Many homes in the US have a switch to turn on the ventilation in a toilet or bathroom. Also, all kitchen range hoods and laboratory fume hoods I have seen must be manually switched on and off. Of course the ventilation could be run continuously, but this would be noisy, waste electricity and remove warm air (or cool air-conditioned air in hot weather) from the building. For toilets, bathrooms and kitchens, the main reason to ventilate the room is only temporary – removing odour or humidity.
An untapped business opportunity is to produce a switch that detects odours or humidity and turns on the ventilation just long enough to remove these. Humidity detection is easiest – just connect a hygrometer to the switch. Detecting smelly gases such as grease vapour in the kitchen, hydrogen sulfide, methanethiol and dimethyl sulfide in the toilet may require spectrometry or a chemical reaction. Laboratory gases are probably the most difficult to automatically detect due to their variety.