Pickled vegetables are easy to be contaminated by microorganisms in the curing process, which reduces them to nitrite. Eating pickles with too much nitrite will turn hemoglobin in the blood into methemoglobin without oxygen function. Symptoms of hypoxia will appear all over the body, and people's skin and lips will be blue-purple, accompanied by poisoning symptoms such as headache, dizziness, nausea, vomiting and palpitation, which can lead to death in severe cases. In addition, in the pickling process, pickles will also produce carcinogenic nitrite compounds, which are easy to induce cancer, especially moldy pickles, and their carcinogenic effect is more obvious. According to the national regulations on the declaration of green food for pickled vegetables, the nitrite content in pickled vegetables should be 4 mg per kilogram, which is the weight of half a grain of rice. If you eat twenty or thirty grains of rice, so much nitrite can cause poisoning. Pickled pickles contain a lot of oxalic acid and calcium. Because of the high acidity, calcium oxalate is not easy to form in the intestine after eating, but is absorbed in large quantities, and calcium oxalate which is not easy to dissolve and absorb will be produced when the kidney is excreted, thus forming stones. In addition, vitamin C is destroyed in the process of pickling pickles. If the human body lacks vitamin C, the ability to inhibit the deposition of calcium oxalate crystals in the kidney and reduce the formation of stones will be reduced, which is also a factor in the formation of stones. Traditional pickled vegetables take a long time, and they can't be taken out for a few days. Generally speaking, the content of nitrite will increase continuously when vegetables are just salted, and will decrease after reaching a peak. This peak is called the nitrite peak. Some vegetables have one peak, while others have three peaks. Generally speaking, the content of nitrite is the highest about one week after pickles, but it is already very low after 20 days. It's safer to eat at this time.