It’s the middle of summer, and hopefully, if you’re heading out in the sun, you’re taking the precaution of applying sun cream beforehand. Sometimes, however, you can end up with sunburn despite your best efforts to prevent it. After sun and moisturisers can help to soothe the burn – here, we take a look at the chemicals that allow them to do their jobs.

Before discussing moisturisers themselves, let’s look at your skin. Skin is composed of three main layers: the epidermis, the dermis, and the hypodermis (also known as subcutaneous tissue). The hypodermis anchors skin to muscles, and also contains fat, but it’s slightly less important to our discussion. The dermis, on the other hand, is very important for water storage – in fact much of the water in our bodies is stored here. It has plenty of other roles too: it contains blood capillaries, sweat glands, nerve endings, and hair follicles. In the context of the skin it’s responsible for providing water, nutrients and energy to the epidermis.

The epidermis is the outermost layer of skin – the layer we can see from the outside. It itself can be subdivided into 5 different layers, the uppermost of which is called the stratum corneum. This outer layer is composed of dead cells called corneocytes, which are filled with the protein keratin. These form protein-based ‘bricks’. In between these are a mixture of compounds, primarily lipids, that act as the ‘mortar’, holding the ‘bricks’ together.

These lipids (mainly a mixture of ceramides, cholesterol, and fatty acids) play an important role in allowing the skin to retain water, as they form a semi-permeable barrier to water. There are also substances in the skin called natural moisturising factors (NMFs) which aids the retention of water in the epidermis. Some is lost, however – the process of its loss is known as transepidermal water loss (TEWL). This water loss occurs as a result of diffusion and evaporation of the water. It’s not something we can control, so in cases of dry skin or sunburn, we have to take steps to temper it.

Damage to the skin, as in the case of sunburn, boosts TEWL; so do high temperatures, wounds, or even just very dry conditions. Moisturisers can assist and help us fight back against this increased TEWL. In order to do that, they contain a wide range of ingredients, but in terms of those that actually help the moisturising process, we can split them into three key groups.

The first of these groups is the occlusive agents. These are essentially the most primitive method of preventing TWEL; they are usually hydrophobic (water-repelling) ingredients which form a non-permeable barrier over the skin to prevent water from escaping. The most common example you’ve probably come across is Vaseline, or petroleum jelly, which is a mix of hydrocarbons containing 25 or more carbon atoms.

Whilst occlusive agents are effective at preventing water loss, they can leave the skin with a greasy, oily feel, and as such their use on their own in moisturisers is usually avoided. The layer they form can also trap the heat given off by sunburn, exacerbating skin damage – so don’t go smearing vaseline on it!

Humectants are the second key group of compounds. These agents function differently, as unlike the occlusive agents they are hydrophilic, and attract water. Some of the natural moisturising factors in the skin, such as hyaluronic acid, are also used in moisturisers as humectants. They draw water up from the dermis to the epidermis to keep it hydrated. Other common humectants include glycerin, sorbitol, and urea.

Whilst humectants can help to moisturise the skin, they’re also a bit of a double-edged sword. By drawing water up to the top layer of the epidermis, they can also lead to increased evaporation of water from the skin surface – and actually worsen any dryness, rather than improving it. As such, like the occlusive agents they’re commonly used in combination with compounds from the other two groups, rather than individually, particularly as at high levels they can cause irritation.

The final main group of compounds used in moisturisers are the emollients. In fact, some compounds that act as occlusive agents can also act as emollients in moisturisers. When applied heavily, emollients can provide a similar layer on top of the skin that prevents TEWL. However they can also help prevent water loss by ‘plugging’ gaps between dead skin cells in the upper layer of the epidermis. Additionally they help smoothen rough skin.

Emollients can often be compounds naturally found in the skin’s epidermis – for example ceramides, cholesterol, or fatty acids. Mineral oil and squalene can also be used. Compared to the other two groups, emollients have no real issues, though like occlusive agents they can sometimes be a little greasy.

Not all of the ingredients in moisturisers are there to moisturise. Another necessary additive is some form of fragrance, to mask the smells of the moisturising ingredients used. Additionally, preservatives must be used to prevent the moisturiser from turning rancid. Water-based moisturisers completely free of preservatives are very hard to come by, because, unless they’re sold in very small quantities, they run an almost inevitable risk of bacterial contamination – not good news!

Now we know all about the compounds that moisturisers are composed of, just one question remains: do they actually work? Reviews seem to suggest some evidence of benefit, but the lack of good quality clinical trials for a large number of moisturisers has been criticised. As such, whilst they seem to be effective, there’s no evidence to categorically prove that one combination of ingredients is better than another.

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