It seems to me WO'N, that the only common thing about these words is the prefix 'ex', which implies, out of or out from. The 'h' in most words that you have as examples come, I suspect, from roots which start with the letter 'h'. Exhale, for e.g., I know is from the Latin halare, which means breathe. Exhume is another that comes to mind; hume is from humus, for ground. It is interesting that 'h' in these words is invariably followed by a vowel, but that might be incidental to the pronounciation of h. It is difficult to enunciate when followed by consonants and is therefore probably not commonly used that way.