John Gill's  Exposition of the Bible 

Proverbs 30:21

For three things the earth is disquieted, and for four which it cannot bear:

For three things the earth is disquieted,.... The inhabitants of it are made very uneasy;

and for four which it cannot bear; they are a load and burden upon it, and are intolerable to those that dwell on it, and make them very uncomfortable.