You've heard it so many times before — wear SPF every day, come rain or shine, and even when you're indoors [1]. But do you know what your SPF really means? Sunscreen, as any expert will tell you, is paramount to protect your skin and cells against harmful UV exposure, as well as guard against premature signs of ageing. But while you might be applying SPF every day, be it a factor 30 incorporated into your face cream or factor 70 when you go to the beach, it's very likely that you're still not getting the protection your skin really needs.

It all starts with knowing what your SPF stands for. Short for Sun Protection Factor, SPF is a measure of how well a sunscreen will protect against UVB rays (the rays that cause your skin to burn) and refers to the theoretical amount of time you can stay in the sun without getting sunburned. But, here's where the confusion lies: that number on your trusty bottle might tell you how many times longer your skin would be protected from the sun but there are many variables that affect a sunscreens effectiveness, including your skin type, how much you apply, the strength of UV rays, and the activity you're doing. To find out more, we called upon a host of dermatologists and experts to discuss sunscreen labelling, the difference between SPF factors, and the importance of understanding how your SPF works, all of which will ultimately help you get the best sun protection possible.

What SPF Stand For, and What the Number on the Bottle Means

"The factor indicates how many times longer your skin would be protected from burning compared to wearing no sunscreen at all," Dr Hiva Fassihi, consultant dermatologist, explains. It measures how long a sunscreen will protect you from UVB rays, for instance: it would take 15 times longer to burn if you were using factor 15; 30 times longer to burn with factor 30; and 50 times longer using factor 50. However, no SPF protects against all rays — SPF 15 blocks about 93% of UVB rays [2], SPF 30 blocks about 97% [3], and SPF 50 blocks 98% of UVB rays.

If you do the maths, it's the time it takes your skin to burn multiplied by the factor you use. "If the SPF is factor 30 and you usually burn five minutes after exposure to the sun without protection, you can stay 30 times longer in the sun without getting a sunburn, that means a total of 150 minutes," explains Dr Marco Lens, London-based plastic and reconstructive surgeon and founder of Zelens. Similarly, if your skin would usually start to go red in 20 minutes without SPF, applying SPF 15 would increase that by 15 times and you could stay in the sun for a total of 300 minutes, or five hours.

So, depending on your skin type and sensitivity to the sun, it's an indication that, in many cases, those of us who apply a low SPF in our foundation or daily moisturiser in the morning or once a day on holiday are not protected from UVB rays throughout the day.

SPF Only Tells You How Well It Protects Against UVB Rays

It's just as important to understand that your SPF doesn't actually protect you against UVA rays, either. These are the rays long enough to reach the dermal layer of skin and can damage collagen and elastin. "SPF doesn't tell you about protection from other rays, such as UVA, infrared, and visible light, which can also damage your skin," says Dr Emma Wedgeworth [4], dermatologist at London's Dr Sam Bunting's clinic. Ultraviolet radiation comes from the sun and is split into UVA and UVB: UVA penetrates deeper into the skin and directly damages skin cells; UVB effects the skin at a surface level and is mainly responsible for sunburn. So, you need to look out for sunscreens with "broad spectrum" labelling as these formulas will protect your skin against both.

Sunscreen's Effectiveness Isn't Just About The Factor Level

Dr Fassihi stresses that while the factor in your SPF does indicate how many times longer your skin would be protected from burning (compared to wearing no sunscreen at all) "this isn't an exact science when applied to everyday use; there are many variables to consider, such as how much you have applied, strength of the UV rays, and your skin's ability to protect itself."

"Swimming, rubbing from clothes, skin-absorbing sunscreen, and not applying enough can mean that you won't receive the full benefit of your sun protection," Dr Fassihi explains. "We recommend that sunscreen is reapplied every two hours (even if your SPF x burn time adds up to more than 120 minutes) after swimming or towelling" she adds. The factor you choose to wear should also depend on your environment and UV index (a measurement of the strength of UV radiation of a particular place or time). "More intense sunlight, as measured by a high UV index can also cause more damage," says Dr Wedgeworth, adding that "a higher UV index occurs in hot sunny environments, particularly between the hours of 11-2."

A Minimum of SPF 30 Is a Must

"SPF 15 offers low protection, and [anyone with] sensitive skin or pale skin should avoid use of it," says Dr Lens. He (and every other dermatologist we've ever spoken with) recommends a minimum of SPF 30 to be used on a daily basis, such as the Zelens Daily Defence Sunscreen SPF 30 [5] (£55), which is formulated with microencapsulation technology and broad-spectrum UV protection. "I would recommend SPF 50, particularly when on the beach and when exposed to the sun on a holiday," he adds.

"From a dermatological perspective, there is no benefit of wearing a lower SPF," adds Dr Wedgeworth. "Given that we know we use way less than we should do, and most people do not reapply as frequently as they should, I advise sun protection as high as possible. People sometimes worry about vitamin D deficiency, but you can make vitamin D even when you are wearing sunscreen, so that's no excuse," she adds. Dr Wedgeworth also discusses the issue presented with tanning, as some people continue to choose a lower SPF when they go on holiday to get a tan, regardless of how dangerous it is. "I think people are also keen to tan their skin, but we advise against intentional tanning as we know it damages the skin, increase your risk of skin cancers, and premature ageing."

The Amount of Sunscreen You Apply Is Key to How Well It Works

"Simply put, SPF tells you how much longer you can be exposed to the sun without burning when wearing correctly applied sunscreen, compared to not wearing any sunscreen — but the key is the 'correctly applied' part," says Dr Wedgeworth, adding that "on average, most people apply only 20-25 percent of the recommended amount of sunscreen and if you want to get the level of protection it says on the bottle, you have to apply 2mg/cm2 of your body." To give you an idea, that's about a shot glass to properly cover the body and should be applied 20 minutes prior to sun exposure. "The amount of UVA and UVB protection a sunscreen provides is related to how thickly it is applied onto the skin, so be generous with your sunscreen application," says Dr Fassihi. Similarly, if you're using SPF and also wearing makeup," wait 20 minutes for the SPF to fully absorb into the skin, only then makeup can be applied on top if desired," she adds.

What to Look Out For When Shopping For Sunscreen

Dr Fassihi explains that when choosing a sunscreen, "it is important to look for a high SPF value that's broad spectrum, or more than 3*UVA or the circled UVA logo." The latter of which is applicable if you're shopping at Boots [6], as it's the UVA star rating system devised by the company that helps determine the strength of UVA protection a sunscreen will provide (one star being the lowest; five stars the highest) and can be useful if you're unsure what to look out for when selecting an SPF. Clare O'Connor, Boots sun-care scientific adviser, says that we "should choose suncare with a 5* UVA rating, such as Soltan," which means that you'll get a product that will give the best possible protection against both UVA and UVB radiation, sunburn, and skin cancers.

"For daily wear under makeup or where topping up isn't always possible, then I would recommend using a higher SPF and applying generously in the morning; the La Roche-Posay Anthelios Anti-Shine Mist SPF50 [7] (£10.50) is a wonderful product that can be applied even over makeup to top up protection throughout the day too," Dr Fassihi advises (she is a spokesperson for the brand). Susie Ma, founder of Tropic Skincare, reiterates the need to wear a higher SPF on your face given that "it's more prone to sun exposure. Tropic's facial sunscreens provide broad-spectrum protection and only come in SPF 50," she adds. The latest Tropic Skin Shade Tinted Facial Sun Cream [8] (£24) and Sun Day Facial UV Defense [9] (£10) also feature added blue light protection and antioxidant-rich protective ingredients to shield against free radicals.

To discover a more comprehensive list of sunscreens that get the thumbs up from the experts, you can check our round up of dermatologist-recommended sunscreens [10], and a list of pro-approved sunscreens for dark skin tones [11].

In Conclusion: Everyone Should Wear a High SPF

"Some people are more at risk of sun damage and need to be extra careful, particularly if you have very fair skin or a family history of skin cancers, however, regardless of skin type or colour, excessive sun exposure will damage your skin over time," says Dr Wedgeworth.

"The answer is simple: everyone should," Dr Fassihi says. It's the single most important product for sun safety as well as our own skin-care routines, with around 80 percent of signs of ageing caused by the effects of the sun. "In the short term, sunscreens protect from painful sunburn; in the long term, they help prevent skin cancer and slow down premature skin ageing."