonic acid.  Acid rain is therefore defined as rainfall (or other precipitation such as snow or hail) with a pH of less than pH5.6.


What are the consequences of acid rain?

As shown in the picture above, there are several biological impacts of acid rain.

Acid rain can cause coniferous trees (e.g. pine trees) to lose their needles (leaves).  This will kill the tree as it cannot photosynthesise and so has led to deforestation in certain parts of the world.

Acid rain leaches minerals (e.g calcium and potassium )from the soil more effectively than normal rain.  This leaves the soil very low in certain essential minerals and so makes it harder for plants to grow.

The minerals leached can then themselves become pollutants in fresh water.  Aluminium ions in fresh water can cause fish to overproduce mucus in their gills.  This will kill adult fish as they cannot get enough oxygen into their blood.  Fish eggs will not survive in acidic water and many small invertebrates are also killed directly by the acidity.  So freshwater ecosystems can collapse.

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