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Is it safe to keep used egg shells in the carton?

No, you should  never put egg shells back in the carton after cracking them if there are still intact eggs left in the carton. Bacteria has the potential to be on the outside, as well as the inside of an egg, and mixing cracked eggs with intact eggs greatly increases the risk of bacteria transfer by hands, utensils, air, etc.  We would also recommend never reusing egg cartons as they can be a reservoir for bacteria. Throw away egg cartons as soon as the eggs are all used.