How to: Understand SPF

The facts about SPF and sunscreen

When it comes to that all-important sun protection, there’s a bewildering array of products to choose from – how do you begin to make sense of all the numbers and letters on the labels? What do they mean and how do they work?

We tan in the sun because of melanin that forms in our skin as a result, in fact, of damage to the skin’s DNA. Some people tan faster than others because they produce more melanin more rapidly, while people who produce the least melanin are those who burn more easily and are therefore at greater risk in the sun. The effects of the sun’s rays also vary depending on how strong the radiation is and on where we are – the nearer we are to the equator, the stronger the sun is.

UV (ultra-violet) radiation is part of the electromagnetic light spectrum emanating from the sun. These wavelengths are classified as UVA, UVB, or UVC, according to how long they are. UVC are the shortest and are almost completely absorbed by the ozone layer before reaching earth, but both UVA and UVB penetrate our atmosphere and play a significant role in premature skin ageing, eye conditions like cataracts, and skin cancers.

By damaging the cellular DNA of our skin, excessive UV radiation can produce the genetic mutations that lead to skin cancer. Although less intense than UVB, UVA rays are more prevalent, accounting for up to ninety-five percent of the radiation reaching the earth’s surface. UVA rays penetrate the skin more deeply than UVB do, and recent studies confirm that they are not only responsible for premature ageing and wrinkling of the skin, but also contribute to the initiation and development of cancerous cells. UVB, the main cause of visible reddening of the skin in the sun, damages the more superficial layers of the skin and has long been known to play a key role in cancers and other negative effects on the skin. Unlike UVA, the intensity of UVB varies depending on season, location, and time of day.

Sunscreen products combine organic and inorganic chemicals to filter the light from the sun so that less of it reaches the deeper layers of your skin. The effectiveness of sun-protection products is measured by SPF (Sun Protection Factor), though rather than being an indication of a specific amount of protection, SPF is a measure of how long it will take skin to redden when using the product compared to how long it takes to redden without the product. If you use a sunscreen with an SPF of 15, for example, your skin will take fifteen times longer to burn than it would without the protection. It doesn’t mean that SPF 30 gives double the protection of SPF 15: an SPF 15 product screens ninety-three percent of the sun’s UVB rays, SPF 30 protects against ninety-seven percent, and SPF 50 screens ninety-eight percent.

SPF ratings, of course, measure only protection against UVB rays and don’t give an indication of how well a product will protect you from UVA radiation. To identity a sunscreen that will protect you from both UVA and UVB, you need to look for one described as ‘Broad Spectrum’, ‘Multi Spectrum’ or ‘UVA/ UVB Protection’. According to dermatologists, a broad spectrum sunscreen with SPF 30 is perfectly adequate protection for most of us, apart from young children, people prone to eczema, or those with especially sensitive skin.

Apply your sunscreen to dry skin, fifteen to twenty minutes before you go out into the sun. It’s important to apply the product thickly – an adult should use between thirty and forty grams, the equivalent of around two tablespoons full – and to reapply it every two hours. Choose a sunscreen that is water-resistant if you’re going to be swimming. And even when thoroughly sunscreen-ed, it’s still best to seek the shade whenever possible, especially between 10am and 4pm when UVB rays are at their most intense.

Other articles you may be interested in:
How to apply your fake tan perfectly
Sunscreens we love
Stop your fake tan from fading
Skin protection tips
The facts about SPF


15 Responses

  1. Since I started using a sun screen, my skin has really changed… now I walk in the sun like a boss.

  2. I prefer to purchase a moisturiser that contains at least SPF 20. Thanks for a wonderful article!

  3. I carry my sunblock to work with me to re-apply when and if I have to leave the office. Its scary how much damage the sun can do.

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