Ventilation controls can sometimes look like ordinary light switches. Exhaust fan and other ventilation controls should be permanently labeled as such, unless their function is obvious or they are labeled by the manufacturer.
ASHRAE. 2010. ASHRAE Standard 62.2-2010, Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Inc.