As already said there are extraction fans and recirculating fans. Extraction is by far the best option as this will remove moisture laden air (steam) from the cooking area and discharge it externally. The recirculating fan with charcoal filter is fine where cooking is a novelty! The filters need to be cleaned/ replaced on a regular basis esp. if a lot of frying is done. In apartments where the developer would be fitting out the kitchen with built in units it is normal practice to supply the extract fan/ hood and duct it appropriately. However in many new houses, where allowances are included for kitchens, tiling etc (as opposed to the builder actually supplying them as part of the contract) it is normal for a vent only to be left in the external wall for the provision of a fan later by the purchaser. In some small one bed apartments there may not be sufficient room on the external wall (with window opes, other vents, flue terminals and drainage pipework) to site a dedicated vent for a mechanical extract hood. In this instance the purchaser will be reliant on a recirculating hood.