Other birds have spread quickly across areas with heavy air traffic.

VOA: special.2009.04.01

They were really gray and there's this heavy feeling in the air,

黑压压的一片，空气中有种厚重的感觉，

I wonder if 实战 - SpeakingMax英语口语达人

The heavy metal enters the body when children breathe the paint dust or fumes in the air.

VOA: special.2009.08.31