The number of the verb is determined by the number of the noun closest to the verb.
In your sentence, the closest noun is “air.” Air is a mass noun, it has no plural form, only singular.
Thus, as you were told, you should use the singular verb “is.”
If you switch the order around where the closer noun would be “trees” or “fields,” then you would use “are.”
One apple, one orange, or two bananas are then added to the blender.
Two bananas, one apple, or one orange is then added to the blender.
Do not allow the mix of the number in the different nouns confuse you.
Just follow the rule and you’ll be fine.