How emollients work?

The science of emollient therapy is constantly developing. This reflects an increased understanding of what happens at a cellular level when an individual experiences dry skin or a dry skin condition. For an in-depth look at emollients, their chemistry and function, Loden and Maibach’s book provides an excellent, detailed overview (Loden and Maibach, 2000).

Skin becomes dry for two main reasons:

1. Natural moisture from within the stratum corneum is lost due to barrier dysfunction.
2. The natural moisturising lipids that are normally found within the skin are for some reason deficient.

The level of water in the epidermis is greater at the interface with the dermis and is least in the stratum corneum. The natural lipids (mainly ceramides) which are found in the intercellular spaces of the epidermis impede the movement of the water from the deeper layers to the stratum corneum. When there is a deficiency of these natural lipids, the movement of water is less effectively impeded and therefore more readily lost from the surface of the skin.

Because the outer layer of the stratum corneum contains only approximately 10% water, any reduction in this quantity causes the skin to loose its flexibility (Marks, 2001). When water is lost from the stratum corneum, the corneocytes become shrivelled, shrink and gaps in between them develop. These gaps allow further moisture and natural lipid to ‘escape’ leading to even drier skin. The analogy that is often used is a brick wall. The corneocytes are the ‘bricks’ which in normal skin are held together with ‘cement’, i.e. the natural lipids. As soon as either the ‘bricks’ or ‘cement’ are compromised, the impermeable, smooth structure of the skin is affected and it becomes dry. It feels rough, often sore and itchy. More detail is given about how dry skin manifests itself in specific dry skin conditions in sections 8 (Psoriasis) and 9 (Eczema).

Desquamation, the loss of skin cells from the surface of the skin, is also thought to be an important factor in skin dryness. Normally corneocytes are shed from the skin surface singly. At the correct point in time (i.e. at the skin surface), they lose the binding forces that hold them together and shed in such a manner that is not visible to the naked eye. However, in dry scaly conditions, the corneocytes ‘stick’ together and shed in a way that is visible to the naked eye. Emollients seem to correct this problem. One suggested mechanism is that water trapped in the skin by emollients activates an enzyme which breaks the desmosomal contacts that keep corneocytes stuck to one another (Marks, 2001). In doing this, corneocytes are shed normally again.

In order to increase the level of water in the skin, emollients exert an occlusive or humectant effect, or both depending on their constituents.

Occlusive effect of emollients
Lipids have an occlusive effect, trapping natural moisture into the skin by mimicking the role played by sebum. Transepidermal water loss is reduced particularly when greasy emollients with a high level of lipid, for example liquid paraffin, are used (Rawlings et al., 2004).

Humectant effects of emollients
Humectants are substances which attract water. The dermis has a high level of water in it (70% of the dermis is water). Thus when humectant substances such as urea and glycerine are added to emollients and then applied to the skin, they attract water from the dermis into the epidermis thus helping to rehydrate it. In this way, they mimic the role of natural moisturising factor (NMF). NMF is comprised of a group of humectant substances (e.g. pyrrolidone carboxylic acid) which occur naturally within the upper epidermis. Because of its water-loving properties, NMF is responsible for maintaining water in this part of the epidermis (Marks, 2001). An emollient containing humectants can only be effective if it also contains an occlusive substance such as soft paraffin. Without this the water drawn into the epidermis would not be trapped there and would be lost transepidermally.

Although not particularly well understood, it does appear to be the case that emollients have an antimitotic, anti-inflammatory and antipruritic properties.

Antimitotic effects of emollients
An experimental study on mice whose skin had been stimulated through tape stripping showed that application of emollient decreased the mitotic activity in the epidermis (Tree and Marks, 1975). The authors suggested this may have been due to either repair of the barrier function of the skin or a reduction in prostaglandin synthesis which is caused by use of emollients high in petrolatum.

Anti-inflammatory effects of emollients
Studies in the field of eczema have shown that even minor damage to the skin (through scratching) can cause the release of powerful inflammatory agents such as IL-1α. Wood et al. showed that by occluding tape stripped skin with polythene, release of IL-1α could be prevented (Wood et al., 1996) and Cork speculated that this action is replicated when a layer of occlusive emollient is applied to the skin (Cork, 1997).


On a practical note, it is worth noting that the physical motion of applying the emollient can make the skin look more inflamed. Two mechanisms are possible. Firstly, whilst vigorous rubbing in of emollients is to be avoided, the very process of emollient application can increase blood flow to the skin surface which mimics the appearance of inflammation. Secondly, if the skin is scaly this will cover up the underlying inflammation and make the skin look a duskier colour. As an emollient will help to reduce the level of scale, it may, in turn, make the skin look more inflamed when it is applied to the skin. Patients should be reassured in both instances that the emollient is not causing further inflammation.

Antipruritic effects of emollients
Experience shows that applying emollients to dry, itchy skin usually provides some transient relief (even those without specific antipruritic agents in them). Emollients containing high levels of water may be more relieving as there is a cooling effect caused by water from the product evaporating from the skin.

Effects of added active ingredients to emollients
A number of different effects can be created by adding further substances to a basic emollient base.

Anti-infective agents: Whilst independent evidence for the impact of anti-infective agents is hard to come by, it would appear that these products do reduce the bacterial load on the skin which can be very helpful for people who suffer recurrent infective episodes of their skin, e.g. those with atopic eczema. Examples of the anti-infective ingredients are chlorhexidine hydrochloride and benzylkonium chloride.

Antipruritic agents: Lauromacrogols have an antipruritic effect which, whilst not entirely understood, probably work by inhibiting the transmission of itch sensations through the unmyelinated C fibres. They also act as an anaesthetic when applied to mucous membranes or bruised skin (Bettzuege-Pfaff and Melzer, 2005).

Descaling agents: An example of these is salicylic acid which is used to break down hyperkeratotic skin and thus remove the build up of scale.