Light-activated toxins scattered through the plant kingdom, notably in citrus peel, parsnips, celery, and parsley. On skin, contact followed by sunlight triggers phytophotodermatitis, the blistering burn familiar to agricultural workers; inside cells, ultraviolet light drives them to latch onto DNA as photoadducts. Normal dietary levels pose little risk to consumers, but handlers of large quantities of the fresh plants take precautions. Plant breeders even select against excessive levels in new celery and parsnip varieties to protect both workers and eaters.