I am looking for a extractor hood, and I see that there exist ones with air recirculation, so a pipe to lead the smoke/greasy air away is not needed.
So I wonder, how good are these?
Can they really clean the smoke/greasy air they pull in?
If you have a gas stove, you need a hood to send the exhaust outside. One that recirculates is not sufficient.
In my experience, the recirculator hoods work OK if you're not doing a lot of cooking. If you do, however, you'll end up with a nasty, greasy dusty coating on everything in your kitchen. I wouldn't bother with a recirculation hood if you ever cook things that involve grease or smoke. For someone who mostly heats up processed foods, it's probably good enough.
In some kitchens external exhaust just cannot be done e.g. a flat in a block of flats, with shared ventilation shaft. In such case the choice is: recirculation hood or no hood at all. In such case the former may be better that nothing, as it catches at least a bit of the unwanted particles and odors, provided the filters are regularly replaced.
Burning gas produces noxious gases such as carbon monoxide and nitrogen dioxide that recirculating hoods do not remove.
Ducted hoods can be made quieter by locating the extractor fan at the far end of the ducting, away from the kitchen.
Recirculating hoods require regular replacement of their carbon filters, meaning expense. Ducted hoods don't have or require filters.
Because of the carbon filters in recirculating hoods they can not achieve the airflow ducted hoods can for equivalent motor sizes. If you're doing lots of frying or high heat wok cooking, your hood must be sufficiently capable or you will just get a horrible nasty grease film EVERYWHERE. This is indeed a fire hazard with under-spec'd extraction often being the cause of Chinese restaurants burning down.
Depending on the brand/model you purchase you may not have to buy replacement filters, but instead clean them in the dishwasher and/or by baking them in oven.