What is an irritant?An irritant in this case is a substance that creates discomfort such as itching, reddening of the skin or dryness. Most detergent based irritants have short lived effects (minutes to under an hour) but a small minority of people have severe reactions.
Why are detergents irritating?I will have to transport you back to the original post in this series. There, I explained that SLS dissolves oil from the surface of hair and skin. With this oil barrier gone, the surface of the skin is more susceptible to water loss causing it to become dry. The dryness can then trigger itching which then in turn triggers redness.
Is this irritation damaging?No the irritation is generally not damaging, just a way of letting you know that the skin is lacking oil (prompting you to apply a cream or oil to the surface). Studies done on skin indicate that the oil barrier is temporarily disturbed but does not affect the ability of cells to multiply and grow as normal. (Archives of Dermatological Research, pp615-620,1998)
When can the irritation be damaging?Eyes are a particularly sensitive surface. For example a gas produced while cutting onions can trigger tears. SLS too can trigger tears but can also damage the eye surface if kept on for long enough. This is why when using ANY shampoo (sulfate free, tear free formulae included and soap!) it is wise to avoid your eyes and rinse immediately with copious amounts of water.
SLS can be extremely damaging to skin if used in high concentrations (such as that used in laboratories NOT I STRESS NOT in shampoo). Also failure to rinse off SLS can cause serious irritation.
How can irritation be reduced?The simplest method is application of an oil prior to use and immediately after use.
Good lab tested shampoos also have a host of very sophisticated anti irritancy methods such as pH control , combination of surfactants, control of molecular size and addition of chemical anti irritants (J. Soc. Cosmet. Chem, pp 667-679,1977).
*Cocobetaine (cocoamidopropyl betaine, cocamidopropyl betaine) a popular SLS substitute is also an irritant and similar to SLS can cause contact dermatitis (Contact Dermatitis, pp 419-422, 2006). All surfactants which act to dissolve oil have the potential to cause irritation. Essentially the irritation is not about the substance applied, but rather its effect (i.e no or reduced oil on skin = water loss = dryness=irritation).