As to lays two traps for the lazy or careless writer.
(i) Like many other prepositional phrases (e.g. as regards, in connexion with, in relation to, in the case of) (see PREPOSITIONS) it offers itself as a trouble-saver to those who cannot be bothered to think of the right preposition to express their meaning. For instance:
Until then it will not be possible to give any indication as to the outcome (of) .
A decision has yet to be taken as to this (on or about) .
Or it may tempt the writer into a more elaborate circumlocution:
The operation is a severe one as to the after-effects. (The after-effects of the operation are severe.)
It is no concern of the Ministry as to the source of the information. (The source of the information is no concern of the Ministry. )
(ii) As to has a way of intruding itself where it is not wanted, especially before such words as whether, who, what, how. All the following examples are better without 'as to':
Doubt has been expressed as to whether these rewards are sufficient.
I have received an enquiry as to whether you have applied for a supplement to your pension.
I am to ask for some explanation as to why so small a sum was realised on sale.
I will look into the question as to whether you are liable.
As to has several legitimate uses, but they are tricky. A sound rule is never to use it except at the beginning of a sentence by way of introducing a fresh subject:
As to your liability for previous years, I will go into this and write further to you.