This is the substitution of an agreeable or non-offensive phrase for one that might be unpleasant or offensive is called

The correct answer is euphemism. Euphemism is being defined as a word or expression that could either be indirect or mild by which they are being substituted for it to be considered blunt or harsh in regards to pointing out a particular thing that could be embarrassing or unpleasant.

i believe

clean air is essential for health and survival

clean air provides nitrogen gas for plant respiration

clean air supplies oxygen that enables living organisms to breathe

hope this

i am brianly

sorry if this doesn't