Understanding SPF: What Sun Protection Factor Really Means
Most people know they should wear sunscreen, but many are unsure what SPF actually means or how much protection they really need. Is SPF 50 twice as effective as SPF 25? Does a higher SPF mean you can stay in the sun all day?
Understanding SPF can help you make informed choices that protect your skin from premature ageing, pigmentation and skin cancer.
What is SPF?
SPF stands for Sun Protection Factor. It is a measure of how well a sunscreen protects your skin from UVB (Ultraviolet B) rays, which are the part of the sun’s ultraviolet radiation primarily responsible for causing sunburn.
Without sunscreen, UVB rays can damage the DNA within skin cells, increasing the risk of skin cancer while also contributing to inflammation and accelerated skin ageing.
The higher the SPF number, the greater the level of UVB protection.
What do the SPF numbers mean?
SPF does not increase in a simple linear way. Instead, each increase provides a smaller improvement in protection.
Approximate UVB protection levels are:
SPF 15 blocks around 93% of UVB rays.
SPF 30 blocks around 97% of UVB rays.
SPF 50 blocks around 98% of UVB rays.
SPF 100 blocks around 99% of UVB rays.
Although the difference between SPF 30 and SPF 50 may appear small, it can make a meaningful difference for people with fair skin, a history of skin cancer, photosensitive conditions, or those spending extended periods outdoors.
Does SPF tell you how long you can stay in the sun?
Many people believe SPF allows them to stay in the sun for a certain number of hours, but this is a common misconception.
In laboratory testing, SPF measures how much longer protected skin takes to develop redness compared with unprotected skin under controlled conditions. However, real life is very different.
Swimming, sweating, rubbing your skin with clothing or towels, and applying too little sunscreen all reduce its effectiveness. The strength of the sun also varies depending on the time of day, season, altitude and your location.
For these reasons, sunscreen should never be used as a way to extend your time in the sun.
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Fun facts – the distance between the sun and the earth is about 150 million kilometers, and travelling at about 300,000 kilometers per second, light from the sun takes about 8 minutes to arrive at the earth.
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SPF only protects against UVB
One important point that is often overlooked is that SPF only measures protection against UVB rays.
The sun also produces UVA (Ultraviolet A) rays, which penetrate deeper into the skin. UVA exposure contributes to:
Premature skin ageing
Loss of collagen and elastin
Some forms of skin cancer
When choosing a sunscreen, look for one labelled Broad Spectrum, as this indicates protection against both UVA and UVB rays, for example LPA recommends Heliocare
How much sunscreen should you apply?
Even the highest SPF sunscreen will not provide its advertised level of protection if too little is applied.
Most adults need approximately:
One teaspoon for the face and neck.
One teaspoon for each arm.
One teaspoon for each leg.
One teaspoon for the front of the body.
One teaspoon for the back.
This is around six teaspoons (approximately 30 ml) to cover the entire body.
Most people apply only around one-quarter to one-half of the recommended amount, significantly reducing the level of protection.
When to apply sunscreen and reapply suncreen?
Sunscreen gradually wears away during the day.
As a general guide:
Apply sunscreen 15–30 minutes before going outside.
Reapply sunscreen every two hours while outdoors.
Reapply sunscreen immediately after swimming, excessive sweating or towel drying, even if the sunscreen is labelled “water resistant.”
Daily application is important, even on cloudy days, as up to 80% of UV rays can pass through cloud cover.
Is sunscreen enough?
Sunscreen is an important part of protecting your skin, but it should be used alongside other sun safety measures.
These include:
Seeking shade during the middle of the day when UV levels are highest.
Wearing a wide-brimmed hat.
Using UV-protective sunglasses.
Wearing lightweight clothing that covers exposed skin.
Avoiding sunbeds.
Why sun protection matters for healthy skin
Sun exposure is one of the leading causes of premature skin ageing. Over time, ultraviolet radiation damages collagen and elastin fibres, weakens the skin’s support structure and contributes to wrinkles, sagging and uneven skin tone.
UV exposure can also trigger pigmentation disorders, including age spots and melasma, and significantly increases the risk of developing skin cancer.
Using a broad-spectrum high spf sunscreen every day is one of the simplest and most effective steps you can take to help maintain healthier, younger-looking skin.
Our advice
In addition to using sunscreen with SPF, for glowing summer skin, we would recommmend a medical range Vitamin C serum and twice weekly exfoliation using a fine grain medical skincare exfoliater such as ZO Exfoliating Polish. We also love the ZO Hydro Mist to add extra hydration during the summer months. At LPA, we encourage patients to think of sunscreen as part of their daily skincare routine rather than something reserved for holidays or hot summer days. Consistent sun protection helps preserve the results of professional skin treatments, supports long-term skin health, and reduces preventable sun damage. We will always recommend you use a suncsreen with spf or a moisturiser with spf (spf 30 or 50) every single day – that way you will not forget on the days when it is really important and you will have consistent best protection.
If you’re unsure which sunscreen is most suitable for your skin type, are unsure about the amount of sunscreen needed or have concerns about pigmentation, ageing skin or sun damage, our team can recommend products and treatments tailored to your individual needs.
Book your consultation today and prepare your skin for this summer.
Why Choose Our London Clinic for Summer Skin advice
At our central London clinic in EC1, we specialise in natural-looking aesthetic treatments and personalised skincare tailored to each patient. With years of experience in acheiving beautiful skin and facial rejuvenation, we guide you through your options and create a bespoke plan for your face.
Find out more about the clinic and team.
