what about a PIR motion sensor?
it could say if there is some moving heat source...
http://arduino.cc/playground/Main/InterfacingWithHardware#motion_pir
what about a CO2 sensor?
1 person in a room needs 7 litres of fresh air per second in order to keep the CO2 level below 1000ppm...
http://senseair.se/wp-content/uploads/2011/12/carbon_dioxide.pdf
i wouldn't use a mains power switch... just about 12V DC to 24V DC from a power supply...
then i would send it through some chains of high power LEDs... via a boost (<40V) or buck converter...
for safety reasons...
but maybe there are mains switches, that r safely isolated (even if some piece of metal in it breaks or in case of over-current)...