是“透透气”

I agree with you,when I learn english ,my english said "go out to get some fresh air " means 出去透透气

1 ) 出去透透空气is the same as 出去透透气. 2 ) In Chinese,透透气 means that you feel someone's room is smelly or stinks, or when there isn't too much air , so you don't want to stay there too long, then you take a breathe outside to avoid the unpleasant smell ,or get your breath back. Sometimes, 出去透透气can be used as a metaphor that means you have been stayed inside for a long time, you play games every day and have been sloven or sloppy or kind of lazy, then one day a friend drops by and asks you to go out with him or her to have a hang out outside, you see he or she doesn't ask you to breathe.

应该是“出去透透气”，而不是“透透空气”， 虽然这里的“气”就是“空气”的意思的，但是常说“透透气”~

供参考： I’m going outside for a breath of fresh air. - 朗文例句