The problem with sulfates
Sulfates are cheap; usually palm oil derived and produce a strong foaming and cleansing action. They are found in most commercial shampoos and toothpastes on the market as well as body washes, dish washing and laundry detergents.
The main problem with sulfates is that they are potentially irritating to the skin. The more that is in a product and the longer you leave it on the skin the worse it will be. SLS is considered safe for use in shampoo and toothpaste because it is on the skin only briefly before being rinsed away.
Products that are in contact with the skin for longer periods (like moisturisers and cosmetics) are not allowed to contain SLS or SLES at levels above 1%.
The three major offenders
SLS (sodium lauryl sufate)
This is the best known, and the most irritating. Human patch tests (typically a 24-hour exposure) confirm that SLS concentrations >2% are considered irritating to normal skin.*
SLES (sodium laureth sulfate also known as sodium lauryl ether sulfate)
Formulated to be less harsh than SLS, but also irritating and has the potential to be contaminated with 1,4 dioxane (see below)
SCS (sodium coco sulfate)
Avoids the more well know ‘L’ in the acronym.
Instead of being produced from one fatty acid it is produced from a blend and so is actually a mixture of sodium lauryl sulfate, sodium caprylic sulfate, sodium capric sulfate, sodium oleic sulfate and sodium stearyl sulfate. This would be too long on the label so they just call it sodium coco sulfate. There is an argument that because it is a blend it is less harsh but really it is almost the same thing as SLS and also causes skin and eye irritation.
I should also mention the carcinogen issue. Let me be clear, there is no evidence that either of these chemicals cause cancer, as has been put about on the Internet.
However, SLES does has the potential to be contaminated with trace amounts of 1,4 dioxane
(a by-product created during the manufacturing process) which is considered carcinogenic.**
Having said this, industry is aware of this problem and manufacturing processes have now improved so that this contamination is really not as much of an issue. So, you are not going to get cancer from this stuff, I certainly don’t want to get into any scaremongering here.
Now skin sensitivity and irritation are very individual things, so while many people will happily be able to use a sulfate containing shampoo with no problem, others will find it causes itching, redness, dryness or irritation. The other factor is how well formulated the product is, if the SLS level is lower and balanced with other components you may find it tolerable even though it contains sulfates.
Bottom line, if you have been finding a product irritates you, makes your skin feel dry or itchy then you may well want to try one that is sulfate free.
Essentially what it comes down to is how sensitive you are and how much you want to avoid harsh chemicals. The more that you fall into one of these categories the more you will want to avoid sulfates.
*https://www.ncbi.nlm.nih.gov/pmc/articles/pmc4651417/
**http://www.ewg.org/skindeep/ingredient/706089/sodium_laureth_sulfate/