adjective
BAD : We opened the window to get some pure air.
GOOD : We opened the window to get some fresh air.
BAD : I always keep a few bottles of pure water on the boat.
GOOD : I always keep a few bottles of fresh/drinking water on the boat.
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Use pure when you mean 'completely clean': 'The water in the lake is so pure that you can drink it.'
The usual word for describing air and water is fresh : 'Her doctor has prescribed a good holiday and lots of fresh air.'