If you’ve ever tried Beats® headphones, you’ll have felt the rich, powerful baselines they give you without

overwhelming the rest of the music. Beats® and its music streaming service, BeatsMusic, have become so popular so

fast that they now recently acquired by Apple for a whopping $3 billion, making co-founder Dr Dre the most

financially successful hip-hop artist of all time. By far.

Here’s how this best-selling product works. Each headphone contains a neodymium magnet (the strongest known

permanent magnet). When an electrical signal from your iPod (or similar) passes through the gold-plated audio cable

to the voice coil,electromagnetic induction gives the voice coil a variable magnetic field. The exact strength and

timing of the variable magnetic field represent perfectly the music being played. The voice coil’s magnetic field then

interacts with the magnetic field of the headphone’s neodymium magnet via magnetic attraction (or

repulsion), which moves the diaphragm, which sits between the magnet and the ear. When the diaphragm moves,

it creates differences in air pressure (sound waves) that are detected by the diaphragm in your ear.

Excellent sound quality requires an air-tight seal between the headphone’s diaphragm and the diaphragm in your

ear. Overstuffed leather guarantees this air-tight fit. Leather requires over 20 treatment processes before it’s ready

to use in manufacturing. One of those processes is dying using polyazo dyes. When used in lower concentrations,

these dyes are brightly-coloured; when mixed together and used in very high concentrations, they give an overall

‘black’ appearance to the leather.

The Beats® headphone frame is made from strong anodised aluminium. Aluminium, a strong yet lightweight metal

perfect for making wearable tech, is anodised to increase its ability to resist wear-and-tear. The aluminium

headphone frame is dipped into anelectrolytic solution with a ~20-volt direct current flowing through it. Bubbles

of hydrogen form at the cathode, and bubbles of oxygen form on the surface of the headphone. This oxygen gas

buildup quickly oxidises not only the surface of the headphone, but deep into pores in the surface, which give the

frame very high resistance to corrosion. ●

Indican is a colourless, water-soluble compound extracted from leaves of the Indigoferaspecies. Indican is

a dextrose molecule conjugated to an indoxyl group by a glycosidic ether (C–O–C) bond.

The indican is hydrolysed at high pH, which separates the dextrose from the indoxyl group. The resulting indoxyl

compound is whisked to aerate it, which causes the indoxyl molecules to oxidise and dimerise into indigotin, which

is the famous blue dye used in Levi’s® jeans.

However, the indigotin blue dye isn’t soluble in water, and must be changed chemically before the jeans are dyed.

Indigotin is subjected to high pH again, which reduces the indigotin, forming leuco-indigotin (also known as indigo

white dye), which is, despite the name, pale yellow in colour.

Jeans are steeped in this water-soluble “indigo white dye”, which is still pale yellow at this stage! However, as soon

as the jeans are removed from the vat of dye, the leuco-indigotin oxidises back into indigotin, which is blue in

colour. The oxidised form (indigo blue) is insoluble in water, which helps the colour stick to the jeans despite being

washed hundreds of times.

Denim is a traditional way of weaving cotton into a thick, sturdy material. Cotton is predominantly cellulose, a strong

polymer of beta-D-glucose monomer units. Several thousand glucose monomers are present in each polymer chain.

Polar hydroxyl groups form hydrogen bonds with hydroxyl groups on adjacent chains to form strong microfibrils,

which the cotton plant then meshes into a strong poly- saccharide matrix. This matrix, and the denim weave, give

high strength and durability to your Levi’s® jeans. ●

LEGO® is made from ABS (acrylonitrile butadiene styrene), a thermoplastic polymercomprised of three monomers.

The first monomer, acrylonitrile, gives the bricks strength. The second, 1,3-butadiene, gives them resilience (i.e.

stops them from snapping so easily) and the third, styrene, gives them a shiny, hard surface. These three ingredients

are mixed with colorants then polymerised (hardened) with the help of an initiator calledpotassium

peroxydisulphate. LEGO® buys pre-made ABS granules and injects them into brick shapes on a massive scale.

LEGO® make 20 billion bricks each year (that’s 35,000 bricks a minute) and according to the Guinness Book of World

Records, they produce more plastic tyres than anyone else.

Firstly, it’s worth considering what it is that makes humans so alluring to mosquitos and the like. 1-octen-3-ol is the compound to blame – it’s present in human sweat and breath, and acts as an insect attractant. It’s so potent in this regard that it’s used in insect traps to lure in their unsuspecting prey. Interestingly, it’s more commonly referred to as ‘mushroom alcohol’, as a particularly optical isomer of this compound is produced by mushrooms and considered largely responsible for their aroma and flavour.

In terms of repellents, there are a wide range of both natural and synthetic compounds that exhibit insect-repelling activity, but in the EU and the US, there are four main compounds that are approved for use: DEET, Icaridin, Citrioldiol, and IR3535. Each of these differ slightly in their effectiveness and characteristics, but all of them work in a similar way, by producing an odour that insects find repulsive.

DEET is by far the most commonly used compound of the aforementioned group. It was originally developed by the US army for use World War 2, and has been available to the public as an insect repellent since 1957. It is sold in a variety of concentrations – at 100%, it has an approximate efficacy of 12 hours, but in the more commonly available 20-30% solutions, this drops to a maximum of 8 hours. Although some controversy has dogged DEET, if used as directed, guidelines have stated that it represents no risk to humans. However, it has been linked with seizures in a very small proportion of people (estimated to be around 1 in 100 million users). It’s also recommended in some countries that children under the age of 12 use a lower concentration of DEET spray than adults.

Though it may be the most common insect repellent, DEET has its drawbacks; it has an unpleasant smell, greasy feel, and can also damage plastics. For this reason, a number of other synthetic alternatives are available. Icaridin, also known as picaridin, is one of these, which has the advantage of being odourless and not damaging to plastics, unlike DEET. It also seems to provide comparable protection to DEET, and is effective against an equally large range of insects. IR3535 is another catchily named alternative compound, which also provides a similar efficacy to DEET, although some studies have shown this to be slightly lower against certain species of mosquito. Though perhaps in part because its use is less widespread, it’s worth noting that, apparently, no negative effects have ever been reported for IR3535.

The only natural compound that is approved in the EU for sale as an insect repellent is citriodiol, or the essential oil of the lemon eucalyptus tree. Overall, studies seem to show that this compound provides slightly shorter protection than DEET, but as well as acting as a repellent, it can also act as a miticide and kill mites and some insects. Despite the oil’s natural origin, some commercial products are made with synthetic versions of the active compound, p-menthane-3,8-diol. This synthetic version is no different chemically from the naturally occurring version of the compound, and acts in exactly the same way.

All this talk of repellents is well and good – but why is it that some people seem to be able to repel mosquitos without them? Whilst science still can’t be sure of the precise reasons, researchers have relatively recently made a discovery which may shed some light on this question. They found that certain compounds, that were either secreted through the skin, or generated by bacteria on the surface of the skin, can actually act to render a person effectively ‘invisible’ from a certain species of mosquito’s senses. One of these compounds is called 1-methylpiperazine, though they have also identified a number of other compounds which contain small heterocyles (rings of carbon atoms also containing atoms other than carbon or hydrogen). Obviously, this could have implications for future methods of preventing insect bites, and differing levels of these compounds from person to person could be one of several factors affecting why mosquitos just prefer some people more than others.

The main chemical used in tanning lotions is dihydroxyacetone, commonly abbreviated to DHA. Tanning lotions can contain up to 15% DHA, though the lotions available to buy in shops max out at 5%, and are usually in the 3-5% range. Obviously, the higher the percentage, the more pronounced the tanning action, though higher percentages tend to be more susceptible to streakiness after application. Another chemical, erythrulose, can also be used, though as its tanning effect is more gradual, it’s more commonly used in conjunction with DHA (if at all).

Although its skin-browning effect was noted by german scientists in the 1920s, DHA’s action on the skin was turned to utilisation for tanning lotions by complete accident in the 1950s. Eva Wittgenstein, a scientist studying the use of DHA orally for children with glycogen defects, noted that spillages of DHA on the skin led to colouration. Research on this aspect culminated with the first tanning lotion being introduced in the 1960s. The first formulations, in fairness, probably still needed a little refining – there are plenty of tales of application leading to bright orange, streaky tans, which probably still colour our perception of fake tanning today.

So how does DHA work to induce the appearance of a tan? DHA actually acts on the dead skin cells on the surface of our skin. The amino acids in this dead layer of skin can react with DHA in order to produce chemicals called melanoidins. The reaction can be classified as a Maillard reaction – precisely the type of reaction that’s responsible for browning in foods such as meat when they’re cooked (and a reaction we discussed previously when looking at the compounds that contribute to the smell of bacon). The melanoidins produced by tanning lotions absorb certain wavelengths of light due to their structure, resulting in a visually browning effect on the skin.

The effect of DHA isn’t instantaneous. It takes around 2-4 hours for the browning effect to kick in, and it can continue darkening for as much as 72 hours. Because it’s the dead cells on the skin’s surface that it affects, the tan it induces lasts for up to ten days, fading as these dead skin cells are shed, so reapplication is necessary to maintain the effect.

Tanning lotions, then, may be a convenient shortcut to tanning without the associated risk of sunburn. However, there are a few caveats. Firstly, studies have shown that, after application of tanning lotions, the skin’s sensitivity to ultraviolet light increases slightly. A 2007 study found that, for 24 hours after a lotion was applied, the amount of UV-induced free radicals produced was 180% higher than untreated skin. Some tanning lotions contain sunscreen to help guard against this, but proper sunscreen should still be applied when using tanning lotions.

You might think that, once a tan from tanning lotions has developed, this will afford your skin some natural protection from the sun’s rays. However, normal tanning is a result of the production of melanin in the skin; the melanoidins produced by tanning lotions do not afford the same protection. It’s estimated that a fake tan only affords an SPF of 3 – much too low to offer significant protection against the sun’s rays, so you need to make sure you still slap on the sunscreen!

Whilst DHA is approved for use in lotions, and the tanning effect it produces is non-toxic, its use in spray tanning booths is a little more of a grey area. This is because it’s entirely possible that ingestion or inhalation of DHA could occur during the process. There are no confirmed effects on the body at the concentrations used in tanning lotions or spray tans, but a study using much higher concentrations did find that DNA damage was observed in bacteria cells. However, the relevancy of the findings of this study to any mutagenic activity in humans has been disputed. Other studies have found that some DHA from tanning lotions is absorbed by living skin tissue (as much as 11%), but it’s unclear if this has any effect on the body.

In a slightly odder study, it was found that DHA can cause severe contact dermatitis in Mexican hairless dogs. Quite why someone decided to apply tanning lotion to Mexican hairless dogs isn’t entirely clear, but if you happen to own one… maybe go easy on applying tanning lotion to it?

In short, there doesn’t seem to be too much risk to using tanning lotions, as long as you remember that the tan they generate won’t really offer you any protection from the sun’s rays. With the evidence of increased UV sensitivity, they should definitely be used in combination with sunscreen!

Lipstick is one of the most commonly used cosmetic products – and a range of chemicals are required for its production. The choice of these ingredients is carefully considered to provide the desired colour, glossiness, and indelibility. A single stick of lipstick will contain several hundred different chemical compounds, but there are a few substances and compounds whose inclusion is essential.

An average composition of lipsticks is given in the graphic, but, in truth, this can be widely varied from lipstick to lipstick. Generally, however, waxes and oils make up the bulk of lipstick’s composition. Waxes are perhaps the most important, as they are crucial for the structure and shape of the lipstick. A range of different naturally occurring waxes can be utilised, with beeswax commonly a major constituent. Beeswax is composed of around 300 different chemical compounds; the principal compounds are esters, which make up around 70% of the composition. The remaining 30% of compounds includes organic acids and hydrocarbons.

Another type of wax used is Carnauba wax, obtained from the Brazilian Carnauba Palm, which at approximately 87˚C has the highest known melting point of any wax. Its inclusion can give the lipstick the rather useful characteristic of not melting in the sun as a result of the lower melting points of some of the other waxes used. As well as beeswax, these other waxes can include Candelilla wax, obtained from the Mexican Candelilla shrub, and lanolin, a wax secreted by the glands of woollen animals. Though they primarily provide the structure of the lipstick, these waxes can also impart other useful properties – they can act as emulsifying agents to help bind together the other ingredients, and can also impart glossiness on application of the lipstick.

As well as the waxes, another important component of lipsticks is oils. The most commonly used is castor oil, which can often comprise the largest percentage of the lipstick, but others, such as olive oil and mineral can also be utilised. The oils give the lipstick emollient, or skin-softening properties; they also make application of the lipstick easier, and contribute glossiness to its appearance. Additionally, they act as solvents for soluble dyes used in the lipstick, or dispersing agents for any insoluble pigments.

The pigments and dyes, though they make up only a minor percentage of the lipstick’s composition, are certainly the most important, as they impart the colour of the lipstick. Pigments are coloured compounds that are insoluble, whilst dyes are more commonly either liquids themselves, or soluble. The manner in which they provide colour can also vary. Carmine red, also known as carminic acid, is a common red pigment, which is derived from cochineal bugs, a variety of scale insects that live on cacti. It is prepared by boiling the insect bodies in ammonia or sodium carbonate solution, filtering, and then adding hydrated potassium aluminium sulfate (more commonly known as alum).

Another common colour imparting component is a compound called eosin. This is a dye that actually subtly changes its colour when applied. In the lipstick, it is red, with a slightly blue tinge; when it is applied, however, it reacts with the amine groups found in proteins in the skin, and this reaction causes its colour to intensify to become a deeper red. Another benefit of this reaction is that it makes the dye indelible, or long-lasting.

Of course, red is not the only lipstick colour, and in order to achieve the wide range of colours available today, other pigments and dyes are needed, of which there are a variety. Additionally, other compounds can be added in order to alter the intensity of the red coloured pigments and dyes. Titanium dioxide, a white compound in isolation, is a common addition, which can be added to red dyes in varying amounts to produce a range of pink coloured lipsticks.

Several other compounds can be added in small quantities to provide moisturising qualities, or to provide a pleasant fragrance that also masks the smells of the other compounds that make up the lipstick. Interestingly, capsaicin, the major capsaicinoid compound in chillis which is largely responsible for spiciness, can also sometimes be found in lipsticks. Its presence is down to its ability to act as a minor skin irritant, which means it can cause lips to appear plumper.

On a final note, in recent years there has been concern over the very small amounts of heavy metals that can be found in some lipsticks. A recent study of 32 popular lipsticks found trace contaminant amounts of lead, cadmium, aluminium, chromium and manganese in many of them. However, this study has come in for criticism in some quarters, as it based its human ingestion estimates on a range of different data for each metal, and also assessed the amounts ingested with the assumption that all of the applied lipstick was ingested – an unlikely scenario. Additionally, the highest levels of the metals in the study were still below the recommended daily intake. The presence of heavy metals in lipsticks is still a legitimate concern, however, particularly with no safe level of exposure to lead being recognised, and as such there is a push for a limit of specific levels of the metals in lipstick to be set. In the meantime, many companies now produce lead-free lipsticks to assuage consumer fears.

If you’re currently a student, then you’ll no doubt often make ample use of highlighters during revision. Even if your studying days are far behind, you probably still use them from time to time. But what are the chemicals behind their luminous colours? This graphic looks at some of the possible dyes that can be used.

Depending on the colour of ink required, a number of different dyes are used in highlighter pens. Yellow highlighters commonly make use of a pyrene-based dye, such as pyranine; fluorescein can also be used. Triphenylmethane dyes are used to make blue highlighters, and these can be mixed with pyrene-based dyes to produce green inks, or mixed with the rhodamine dyes used to make pink highlighters to produce a purple ink. Finally, a combination of a coumarin dye and a xanthene dye can be utilised for orange ink.

Knowing the chemicals behind the colours is all well and good, but why do they produce these different colours? To explain that, we need to talk about the interaction between light and the various chemical structures. In general, chemicals are coloured because they absorb some wavelengths of light, but not others. Highly conjugated molecules – that is, molecules with a large number of alternating double and single bonds – can absorb wavelengths of light in the visible range of the spectrum, causing them to appear different colours depending on the precise wavelengths of light absorbed.

So, the dyes in our highlighter inks are coloured due to their large number of alternating double and single bonds. But that doesn’t really explain their fluorescent appearance. After all, there are plenty of chemicals out there that contain a large number of alternating double and single bonds and are consequently coloured, but significantly fewer of them are fluorescent in the same manner as highlighter inks. This is, however, also possible to explain with chemical structure and absorbance.

As well as absorbing visible light, the chemical structures of the dyes used in highlighter inks also absorb light in the ultraviolet portion of the spectrum. When the electrons in the molecule absorb this light, they are ‘excited’ to a higher energy. The electrons do not remain in this higher energy state, but ‘relax’ back to their original state, releasing the excess energy in the form of light. This light generally has a longer wavelength than the original absorbed light; as such, despite original absorbed light having a wavelength in the ultraviolet portion of the spectrum, when it is emitted, it can be in the visible portion.

These fluorescent pigments, then, are constantly undergoing this process, and emitting visible light as a consequence of absorbing UV light. This isn’t too noticeable in normal daylight, but under a UV light, it’s exceptionally pronounced, and gives highlighter inks their fluorescent appearance.

Finally, though the dye is the vital ingredient, it makes up no more than 5% of the ink. The vast majority of the rest of the ink is a combination of a glycol solvent and water; it can also contain a biocide to prevent the growth of bacteria or fungus in the ink.

Swimming pools are a brilliant way of cooling off during a hot summer. Of course, this isn’t a particularly original idea, and hundreds of people might use a particular pool every day. Chemistry is on hand to help prevent us from swimming in water that harbours potential water-borne infections. It can also help out with the cardinal sin of pool-peeing, though not without consequence. It does this, as you likely already know, through the chlorination of pool water – although it’s less simple than you might think!

Firstly, how is water chlorinated? You might expect that it’s accomplished using chlorine, but it’s actually become quite rare to chlorinate pools using chlorine itself. This is because of the toxic nature of chlorine gas, which makes it tricky to store, and potentially hazardous to health if an accident were to occur. Instead, other chemicals which can also accomplish water chlorination are used instead.

Chief amongst these are the hypochlorites, and some of the most commonly used are sodium hypochlorite and calcium hypochlorite. These compounds have an advantage over chlorine, in that they are solids at room temperature, and can be dissolved in water, making them much easier to store and use. Sodium hypochlorite is a compound you’ve probably come across in your household, too, as it’s a component of chlorine based bleaches. It’s sold in solutions of slightly higher concentration to chlorinate swimming pools, and can also be obtained in tablet form. Of course, once it’s in the water, it’s at a much lower concentration than that found in bleach, so the fact that it’s found also there isn’t a concern.

Both chlorine and hypochlorite salts react with water to produce a different compound called hypochlorous acid. This is a relatively weak acid, but also a strong oxidising agent, and actually largely responsible for the bactericidal effects of water chlorination. Exactly how it helps kill bacteria has been debated, with it thought to effect a number of factors at the bacteria cell membrane including suppressing the metabolic function of the cell, preventing its DNA replication, and stopping proteins in the cells from being able to group together.

Hypochlorous acid partially dissociates (splits up) in water, forming the hypochlorite ion. This is an oxidant around 60 times weaker than hypochlorous acid, so it isn’t as good at helping remove bacteria from water. Luckily, the dissociation of hypochlorous acid is reversible, and we can tweak it in our favour by monitoring the acidity of the pool. Whilst we want to keep it at a pH that’s still pleasant for us to swim in, by keeping it between 7.2–7.8, we favour most of the hypochlorous acid staying put, rather than breaking up to form the hypochlorite ion. The combined concentrations of the two are often referred to as ‘free available chlorine’ (FAC).

Another issue, particularly with outdoor pools, is that of UV photolysis. This is the break up of chemical compounds in the presence of UV light. As we well know, the sun gives off UV light, which is what we try to protect ourselves from using sunscreen. This UV light can also cause the break-up of hypochlorite ions (and, to a lesser extent, hypochlorous acid). This causes 90% of the FAC loss from outdoor pools, and means that outdoor pools require more frequent chlorination. Other chemicals can also be added to the pool water to help prevent this, which is something we’ll discuss shortly.

A common side effect of a day of swimming in a chlorinated pool is stinging eyes. Often, this is blamed on the levels of chlorine in the pool being ‘too high’; in fact, the opposite is true, as we’ll discover. Firstly, it’s not actually the hypochlorous acid or hypochlorite ions that cause these sore eyes. In fact, it’s the result of compounds produced by the reaction of these with chemical compounds contained in your sweat – or in your urine.

Both you sweat and urine contain ammonia or ammonia-derived compounds. Urea is a compound we associate with urine, but it’s actually also found at very low levels in sweat too. Uric acid is another compound that’s found in both. When these compounds react with the hypochlorous acid in chlorinated water, a range of compounds are produced, including some known as chloramines.

Chloramines are the compounds responsible for the eye irritation that any frequent swimmers reading this have no doubt experienced at one point or another. They’re also responsible for the smell we associate with swimming pools. Though we often refer to this as the smell of the chlorine, it’s actually these byproducts that produce it, so if a pool smells strongly of ‘chlorine’, it indicates a higher level of these compounds in the water – which is obviously not a good thing!

The percentage of people willing to confess to peeing in the pool is actually higher than you might expect: a whopping 19% of Americans asked in a 2012 survey admitted they have, at some point, relieved themselves in swimming pool water. In light of this, the fact that 79% of those asked in the same survey suspected that other people urinate in the water. This isn’t great news, because urine increases the amount of trichloroamine present in the pool water. Trichloroamine has been accused of causing respiratory symptoms in frequent swimmers and pool workers, with debate on whether it might be responsible for inducing asthma in some.

Another chemical produced as a consequence of pee in the pool is cyanogen chloride, a chemical which can also have some pretty unpleasant effects – although, at the concentrations produced in swimming water, it’s been questioned as to whether any ill effects would be seen. Of course, there’s a simple way to help prevent production of these compounds, and that is, obviously, not peeing in the pool. If you’re a dedicated pool pee-er, you may want to reconsider your position! Michael Phelps, take note.

The chlorine contained in these byproducts of chlorination is referred to as ‘combined chlorine’ (CC). The total amount of chlorine in the pool is the sum of the free available chlorine (FAC) and combined chlorine, and it’s recommended that the FAC level should remain between 1-4 parts per million. An Olympic swimming pool contains 2,500,000 litres of water, so this is actually an incredibly small amount.

Other compounds can be added too, as well as those added for the purpose of chlorination. Once such compound is calcium chloride. This is added to prevent the slightly soluble calcium sulfate, a component of the grouting between the tiles in swimming pools, from slowly dissolving away. It prevents this via something known as the common ion

effect. Essentially, the calcium ions present in calcium chloride raise the concentration of calcium ions in the water, preventing much of the calcium sulfate from dissolving.

Another compound that can be added is isocyanuric acid. This compound is a herbicide, and so levels must be kept below 200 parts per million; it’s usually present at a much lower level. The reason that it’s added is that it can help stabilise chlorine levels, particularly in outdoor pools where they’re depleted by the action of UV light. Isocyanuric acid reacts with hypochlorite ions, producing dichloro(iso)cyanuric acid. However, this is a another reversible reaction, and as hypochlorite ions are depleted by UV photolysis, the breakdown of this compound back to isocyanuric acid and hypochlorite ions is promoted. It therefore acts as something of a chlorine ‘reservoir’, replenishing the lost hypochlorite ions.

It’s probably become clear through the course of this article that there’s a lot of chemistry behind keeping a swimming pool clean. The chemical landscape in a swimming pool is a constantly changing one, and careful management of it is required to maintain a safe, clean pool.

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.

There are many careers that require some knowledge of chemistry.

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