How dangerous is a magnetron?
Some magnetrons use beryllium oxide as the &quot;ceramic&quot; looking insulators inside of the ring magnets on both the &quot;Stem&quot; and the &quot;Antenna&quot; ends. Reference the image below, the beryllium oxide parts are the pink items in the middle. They are totally inert if undisturbed.

Image Source: Toshiba Hokuto Electronics Corporation - Magnetrons for Microwave Oven
Not all magnetrons use that for the insulators, but it's virtually impossible to tell if they did so you must assume they do. It has to get airborne to become dangerous. So just don't go crushing and snorting the ceramic dust and you will be fine. If you do happen to break one, don't use a vacuum cleaner, clean up with a damp rag and get ALL of the dust, then dispose of the rag while still wet by putting it in a plastic zip-lock bag.
I take apart magnetrons from old microwaves that I get for free and harvest the magnets, they are cool and powerful. I then put that center assembly into a thick plastic zip-lock bag before disposing of it.