I had always thought that not touching something that is sterile keeps it sterile.  Apparently even placing your hand or arm above a sterile field (unless your sterile)  contaminates the field.  I never knew that.   I found this out when practicing packing a wound in class yesterday.

I found some info to back this up.

Proposed recommended practices for establishing and maintaining a sterile field  (AORN journal,  Jan 1996)