can be caused by MANY different things, and usually by a combination of several things.
Causes of coughing can include (at least):
--SINUS PROBLEMS, due to air pollution, infections, etc.
--LUNG PROBLEMS, such as infections like pneumonia, bronchitis, (caused by bacteria, viruses, mycoplasmas, etc.) asthma, lung damage, etc.
--AIR POLLUTION, due to irritants, toxic CHEMICAL fumes/gases, etc.