Sunscreen Explained - Everything you need to know about skin's best friend
How does sunscreen work? | What does SPF mean? | 15,30,50-which should I use? | Is the '+' necessary? | How much do I need? | When should I apply? | Definitions | Nano Particles
Sunscreens, which can be sprays, lotions, gels or waxes, are usually made up of a mix of chemicals. Inorganic chemicals in sunscreen can reflect or scatter the light away from the skin, and organic (carbon-based) chemicals can absorb UV rays, so that our skin doesn't.
How does sunscreen work?
Some inorganic chemicals that are found in sunscreen, including minerals such as zinc oxide or titanium dioxide, act as a physical sunblock. They reflect UV rays, similar to how white paint reflects light. In the past, sunscreen showed up on the skin as a visible white layer however, now sunscreen manufacturers make the inorganic particles much smaller, so we don't see the visible white. (See below re nano-particles.)
Along with inorganic chemicals, sunscreens often contain organic chemicals, with names such as avobenzone or oxybenzone. Instead of physically deflecting UV light, these molecules absorb UV radiation through their chemical bonds. As the bonds absorb UV radiation, the components of the sunscreen slowly break down and release heat.
What does SPF mean?
The SPF on sunscreen bottles stands for Sun Protection Factor, and refers to how well the sunscreen protects against one type of UV radiation, called UVB (think B for burning). UVB rays cause sunburn and several types of skin cancer.
Another type of raditation, UVA radiation, penetrates deeper into the skin and can cause premature wrinkling, age spots and also heighten the risk of some skin cancers.
Sunscreen lotions labelled broad-spectrum block against both UVA and UVB, but currently there is no standard listing for UVA block power. Inorganic chemicals that deflect sunlight will deflect both UVA and UVB rays.
SPF 15, 30 and 50
Because some UV radiation still gets through the sunscreen and into your skin, the SPF number refers to roughly how long it will take for a person's skin to turn red. Sunscreen with an SPF of 15 will prevent your skin from getting red for approximately 15 times longer than usual (so if you start to burn in 10 minutes, sunscreen with SPF 15 will prevent burning for about 150 minutes)*. However, because most people don't use enough sunscreen and because sunscreen tends to wear or wash off after time, you should reapply an SPF 50+ sunscreen every two hours, regardless of its strength.
The Skin & Cancer Foundation Inc recommends using a sunscreen with an SPF of 50+.
A sunscreen with an SPF of 15 protects against about 93% of UVB rays, one with an SPF of 30 protects against 97% of rays and a sunscreen with an SPF 50 blocks 98% of UVB rays, according to the Mayo Clinic. Or, another way of looking at it is:
- SPF 15 (93% protection) allows 7 out of 100 photons through
- SPF 50 (98% protection) allows 2 out of 100 photons through
So, while it may not seem that you're increasing your level of protection by a significant amount, an SPF 50 sunscreen will block three times the radiation than an SPF 15 sunscreen would let through to your skin.
There is no sunscreen that can block 100% of UV rays, which is why it is important not to spend prolonged periods of time in the sun, even whilst wearing sunscreen.
The '+' sign
The plus sign means 'more than'. For example, SPF50+ sunscreen must provide at least SPF60 in testing. This is because the same batch of sunscreen will test slightly differently in different laboratories with different methodology. By testing at SPF 60, it removes any margin for error.
How much sunscreen should I apply?
An average sized adult should apply the equivalent of one teaspoon of sunscreen to each arm, leg, front of the body, back of the body, and face (including the neck and ears). Most people apply less than half this amount, which means they get far less protection than the SPF stated on the bottle.
Sometimes it is good to hear what the experts have to say. The following video has Associate Professor Peter Foley briefly explaining how to apply sunscreen effectively:
When should I apply the sunscreen?
Ideally, sunscreen should be applied about 20 minutes before being out in the sun, so it has time to absorb into the skin. Once you are out in the sun and are sunburnt then sunscreen will be useless.
Is sunscreen by itself enough?
Sunscreen should form part of overall sun protection that includes clothing, hats, seeking shade and avoiding being out in direct sunlight when it is brightest (SLIP on a shirt, SLOP on some sunscreen, SLAP on a hat, SEEK shade, SLIDE on sunglasses).
Water resistance sunscreen means that it does not come off the skin during swimming or exercise, provided it is not wiped off. Some labels will state that the sunscreen is '4 hours water resistant' but you should still reapply sunscreen every 2 hours to maintain the same level of protection.
Without a broad-spectrum sunscreen With broad-spectrum sunscreen
Broad-spectrum sunscreens filter both UVA and UVB rays. UVB is the prinipal cause of sunburn, but both UVA and UVB contribute to increased skin cancer risk.
Learn more about how UVA and UVB damage your skin.
Waterproof, sunblock, sweat proof
Revised Australian Standard on sunscreen means these words are no longer permitted on sunscreen labels.
The term 'waterproof' is misleading and not permitted. The Standard acknowledges that sunscreens will wash off when immersed in water.
The term 'sunblock' is misleading and not permitted because it may be interpreted to mean that 100% of the sunburning radiation is blocked.
The term 'sweat proof' is misleading and not permitted. 'Sweat resistance' is not a substitute for 'water resistance'.**
For many years sunscreen manufacturers have used the inorganic compounds Titanium Dioxide and/or Zinc Oxide to deflect or filter UV rays. In its bulk form, this would leave an opaque or white film on the skin, which resulted in consumers being reluctant to use them (although it's quite an Aussie thing to wear zinc cream on the nose). Since those early days, science has evolved to allow the manufacturer to decrease the size of these metal oxides to be nano particles. When used in a sunscreen, they cannot be seen on the skin, and they have even enhanced the effectiveness of the sunscreen. The nano particles are also particularly effective in filtering UVA as well as UVB rays, thus providing very effective broader spectrum protection.
However, some have feared that these nano particles can penetrate the skin and get into the blood stream. This has stimulated quite a wide range of research that has focussed on these safety concerns. The two issues addressed have been the ability to penetrate the skin, and the toxicity of these nano particles.
There have been a number of studies published, and the Federal Government's Therapeutice Goods Administration (the TGA) has been consistently reviewing that scientific research. Their first review was published in 2006, and their latest was published towards in January 2017. The TGA has accepted the overwhelming conclusions in those research papers - that there is no cause for these concerns.
"The majority of in vitro studies (using both animal and human skin) and in vivo studies have shown that both ZnO and TiO2 NPs [nano particles] either do not penetrate or minimally penetrate the stratum corneum and underlying layers of skin. This suggests that systemic absorption, hence toxicity, is highly unlikely," the TGA reported.
"In conclusion, on current evidence, neither TiO2 nor ZnO NPs are likely to cause harm when used as ingredients in sunscreens and when sunscreens are used as directed." (1.)
For a copy of that report, click here.
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(1.) Literature review on the safety of titanium dioxide and zinc oxide nanoparticles in sunscreens, Therapeutic Goods Administration (TGA), January 11, 2017. See https://www.tga.gov.au/literature-review-safety-titanium-dioxide-and-zinc-oxide-nanoparticles-sunscreens