Published by Australian Academy of Science
KEY TEXT Putting on a good face – the chemistry of cosmetics

The use of cosmetics is not a modern phenomenon. Ancient Egyptian women used kohl to darken their eyelids, and Cleopatra is said to have bathed in milk to whiten and soften her skin. More than 3000 years ago Greek women used poisonous lead carbonate to achieve a pale complexion, costing some wearers their lives.

Today, cosmetics are big business. According to the Australian Bureau of Statistics, Australians spent A$4.1 billion on cosmetics, perfumes and toiletries in 1998-99. Cosmetic advertising is now targeting a wider audience than ever. Most of us – males and females – care about our appearance.

What is a cosmetic?

The Australian Trade Practices Act 1974 defines a cosmetic product as ‘a substance or preparation intended for placement in contact with any external part of the human body' (this includes the mouth and teeth). We use cosmetics to cleanse, perfume, protect and change the appearance of our bodies or to alter its odours. Products that claim to ‘modify a bodily process or prevent, diagnose, cure or alleviate any disease, ailment or defect’ are called therapeutics (Box 1, Ingredient labelling). This distinction means that shampoos and deodorants are placed in the category of cosmetics, while anti-dandruff shampoos and antiperspirants are considered to be therapeutics.

What do cosmetics contain?

Most cosmetics contain a combination of at least some of the following ingredients: water, emulsifier, preservative, thickener, colour, fragrance and pH stabilisers.

Emulsifiers

Many cosmetic products are based on emulsions – small droplets of oil dispersed in water or small droplets of water dispersed in oil. Since oil and water don't mix, emulsifiers are added to produce the small droplets and to prevent the oil and water phases from separating. Emulsifiers work by changing the surface tension between the water and the oil, thus producing a homogeneous product with an even texture.

Preservatives

Preservatives are added to cosmetics to prevent the growth of microorganisms (eg, bacteria and fungi), which can spoil the product and possibly harm the user. Preservatives used in cosmetics can include parabens, benzyl alcohol and tetrasodium EDTA (ethylenediaminetetra-acetic acid).

Thickeners

Thickening agents such as polymers are often added to cosmetics to change their consistency. Polymers can be synthetic (eg, polyethylene glycol) or derived from natural sources (eg, polysaccharides). Seaweeds are a common source of natural polysaccharides – carrageenans are extracted from red algae and alginates from brown algae. Cosmetics that are too thick can be diluted with solvents such as water or alcohol.

Fragrances, colours and pH stabilisers

The ingredient list of a cosmetic product might also include chemicals that give a pleasant smell to the product, provide an appealing colour, or adjust the pH (the acidity).

Some types of cosmetics and their ingredients

Moisturisers are generally used to treat dry, scaly skin. Our skin becomes dry when water is lost from the top layer of dead skin cells faster than moisture can enter it from the living layers of skin below (Box 2, Only skin deep?). Moisturisers can correct this problem in two ways: by preventing further moisture loss (occlusion) and by adding substances that increase the water-holding capacity of the skin (humectants). Occlusive moisturisers may contain oils such as isopropyl palmitate, stearyl alcohol or light mineral oil. The oils form a waterproof layer on the skin, reducing evaporation and allowing the body’s natural process of rehydration to return the skin to a normal water level. Humectant moisturisers may contain substances like glycerine or alpha hydroxy acids (fruit acids such as glycolic acid, citric acid or lactic acid), which add water to the top layer of skin.

Related site: Introduction to surfactants Uses diagrams to describe surfactants and their properties. (Key Centre for Polymer Colloids, Australia)

Shampoos and soaps clean by the use of surfactants (surface active agents). Surfactant molecules have both fat soluble (lipophilic) and water-soluble (hydrophilic) parts. The lipophilic part of the molecule sticks to oil and dirt, and the hydrophilic part allows water to then carry away the otherwise water-insoluble grime. Washing-up detergents work in the same way, although it isn’t generally advisable to wash your hair with dishwashing liquid - they are formulated to remove thick grease from plates, not to gently clean your hair!

Water solubility - or the lack thereof - is an important factor in creating lipstick. Lipsticks are generally made by combining a water-insoluble dye with wax and a non-volatile oil (beeswax with castor oil is a common formulation). This results in a substance that is stiff, but will spread easily on your lips. Because it’s water-insoluble, the lipstick won’t be dissolved by saliva or by the drink you’re sipping. Some lipsticks also use dyes which react with the amino acids in the protein of your skin - this is why some lipsticks appear blue or green in the tube, but turn a deep shade of red when applied to your lips.

Fake tans also change colour on contact with skin. The active ingredient in most fake tans is dihydroxyacetone, a colourless compound that darkens when it reacts with the amino acids in the top layer of skin. The colour change is permanent, but because skin cells are constantly being shed the tan is usually gone after about a week.

Some unintended effects of chemicals found in cosmetics

Unfortunately, sometimes the ingredients in cosmetics can have unintended side-effects. For example, skin allergies (allergic dermatitis) to specific ingredients can be a problem. Allergies to cosmetic products can be due to chemicals such as added fragrances and preservatives. This can lead to a skin rash where the product is applied. If you think you may be allergic to a cosmetic product, it is important to determine which ingredients may be causing the problem. A specialised allergy test, called a patch test, may be helpful in this. Chemicals causing the allergy can then be avoided by reading product labels. Other people, while not allergic to a specific ingredient, may nevertheless find that a product irritates their skin because it damages the outer layers - a condition known as irritant dermatitis.

Exfoliants and skin peels leave the skin underneath temporarily more vulnerable to sun exposure because they remove the outermost protective layer of dead skin cells. Over-washing of hair or skin with soaps and detergents can strip the skin’s natural protective oily layer, resulting in dry and scaly skin. Alternatively, excessive use of make-up or oily moisturisers can block pores and aggravate acne.

Related site: Parabens in deodorants and antiperspirants linked to breast cancer Assesses claims of a link between parabens and breast cancer. (National Industrial Chemicals Notification and Assessment Scheme, Australia)

More serious side effects have been suggested for certain cosmetic ingredients. For example, a recent study was published that linked breast cancer with deodorants. The focus of the study was on parabens, a class of chemicals commonly used as preservatives in deodorants and antiperspirants. While parabens were found in breast cancer tissue, the study did not establish that they were the source of the cancer nor did it identify underarm cosmetics as the source of the chemicals.

Related site: Skin deep A safety assessment of ingredients found in cosmetic products. Includes a searchable database on 25,000 different products (Environmental Working Group, USA)

A recent US study found that many cosmetics and toiletries used worldwide contained chemicals that were either known cancer-causing agents (carcinogens) or were untested for their effect on human health. More research into the safety of cosmetic chemicals is needed.

In our pursuit of beauty, it is wise to remember that cosmetics can be complex combinations of chemicals. Reading the label and understanding which ingredients are used in a product are helpful when putting on your best face.

Boxes

1. Ingredient labelling

2. Only skin deep?

CREDITS

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