health reasons. MMW scanners are authorised but not those that involve ionising radiations (i.e. back scatter).

As to national security, even if it could be argued, you would have a hard time showing that nat sec required backscatter rather than MMW so this argument is clearly dead in the water.

As it happens, the relevant rules allow Member States to adopt more stringent security rules than specified in the reg and Justine Greening used that argument to deny opt-outs from scanning. Highly dubious legally speaking and I doubt that it would survive a challenge in a court but there you go.