What's the deal with sulfites?

Are they substances we need to be concerned about?
When I buy a bottle of wine, it says, “contains sulfites.” Are sulfites harmful?
Sulfites are a commonly used preservative found in foods, alcoholic drinks (especially wines), and in medications. They are anti-oxidants that are often used in salad bars to prevent the lettuce from browning and on dried fruits to prevent them from becoming very dry and stiff. It is also produced by the human body at the level of about 1000 mg (milligrams) per day. 
Basically, sulfites are a derivative of sulfur and a natural by-product of fermentation. It is a natural chemical used almost universally in small quantities in winemaking to prevent spoilage and oxidation. Wines sold in the United States that contain more than ten parts per million of this preservative must be labeled “Contains Sulfites.”
Most wines contain very low levels of sulfites, which have been used for hundreds of years by winemakers to clean and sterilize equipment and barrels; to kill off bacteria; and to prevent browning and possible spoilage. Sulfur is also sometimes sprayed in a vineyard to prevent disease and pests. Most wines still contain low levels of sulfites even when the winemaker doesn’t utilize sulfur anywhere!
Sulfites can be considered a respiratory hazard in the undiluted state, necessitating careful handling, but are entirely safe at the levels in which they are used for winemaking.
Consumption of food preserved with sulfites generally is not a problem except for a few people who are deficient in the natural enzyme to break it down. The Food and Drug Administration estimates that one out of one hundred people is sulfite-sensitive, and that 5 percent of those who have asthma, are also at risk of suffering an adverse reaction to sulfites. For these people, the additional sulfites from food can be a problem. If an asthmatic lacks the enzyme sulfite oxidase, they shouldn’t consume foods or beverages that contain sulfites. About 1% to 2% of people will have an allergic reaction to sulfites, which can consist of nasal congestion and sneezing, skin hives, or wheezing and difficulty breathing. Persons with asthma and/or allergies to aspirin are particularly sensitive to sulfites and could have a serious anaphylactic reaction, in which there is severe swelling of the throat, tongue, and airway, obstructing breathing.
There have been reports of severe and life-threatening reactions when sulfites were added at erroneously and enormously high levels (100 times recommended usage) on salad bar vegetables. 
There are many erroneous ideas about sulfites, so to put the record straight:
  • All wines contain sulfites. Yeast naturally produce sulfites during fermentation so there is only a rare wine which contains none.
  • The US requires a “sulfite” warning label and Australia requires a label indicating “preservative 220,” but nearly all winemakers add sulfites, including those in France, Italy, Spain, Australia, Chile, etc. The wine you drink in foreign countries contains sulfites, but you are not being warned about it when purchased abroad. Survey studies show that European wines contain an average of 80 mg/L sulfites just as in the US.
  • There are a very few winemakers who make wines without adding sulfites. In the US, organic wine must be made without added sulfites. These are unusual because the wine is very perishable and often have unusual aromas from the aldehydes that are normally bound and rended aroma-less by the sulftes.
  • Sulfites do not cause headaches! There is something in red wine that causes headaches, but the cause has not yet been discovered. Many people seem to connect their headache with the sulfite warning label, but sorry, there is no connection. To avoid headaches, try drinking less wine, and drink with food. If you think sulfites are causing your headache, try eating some orange-colored dried apricots, to see if that induces a headache. These bright colored dried fruits typically have 2000 mg/kg sulfites, so a two ounce serving (56 gm) should contain about 112 mg sulfites
In the US, the law states that…
  • Wines cannot contain more than 350 mg/liter
    sulfites. Wines with more than 10 mg/liter must have a “Contains Sulfites” warning label
  • Producers must show levels below 10 mg/liter by analysis to omit the label
  • Wines must have less than 1 mg/liter to have a label that says “No Sulfites”. This level must be shown by analysis
  • All wines must carry the label whether made in the US or abroad