(CNN) -- What rules must a turkey sandwich follow in order to board an airplane?

With the six needles found in sandwiches on four international Delta Air Lines flights from the Netherlands to the United States, it's a question already stressed travelers are starting to ask. Assuming you get any food on your flight these days, how do you know it's safe to eat?

Each country is responsible for setting its own in-flight catering rules, says Perry Flint, a spokesman for the International Air Transport Association, an airline industry group. The United Nations' International Civil Aviation Organization also has standards and guidance on catering security.

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