figures ... here.
Tried using google? (Include sv in your search words to pick up scientific references quoting radiation dosages, rather than press rubbish by people who don't know what an sv might be).
http://www.dft.gov.uk/pgr/security/aviation/airport/securityscanners/securityscanner/
There's a table about a page in.
Rapiscan Secure 1000 x-ray backscatter body scans (return flight with one set of scans at each embarkation)
Effective radiation dose = 0.12 micro Sv
1.4 minutes flying at airline cruising height
Effective radiation dose = 0.12 micro Sv
Your annual average background radiation dose in the UK is 2700 micro Sv.
Incidentally, in common with all X-ray equipment (and for that matter, aeroplanes) the people most at risk are the staff operating the machines. Scanner operators are exposed every time a person is scanned (if they stand back they get a fractional dosage, but even if security staff are allowed to do that, it's cumulative, for hundreds or thousands of scans per working day. Likewise, airline flight crews get the enhanced radiation dose every working hour at altitude.
(per annum, something like 2-3 times the natural background, based on the above figures).
I'll pass on backing up the flip comment about Aberdeen Railway station. It's well-known that it is built of naturally radioactive rock, and that if it were a nuclear power station it woiuld have to be closed down -- but I don't have time to chase up a reference.