"I'm going to need you to change your gloves." in a polite, firm voice always works.

Make sure that your tone of voice indicates that it is a statement and not a request. Be prepared for childish reprecussions such as a delay or a more thourough search, but do not take no for an answer, even if they have to go and find a box of clean gloves.

They should not even be going through your belongings unless there is something that cannot be identified on the x-ray. Going through your stuff with filthy gloves is completely unacceptable.

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"Give me Liberty or give me Death." - Patrick Henry