Airbrick as a single word is in the online Oxford and Collins dictionaries and Wiktionary. It's even in the Cambridge Advanced Learners' Dictionary. However I didn't see it in any US dictionary. Oxford labels it British, and Collins mainly British.
Most usage examples I found were indeed from British sources. Some people write it as two words and some as one.
I don't know whether such bricks are called something else in other parts of the world, or if they're simply not used.
At any rate, airbrick will be accepted in future, as a rare word.
