Sulfates are commonly occurring ionic salts, mostly derived from sulfuric acid. They have a wide variety of industrial applications, and can be found in differing forms in applications as diverse as construction materials and home health products. The natural mineral form of hydrated calcium sulfate is gypsum, used to make the plaster in drywall. Iron sulfate is found in mineral supplements designed for both humans and animals, while copper sulfate can be used to kill algae. Epsom salts are a common form of magnesium sulfate, and lead sulfate is used in car batteries.
The form of sulfates used in home care products is most often a sodium laureth sulfate, or SLES. These chemicals are cheap foaming agents, found in many soaps, shampoos, and toothpastes. They are surfectants, used in cosmetic products for cleansing and emulsifying properties (They bond to oils on your skin and allow them to dissolve in water). Along with sodium lauryl sulfates, a slightly more powerful variation, SLES is a known irritant, an effect that increases with concentration.
While sulfates are naturally occurring, the real issue stems from a manufacturing process, ethoxylation, which results in sulfates being contaminated with 1,4 dioxane, a carcinogenic by-product. A leading groundwater contaminant, 1,4 dioxane is referred to by the CDC as "probably carcinogenic to humans," toxic to the brain and central nervous system, kidneys, and liver.
A major problem with sulfates is fact that they are absorbed through the skin and membranes. Since our bodies don't have the proper enzymes to break the chemicals up and metabolize them, contamination is a gradual but cumulative process. This is a major source of confusion when reading studies on SLES: High levels of exposure are unusual, yet repeated, slight exposure, combined with repeated subjection to other chemicals in health and beauty products, has been linked with a number of negative effects in humans.
According to Environmental Working Group's Skin Deep database (which is seriously worth browsing time), SLES/SLS have been linked to:
Lest you think that your hands are tied, however, there is much that you can do to protect yourself from harmful effects. With increased public awareness comes a new generation of products aimed a chemical-wary consumers, though some have other drawbacks. Sulfate-free shampoos and soaps, for instance, are commonly marketed, although some contain other harsh chemicals.
It's always a wise idea to investigate what products you're using, and in the age of the internet, we have resources like Skin Deep and Good Guide to help us make our way through the sea of information. Bookmark those sites and use them next time you're thinking of buying a product. I promise you'll be happy you did (added benefit: Good Guide is awesome for evaluating food products as well).
Also, when you are investigating products and reading labels, pay attention to the order in which they're listed. Manufacturers are required to list ingredients in descending order by volume, so the first few things you see are the most prominent in that product. As a good rule of thumb, if the first few ingredients are things you can't pronounce, you probably don't want them in your body.