There was an increasing fear of foreign people after WW1. This was due to the war, but mainly the fear that they would take American jobs (because they accepted lower wages.) ... RESULT The USA became isolated from other countries both economically (trade) and politically (by staying out of the affairs of other countries.)8 May 2007
Explanation: The use of machines in production became more and more prevalent, eventually many factories were built in cities which caused pollution therefore making the air gross.
