An over-the-range microwave only needs an exhaust fan vent if it is required by code. However, exhaust fans are recommended for removing odors, heat, smoke and fumes that collect above the stovetop.
Over-the-range microwaves can vent to the outside or can recirculate the air. When a microwave is replacing a standard range hood, it is usually best to use the existing venting method. Microwaves that vent to the outside force the air through duct work to the outdoors. Microwaves that recirculate force the air through a filter and then vent the air back into the room. Homeowners who install over-the-range microwaves should follow manufacturer's instructions to ensure proper ventilation.