Science of Skin

Sunscreen

Sunscreens-generic

Skin cross section demonstrating how sunscreen reflects UV light (left) compared to unprotected skin (right)
Skin cross section demonstrating how sunscreen reflects UV light (left) compared to unprotected skin (right)

Sunscreens are the most important product in the skin care routine. They are designed to block and absorb UV light and have the potential to delay the appearance of photoaging and decrease the risk of developing skin cancer.

The first sunscreens were developed for military personnel on aircraft carriers to prevent heat stroke and extreme sunburn. They were made from red veterinary petrolatum which was quite uncomfortable and aesthetically unappealing.  Further developments in sunscreen ingredients lead to the use of para-amino-benzoic acid (PABA) to absorb UVB radiation. In the early 1980’s it was believed that PABA caused adverse skin conditions such as contact dermatitis and that it may have had carcinogenetic properties.  This led to the development of a “PABA free” sun care range.

Modern sunscreens have been designed to protect the skin from both UVB and UVA radiation.  UVB radiation affects the epidermis, the skin’s top layer, and is the primarily responsible for sunburn. UVB is at its most intense between the hours of 10:00 am and 4:00 pm but unlike UVA, is unable to penetrate glass.  UVA is a major contributor to skin damage as it penetrates into the dermis; deeper layers of the skin. The intensity of UVA radiation does not fluctuate like UVB during the day and is constant throughout the year. A simple way to remember the effects of UVA and UVB on the skin is: UVA is Aging and UVB is Burning.

Sunscreens protect the skin in two ways. Firstly, by physically blocking or reflecting UV light with ingredients such as titanium dioxide or zinc oxide and secondly with a chemical barrier that absorbs the UV light, with ingredients such as benzophenones and salicylates. These physical and chemical barriers are often referred to as UV filters.  According to Nohynek and Schaefer (2001) complete protection over the entire range of UVA and UVB radiation is rarely possible with a single UV filter. Therefore, most modern sunscreens use a combination of several physical and chemical filters to protect against the broad spectrum of damaging UV light.

Sun Protection Factor (SPF)

When choosing a sunscreen, the higher the Sun Protection Factor or SPF the better. An SPF rating indicates how long it will take for UVB radiation to cause redness (erythema) on skin when using a sunscreen, compared to how long skin would take without sunscreen. So if it takes 10 minutes for your skin to start to burn, then an SPF 30 will theoretically give you 30 x 10 minutes or 300 minutes. You are, however, very unlikely to get the full 5 hours of protection due to normal wear rubbing off on clothing, sweating or swimming etc. Even so-called "water-resistant" sunscreens may lose their effectiveness after 40 minutes in the water. It is recommended to re-apply every two hours or after any physical exercise to ensure adequate protection.

An SPF 15 sunscreen is said to block 93% of the sun’s UVB rays, an SPF 30 to protect against 97% and an SPF 50 to block 98% of UVB radiation but the true effectiveness of an SPF rating is may be much less than this. The SPF is calculated under laboratory conditions with specific amounts and exposures. However, the actual application of sunscreens by the general public has been shown to be much less than required, with an average of half the recommended amount applied to achieve photoprotection.

All sunscreen labels state that the product should be applied liberally, but few detail the exact amount required. According to the American Academy of Dermatology (AAD), one ounce (30 grams), or enough to fill a shot glass, is the appropriate amount needed to cover the exposed areas of the body properly.  One gram is equivalent to covering the fingertip of the forefinger.  It should also be remembered that it takes 20 to 30 minutes for sunscreens to become active and you can still burn within this period.

Nanoparticles

There has been a lot of interest in the media recently on the use of nanoparticles in sunscreens. Nanoparticles are smaller than 100 nanometres and invisible to the human eye - a nanometre is 0.000001 millimetre. Nanoparticles of titanium dioxide have been used in sunscreens in Australia since 1990 and zinc oxide since 1999. These two ingredients have the ability to filter both UVA and UVB radiation, giving broader protection than other sunscreen ingredients.  The concern over nanoparticles is that they are so small they may be able to penetrate the stratum corneum, the dead outer layer of the skin. A review was undertaken by the Australian Therapeutic Goods Administration (TGA) in May 2009 on the safety of nanoparticles in sunscreens and it found “To date, the current weight of evidence indicates the particles remain on the surface of the skin and in the outer dead layer (stratum corneum) of the skin.”

The use of nanoparticles will be closely watched by those in the skin care industry and regulators alike and research into their possible effects is ongoing.

Sunscreens and Children

According to the AAD, Infants under six months of age should be kept out of direct sun and be covered by protective clothing. Apply sunscreen from six months of age. Children under six months of age should not have prolonged sun exposure, but if this occurs then sunscreen should be used.  Sunscreens should be considered a third line defense from UV light behind avoidance of the sun and UV protective clothing. The Australasian College of Dermatologists agrees and recommends the use of a sunscreen at any age when there is exposure to the sun. Shade, clothing and broad rimmed hats are the best sun protection measures for babies. Sunscreens should be applied to areas of the skin not protected by clothing. Titanium dioxide or zinc oxide based sunscreens are less likely to cause skin irritation in babies and children with sensitive skin.  

A good sun protection routine should begin in infancy and continue throughout life. The AAD estimates that we get about 80 percent of our total lifetime sun exposure in the first 18 years of life. It can take another 20 to 30 years after this exposure for the real damage to begin to show.  Sunburn, darkening of freckles and even the much sort after tan are all immediate signs of skin damage from UV light and are a pre-cursor to photoaging and certain skin cancers.

Know your skin type - Sunscreen

Combination/Normal skin. Combination/normal skins can use any type of sunscreen whether it is a cream, gel, lotion or alcohol solution. Anything less than SPF 15+ is not going to give adequate protection from sun damage and skin cancer

Dry skin. A dry skin should avoid using an alcohol based sunscreen as it could have a drying effect on your skin.  Anything less than SPF 15+ is not going to give adequate protection from sun damage and skin cancer

Oily skin. Oily skins are able to use any type of sunscreen but if you are prone to breakouts or acne then a gel is easier to apply and is less likely to irritate this skin condition. Anything less than SPF 15+ is not going to give adequate protection from sun damage and skin cancer

Sensitive skin. Sensitive skins should use a product specifically designed for sensitive skins, try to avoid using a gel or alcohol based formula as these can irritate further. Anything less than SPF 15+ is not going to give adequate protection from sun damage and skin cancer

Mature skin. Mature skins can use any type of sunscreen whether it is a cream, gel, lotion or alcohol solution. Anything less than SPF 15+ is not going to give adequate protection from sun damage and skin cancer

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