The "microwave oven" has taken over kitchens all over the world: at home, in the office, in institutions, and in restaurants. For some reason, it seems to be generally "accepted" that this can be used wi
thout any restrictions for both meat and dairy dishes (one after the other, of course). But this practice has no real halachic foundation, and it is based on the mistaken assumption that the walls of a microwave oven do not absorb or discharge vapors. Any person who regularly uses such a device can feel that the walls, the roof and the bottom of the oven become greasy, and that the flavors are indeed absorbed as a result of the heat caused by the radiation.
In general, the policy of strict separation between meat and dairy is one of the most basic tenets of the Jewish kitchen. It is not a good idea to create a "mixing point" in the main heating device in the kitchen, one that creates vapors every day, without maintaining some elementary type of separation. (Remember that there is a halachic concept of relatively strict rules for a vessel that was last used less than one day before – ben-yomo.)
For anybody who cannot afford two separate microwave ovens (or who does not have enough room in the kitchen), the recommended procedure is as follows: One type of food – either dairy or meat – will be heated without any limits, in an open dish. The auxiliary devices of the oven, such as browning, etc, can be used for this type of food. The microwave will always be considered as ben-yomo for the defined type of food.
Any food of the other type, or parve food that will be eaten together with the other type of food, will be heated within a closed vessel – either covered by an upside down dish or inside a plastic bag. The standard microwave covers are not sufficient, because they have holes. According to the halacha, the cover does not have to be hermetically sealed, and there is no problem even if some vapors "escape" from the wrapping. One who wants to be especially strict can use two covers – an upside down dish, with an additional plastic bag surrounding everything.
Anybody who visits an institution with a publicly used microwave oven (such that there is no way to control how the device is used) should heat his own food inside a plastic bag.