I'd say this is one of the most poorly understood items in most UK domestic kitchens.
Ideally, a cooker hood ought to be vented to the outside. But given the realities of shared walls, restricted spaces, and limited ducting routes, this is rarely achieved. I don't have hard data to back this up, but I'd guess that a very high percentage of domestic cooker hoods are installed in the "recirculation" mode. Meaning cooking smoke, vapour, etc. is drawn through some series of filters, and then...
Cooker Hoods
Ideally, a cooker hood ought to be vented to the outside. But given the realities of shared walls, restricted spaces, and limited ducting routes, this is rarely achieved. I don't have hard data to back this up, but I'd guess that a very high percentage of domestic cooker hoods are installed in the "recirculation" mode. Meaning cooking smoke, vapour, etc. is drawn through some series of filters, and then...
Cooker Hoods