Googled it, found an explanation on
WikiMustard oil irritates mucous membranes; therefore, excessive internal use has been known to cause stomach problems and kidney irritation[citation needed]. Breathing the vapors of a mustard plaster can trigger sneezing, coughing, asthma attacks, and/or eye irritation. Leaving a mustard plaster on the bare skin for too long will lead to burning, blisters, or potentially even ulcers. A mustard plaster should never be left on for longer than 30 minutes. The actual mustard paste never comes in direct contact with the skin, just the cloth on which it is spread. The paste resembles the Chinese mustard sauce served with eggrolls in many American Chinese restaurants.