Hey Russ, you really missed the point on this reply. He did not say a word about air flowing up through the drain pipe. He said make-up air has to flow into the base of the shower, hence the inward pulling of the shower curtain. Drains were not mentioned or implied.That can't be. Besides the fact that the drain opening is tiny and a significant cross section is covered with water, the drain is trapped specifically to prevent airflow in the other direction. When you partly fill up a previously empty pipe with water, the air has to go out.