The ozone layer is constantly formed and destroyed by the same reaction
with UV light. Humans have unbalanced this reaction by adding certain chemicals
to the air. These chemicals are nitrogen oxides from combustion engines,
chloroflourocarbons (CFC) and, halogen compounds. They are found in factory
fumes and products used in homes such as chemicals used in refrigeration
and air conditioning as well as in common products like aerosols and styrofoam.
In 1990, the Montreal Protocol was signed by many countries which banned
the use of CFC in manufacturing in these countries. The United States and
Canada were two countries which participated.