The structure of the sulfite anion can be described with three equivalent resonance structures. In each resonance structure, the sulfur atom is double-bonded to one oxygen atom with a formal charge of zero (neutral), and sulfur is singly bonded to the other two oxygen atoms, which each carry a formal charge of -1, together accounting for the -2 charge on the anion. There is also a non-bonded lone pair on the sulfur, so the structure predicted by VSEPR theory is trigonal pyramidal, as in ammonia (NH3). In the hybrid resonance structure, the S-O bonds are equivalently of bond order one and one-third.
In the US, wine bottled after mid-1987 must have a label stating that they contain sulfites if they contain more than 10 parts per million.
Most beers no longer contain sulfites. Although shrimp is sometimes treated with sulfites on fishing vessels, the chemical may not appear on the label. In 1986, the Food and Drug Administration in the United States banned the addition of sulfites to all fresh fruit and vegetables which are usually eaten raw.
Some humans are allergic to sulfites. It is an undeclared allergen that may cause breathing difficulty within minutes after eating a food containing sulfites. Asthmatics and people with allergies to aspirin (also known as salicylate sensitivity) are at an elevated risk for reaction to sulfites. The reaction can be fatal and requires immediate treatment at an emergency room, and can include sneezing, swelling of the throat, and hives. Those who are allergic to sulfites are urged to avoid products that could contain them.
Sulfites are also known to destroy vitamin B1 (thiamin), a vitamin essential for metabolism of carbohydrates and alcohol.