Many authors tested this by running their negatives and slides with x-ray machines. Tests were done some 8-10 years ago. General guideline is don't run anything faster than iso 400 on the safe side. These folks put their film through x-ray machines minimum of 6 times (cos of interstate flight transfers etc) and still no obvious colour change. Additional risks arise if you put them through those very old x-ray machines cos they use higher radiation dose.
Controversy over lead lined bag rages. Cons include:
1. operators will force you to take film out of bag to scan them anyway
2. operators will increase radiation dose until they can see something
3. operator will pull film out to manually check that is really film, thus exposing your film
Personally, I've brought provia100 film through x-ray machine 6 times in all before developing, including that in China and Mongolia in a single trip which may be using older machines. No obvious problems.