To be 100% safe you need a Faraday cage.
If you know the frequency of the radiation, then you can compromise with a mesh of conductor instead of solid conductor, but the holes have to be less than the wavelength of the RF you are trying to block.
Observe a microwave oven door for a good example of this in practice.
In the context of the thread, I'd say have a few candles around just in case
A..