A downdraft hood is ducted downwards and out of the house just like any other hood, or  you can get one that uses charcoal filters which are very efficient and you don’t have any location restrictions on your new hood.  If you are ducting downwards, then you need to make sure you’ve taken extra measures to ensure that grease isn’t collecting at the bottom of the duct, as this becomes a fire hazard and a food source for rodents. If they then get stuck in your ducting, your hood won’t be able to get rid of kitchen doors as a very different one will start emanating into the kitchen.