Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. Compared to conventional cooking, the nutritional content of some foods may be altered differently, but generally in a positive way by preserving more micronutrients - see above. There is no indication of detrimental health issues associated with microwaved food.