Sodium Lauryl Sulphate (SLS) is a very widely used chemical as it is found in a great many foam producing products. (ie) shampoo, bubble baths, liquid soaps, some toothpastes, face washes, but also car washes, floor cleaners etc.
It was originally used as an industrial detergent and there has long been controversy over how toxic it actually is. There is little doubt that it does strip oil from the skin and hair and can cause the skin to become irritated. It is also said by some sources to impair the growth of the hair and because of its low molecular weight it is possible that it can be absorbed into the body via the skin, inhalation and ingestion and accumulate in the tissues.
The same sources also claim that it has been associated with the development of cataracts and with reoccurring mouth ulcers (via microscopic damage done by SLS in toothpastes).
I have not come across any evidence that it can cause cancer directly but it is possible that it can react with other common chemicals that contain nitrogens to form nitrosamines that have been associated with cancer.
Some products contain Sodium Laureth Sulphate, which is similar but considered a milder version as it has larger molecules that are supposedly less able to penetrate the skin and is considered less irritating.
Also many so-called natural products will also contain SLS that has been derived from coconut oil, but I have not been able to find any evidence as to whether this is any less irritating than the product derived from petroleum.
There is a huge amount of very confusing information on the internet about SLS much of it disputing derogatory information put out by several marketing companies. It does appear to be agreed however that this chemical is a skin irritant but unlikely in its own right to be cancer causing.
It is therefore possible that this chemical contributes to dry scalp problems.
I would try a product that is SLS free and find out!
If the problem continues then it may well be due to a deficiency of Zinc, in which case supplementing with either Zinc or better still with Colloidal minerals should eliminate the problem.
See page on ‘Zinc’