Under cabinet hoods should vent outside through a 4-6" vent typically hidden by the cabinet above that runs up through the roof. It's always a good idea to double check that the vent runs all the way up--you would be suprised how many vents are never connected in the cabinet above the hood or even up through the roof. The same holds true for any exhaust fans in your home as well.

Over the range microwave vents just kind of recirc--they pull the air up through the filters under the microwave then push the filtered air out the top vent on the front of the unit. Most if not all the grease in the air it pulls in should get trapped in the filters & not released back in through the upper vent.

Hope this helps!

Yes, it does, thank you!

Only if I could understand the downdraft one now... LOL

Surely OTR micro vents can also be ducted to the outside? Not saying that makes them effective but I am pretty sure my neighbor has his vented outside.

eleena: Downdraft is where you have a vent either mounted in the cooktop or a few inches above it. The effluent from cooking is supposed to go DOWN the vent. I used to have one. I think the only thing that is essential to know is that you DON"T want one. I found it noisy and almost useless.

Thank you, buffalotina!

Yes, it seems that "everyone" thinks they are useless (I searched GW). And I don;t have a basement, so that I out. I am trying to figure out if I could cook on my open peninsula w/o installing a visible ceiling hood (which would be an eye sore). No such luck I guess. :-(