In the following sentence:
Но насколько на самом деле опасна радиация?
This sentence does not seem to work if I use опасная (long form). I checked the rule for long/short forms, but both forms seem to work. I'm not usin any complement to the adjective (like готов к экзамену) nor am I using any infinite verb (like намерен выполнить всё), so I thought both forms work.
But why is опасная not correct here?