Ever burned something while cooking? Then you know that if you have a stove, you need an exhaust.
With a kitchen island, you do not have the usual option of an exhaust hood against the wall (with the vapors drawn straight out a hole in the wall). Kitchen islands are removed from walls. So what's the solution?
Two solutions, really. Up or down. Higher-end stove installations in islands hang a hood over the stove, with the smoke being drawn straight up a tube and out of the house. Other arrangements might use a downdraft cooktop. A grate in the stove next to the burners sucks the smoke straight down, and then out of the house through the crawlspace.
Up or down: which is better? The above-the-stove hood does a better job, since smoke and steam rise. However, the downdraft element is usually less expensive.