Sulfur Dioxide (SO2) is a food preservative and is part of a group of sulfiting agents which includes sodium sulfite, sodium and potassium bisulfite, and sodium and potassium metabisulfite.  Preservatives serve as either antimicrobials or antioxidants, or both.  Sulfur Dioxide does serve both purposes.  
    1.  SO2 acts as an antimicrobial - preventing the growth of molds, yeast, and bacteria
    2.  SO2 acts an an antioxidant - which suppresses the reaction that occurs when food combines with oxygen.  SO2 prevents food from becoming rancid, turning brown or developing black spots, in other words - discoloration.  The antioxidant properties of SO2 also minimizes the damage to some essential amino acids and prevents the loss of some fragile vitamins like vitamin C.

SO2 can be counted as safe because the human body converts any surplus into harmless sulfate.  This process of detoxification can easily cope with any SO2 that we consume.  The only exception are for those who are sensitive or allergic to SO2.  Asthmatics, for example, are particularly sensitive to the smell of freshly opened wine, which contains a moderate amount of residual SO2.

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