In a metaphysical sense, radiation usually propagates invisibly through air but can burn like fire so philosophers (assuming they are aware of it) would likely considered it a combination of air and fire; a "burning wind" of sorts. Since only it's effects are easily observable, in the form of radiation burns and poisoning, primitive folk would likely consider it to be some form of sickness caused by tainted air (much like malaria).
From a mechanical standpoint, the damage radiation causes could most easily be equated with fire damage, though I suppose a case could be made that it is as similar to electricity damage as it is to fire.
Not sure if this helps answer your question. :\