No, they aren’t.
While the internet is filled with theories on the harmful nature of microwaves attributed to the rays it uses to heat food, there is no solid scientific proof about it. The only danger posed occurs when you place items such as metal, plastic, or tin foil in them.
You can check if the material you intend to use is harmful by placing it in the microwave for about 45 seconds and if it is hotter than the food you are preparing then it is harmful.