Urban air quality worsened during the Industrial Revolution 'an age of smoke'.
After world war II a number of invisible threats began to emerge.
Acid rain
Photochemical Smog
Ozone depletion
Climate change.
Air pollution - most sources are the result of human activity
Shift from fuelwood to coal then to oil.
CFC's didn't exist before the 20th century.
Outdoor pollution only became a magor issue with the rise of cities.
The data from artic ice cores shows that the cumlative deposition of copper pollution in the northern hemisphere was much greater before the industrial revolution than after.
Just a little info there for anyone who wants to read the it further