When spending a lot of time with someone, in an environment that they are often in (such as their bedroom), the little skin particles that become dust will eventually become agitated, and find with way into the air and into your lungs.

Hence, while the expression "you're the air that I breathe" might be a silly thing to say, in some cases it is accurate. More often, the expression would be that "I'm the air I breathe" if you breathe in dust of your own creation, but life is funny like that.