High pollution was observed prior to the 1980s.
Which period does 'Prior to the 1980s' refer to, 1)include, 2)partially include, 3)exclude the period of the 1980s?
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Prior means 'before'. If something happened prior to the 1980s, it happened before that decade.
prior to sth
before a particular time or event:
the weeks prior to her death
"Prior to" is an exact synonym of "before." So strictly speaking, the phrase "prior to the 1980s" does not include the 1980s.
That said, "the 1980s" is a vague term. It doesn't necessarily mean the precise period from 1 January 1980 to 31 December 1989. The phrase therefore has some inherent gray area and only means that the 1980s were the first decade during which a noticeable change occurred.