OK - as a radiation oncology physicist I'll have a go at clearing up a few things....
Firstly, as far as irradiation of food etc goes, there is absolutely no 'residual radiation' or radioactivity emanating from anything which has been exposed to low-energy (KiloVoltage) x-rays such as used in security scanning equipment. Its only when you get up to really high energy stuff (10 MegaVolts and above) as sometimes used in radiation therapy that any 'aftereffects' are seen in the form of neutron activation, and thats way out of the league of any airport scanner. Anything you touch or use after it has passed through is not having any effect on you whatsoever.
At the atomic level, the item passed through the scanner may suffer some ionisation and subsequent production of free radicals which, from a single scan, would have no significant effect on it. If it were sensitive electronic equipment then theoretically it could be damaged by undergoing a huge number of scans, but in the case of food there's not going to be anything different about it. Industrial irradiation of food to much higher levels in order to destroy microorganisms has been common practice for a while and the effects of this carefully studied - again, the doses involved are orders of magnitude larger than the scanner doses we're talking about here. This aspect of your occupation is much less likely to be a hazard than the 'western' dietary aspects which are an acknowledged risk factor for all of us eating too much fat and not enough fibre.
Secondly, your dentist is only doing exactly as he should as an occupationally exposed worker. You may only have maybe 20 medical x-rays in your life to accumulate radiation exposure - he may end up x-raying thousands of patients over his career so even though he wouldn't be directly in the beam he'd cop a bit each time and it would add up. There are well defined rules and dose limits for all radiation workers to minimise the risks involved, and there are also limits on how much members of the public may be exposed too, although these limits exclude justified medical exposures such as your dental x-ray.
Now, as for exposure of flight and cabin crews to cosmic ray irradiation...that's been covered elsewhere I think!