According to the WHO* it is defined as the contamination of the environment by any agent (chemical, physical or biological) that modifies the natural characteristic of the atmosphere. 

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In our experiment the particular pollutant measured was the nitrogen dioxide* which is mainly formed as the result of fuel combustion processes.

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Many of studies have shown that the nitrogen dioxide may cause damage to the human respiratory system, aggravate respiratory diseases (such as asthma) and even cause premature death. 

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*WHO = world health organization

*Nitrogen dioxide = NO2