Microwave ovens,if not a stand alone, that include venting for the stove as well, are engineered to make cooking and reheating foods a fast process. An important component in its operation is the exhaust function, but heat, along with grease or other liquid particles sometimes blow through the exhaust vents with the air due to the steam created when cooking usually on the stove not so much on the microwave. Steam/heat has to go someplace when cooking. Usually when this happens, the grease can adhere to the vents and cabinets eventually cause the vents to clog. The heat escaping from the vent can cause discoloration of wood cabinets. It doesn't happen overnite but I've experienced it first hand and so have friends I know, but this is mainly inside a house not a TT of MH so perhaps its' the frequency of use but after reading all the replies I won't worry about it thanks or rather it will help my DW not....:-) Thxs all