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If there are a few crisp-ish stalks, look and see if they are turning white or brown... if they are still green, they are, at least according to my research, still all right. But if they are getting limp and turning colors, let 'em go to the compost bin. Old celery can cause food poisoning long before you get enough to cause cancer... but Thalia went for the biggest gusto she could to get that owner to do what he needed to do.