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What is SPF and what do the numbers mean?

The topic of sun protection is filled with a confusing amount of jargon. Do you know your UVA from your UVB? Your chemical from your mineral filters? Allow us to decode the science, explaining what SPF is and how to use it for the best possible protection.

What is SPF?


The term SPF has become a shorthand for sun creams and sunscreens in general, and the three terms are used somewhat interchangeably. But, understanding that the abbreviation SPF actually means something quite different to sunscreen is the first hurdle to jump. SPF stands for Sun Protection Factor, and is a metric designed to indicate how much UVB protection you’re getting from your sunscreen. If you’re eagle-eyed, you may have noticed that we said UVB protection, not UV in general, as SPF is only concerned with these rays, and doesn’t take UVA into account.


UVB rays make up around 5% of the UV rays that reach the earth. They are relatively short, with high energy, and most prominent during the summer months. UVB is the biggest cause of sunburn and skin cancer, reaching the epidermis (top layer) of the skin. UVA rays, on the other hand, make up an impressive 95% of the UV rays that reach the earth. They are present all year round, and UVA rays can penetrate through glass and clouds. UVA rays are longer than UVB, allowing them to travel into the dermis where they act as the biggest cause of premature ageing.


If you want to shield your skin against UVB and UVA rays (and trust us, you really should) you’ll also need to look out for the term “broad spectrum”. This means that the sunscreen also protects against UVA rays. Currently, there isn’t one globally accepted test for measuring UVA protection, and the terminology used to indicate this isn’t consistent around the world. But, if your sunscreen mentions broad spectrum or UVA protection on the packaging, this is a good indication.


What is the highest SPF sunscreen?


If you’re serious about sun protection, you’ll want to be using the highest factor available, but keep in mind that your application technique has a big impact on how much protection you’re really getting. SPF15 should block around 93.3% of UVB rays, SPF30 around 96.7% and SPF50 around 98%, but only if you’re using it exactly as directed. That means applying a generous amount (roughly ¼ teaspoon for your face, ½ teaspoon for each arm and 1 teaspoon per leg, front torso and back torso) and topping up regularly – especially if you have been swimming or sweating. If you’re taking a slapdash approach by applying too little, or chancing your arm with a once-a-day application, then you’ll be getting far less than the protection listed on pack. Opting for the highest factor possible is a smart move, as it gives slightly more margin for natural error, like applying too little.


In the UK and EU, the maximum SPF you can claim is 50+, but other countries allow you to claim more. Even if you see an SPF 100, remember that this does not mean 100% protection, rather an indication of the amount of UV protection you’re getting. Everyone’s skin is different, and some people will burn quicker than others.


What is the difference between chemical and mineral sunscreen?


UV filters are elements within sunscreen formulations that work to filter UV rays from your skin. There are two types of UV filters – mineral (also known as physical) and chemical, but the names are slightly misleading, as both are technically chemicals. Sunscreens can be formulated with only mineral filters, only chemical filters or a mix of both. Many use a mix of filters, as different filters are more effective at specific parts of the UV spectrum. So, for example, some might work best on UVA, and others work best on UVB.


Chemical filters work by absorbing UV rays and converting their energy into small amounts of heat – so small that you can’t feel it. Mineral filters work in the same way, but can also reflect and scatter UV rays. There are only two mineral filters available (zinc oxide and titanium dioxide) but many more chemical filters (including octocrylene, avobenzone and oxybenzone). Chemical sunscreens tend to be more lightweight than mineral, absorb quickly and dry fast. Mineral sunscreens are generally better suited to sensitive skin as they have soothing properties, but can have a white cast, which makes them unsuitable for those with darker skin tones. Which type of filter you use is down to personal preference but ultimately, the best sunscreen is the one you want to wear and reapply.