According to Anne Marie Helmenstine, Ph.D. of About.com Chemistry, potassium alum is normally safe but can be toxic in large doses. It is commonly used in deodorant, and (as an FDA approved food additive) it's the aluminium in alum makes vegetables and fruits sturdier, producing a firm cherries and crisp pickles. Long-term exposure to the aluminium ions in alum can cause sickness.
Potassium alum can irritate mucous membranes and the skin, and breathing alum can damage the lungs. Alum is a form of aluminium sulfate, which can cause vomiting when ingested. It occurs naturally in rocks that contain aluminium minerals and sulfide. Alum is common in areas of oxidation and weathering. It is an electrolyte that can alter the ionic equilibrium of blood.
Alum inhibits proliferation of odor-causing bacteria, but does not reduce sweat production. It is also used as an aftershave and in reducing minor bleeding. Additional uses for alum include the preparation of cheese and flour, water clarification, use as an astringent, tanning leather and sea food preservation. Baking powder contains a type of alum called sodium aluminium sulfate.
Some people are allergic to potassium alum, and long-term use of alum can cause degeneration of nervous tissue. Exposure to aluminium increases risk of brain plaques, Alzheimer's disease and cancer, according to About.com. People experiencing discomfort on application sites should discontinue use of potassium alum.