What is the economic impact of air pollution?

Air pollution is what economists call a "negative
externality" or, as in the link below, a "external diseconomy."  This is defined in the
link as a

cost
arising from an economic activity which does not fall on the person or firm controlling
the activity.

When air
pollution happens, it is not necessarily those who pollute who have to pay the costs. 
This is a major economic impact of pollution.

For example,
one impact of air pollution is an increased need for health care.  When people breathe
polluted air too much, they often end up with respiratory problems and need medical
treatment.  They have to (or their insurers do) pay for the effects of air pollution
even if they did not cause it.

Air pollution can have more
widespread economic impacts when or if governments try to reduce it.  Governments then
have to impose regulations on firms, forcing them to pollute less.  In the short term,
at least, these regulations make those firms' costs go up and that reduces the supply of
whatever those firms produce.  This is a major reason why air pollution (greenhouse
gasses are a huge example of this) is not better regulated by the
government.

Air pollution, then, can economically impact
both individuals and whole economies.