The frequency of the electromagnetic radiation in a micro wave oven is thousands of times lower than gamma and x rays, so negligible ionization is produced in the food unless you heat the food to very high temperature. Typically food that has been microwaved is safer than food heated by other methods, but I suppose rare health exceptions occur. If heated in a plastic container, there is some possibility that toxic gases from the plastic will enter the food, but that is also true of heating food in plastic containers by other food heating methods. Neil