“as someone smoking”에서 as의 뜻이 궁금해요

“There is plenty of evidence that second-hand smoke — breathed when you are in the same room as someone smoking — can be harmful, particularly to children.”라는 문장이에요.
“as someone smoking”은 어떤 구조인가요?
여기서 as는 ‘~때’라는 뜻인가요?