A good question and a good answer are also there. There are many misconceptions about cooking in microwave ovens. There is nothing bad or unhealthy in cooking in microwave whether it is covered or not. You may cover food while cooking in a microwave oven just for preventing the food content splashing inside the oven while heating. That's all I can find there as a food health researcher. Actually, the dangerous things while cooking is nothing but those preservatives and taste makers. Never use those kinds of stuff. You know one thing the manufacturers of these kinds of taste add-ons are the owners of medicine companies too. So beware of colors,taste-makers, and preservatives you are using in your food.