Victoria:  Several years ago at another institution. I did have this issue come up in a general chemistry lab.   We assigned the student to a group whose workstation was closest to the door and the dog stayed just inside the door while the student was working. The dog was not at the lab bench, the student partners took care of any needs there.  The biggest concern was broken glass fragments on the floor.  In that particular lab hazardous fumes where not an issue.  We did a walkthrough of the lab with both the campus disability coordinator and a representative from the local service dog organization to establish procedures and get their OK. It worked well.  Good Luck.