From knowing how long your SPF really lasts, whether or not to put moisturiser on first and whether chemical or mineral is better, our expert doctors tell you all your need to know for healthy summer skin

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"If you only use one product in your skincare regime, please let it be an SPF," writes top aesthetic doctor David Jack in his guide to looking good  Vain Glorious, £7.79  co-written with Jeremy Langmead

As we all know by now, sun cream is vital to protect us against prematurely ageing UVA rays, sunburn-inducing UVB rays and ultimately, skin cancer. (For the best everyday sunscreens for the face, we've got you covered). But that's only if we apply it correctly and often enough, which most of us don't.

So what does using sun cream properly look like? We took the advice of Dr David Jack, as well as cosmetic dermatologist and Get The Gloss Expert  Dr Tapan Patel and dermatologist and skin cancer surgeon Dr Andrew Birnie, founder of Altruist Sunscreen, speaking on the BBC's Sliced Bread podcast, to debunk some of the common misconceptions around SPF.

From whether water-resistant suncream does what it says, to the truth about the SPF protection in our cosmetics and mositurisers, here’s how to stay safe and sunburn-free this summer and beyond.

What does the sun protection factor actually mean?

“‘SPF’ stands for Sun Protection Factor and is used to how long the protection provided by the sunblock will last," says Dr Patel. "If you burn within 20 minutes of exposure to the sun, an SPF 15 is formulated to protect you 15 times longer (around five hours).”

So, the SPF factor denotes how many times longer you can stay out in the sun before your skin turns red. If you normally go red after 20 minutes in the sun in June, an SPF 50 will allow you to stay out for 50 times that length (i.e. 50 x 20 minutes). The protection factor depends on how quickly you personally turn red in the climate you are in.

“An SPF 50 will provide you with longer protection, meaning it should theoretically need to be applied less frequently than say, an SPF 30," says Dr Patel. "It also blocks a higher percentage of UVB rays from the sun, meaning you are less likely to burn or experience skin damage.”

Is SPF 15 double the protection of SPF 30?

Simply, yes, says Dr Birnie, when it comes to the length of time you can stay out. As for the percentage of UVB rays that they block, they both keep out surprisingly high levels of rays. "SPF15 allows 93.3 per cent UVB to be blocked, SPF 30 allows 96.7 per cent to be blocked, " says Dr Birnie. "If you spin that around SPF30 allows 3.3 per cent of UVB through, SPF15 allows 6.7 per cent through, so it is double the protection."

What's the difference between mineral/physical SPFs and chemical sunscreens and which is better?

Neither is better or worse than the other when it comes to warding off rays, they just act differently (however those with sensitive skin often fare better with mineral sunscreen )

"Mineral means it comprises of mineral sunscreen, usually zinc oxide or titanium dioxide," Dr Birnie explains. "Mineral works by reflecting or scattering UVA rays as they hit skin while chemical causes a chemical reaction which causes the UV energy to change into heat which is then lost."

Chemical sunscreens include compounds such as avobenzone, octinoxate and oxybenzone, as well as many, others writes Dr David Jack in Vain Glorious. Is oxybenzone toxic? Despite what David Jack calls a sensationalist piece in the New York Times and elsewhere, "oxybenzone, has not actually been shown to cause any harm to humans. I would therefore say: be sensible. What we do know for sure is that skin cancers can be lethal and that UV is the main cause of them, so we would be particularly silly to not use UV filters in our skincare based on some overhyped and underresearched hearsay. At least until we have further solid evidence to the contrary."

Which is better? That comes down to personal preference, says Dr Jack. "I prefer chemical screens. They are lighter in weight, tend to be less comedogenic (less likely to block pores and cause breakouts) and you don’t need to use as much. Physical screens need to be applied quite thickly to achieve the level of protection they say they provide. In fact, you may need to apply as much as two teaspoons to the face and neck alone to obtain the cover stated." He adds that mineral sunscreens may leave a reflective white film on the skin, "which might be problematic for darker skin types."

Sometimes brands will offer sunscreens that contain a mixture of both. Find out more in our guide to sun cream ingredients for sensitive skin.

Which comes first - sun cream or moisturiser?

Usually, SPF is your last step after moisturiser. Especially if it's a mineral suncream it acts as a barrier. If you put moisturiser on top it can dilute the effectiveness of your SPF underneath or even rub it off.

MORE GLOSS: 10 SPFs Caroline Hirons buys at Space NK

Why do I need UVA and UVB protection and what do they mean?

Sunlight contains UVA and UVB rays both of which can cause skin damage and cancer. An easy way to remember what they do is UV-A is 'Ageing' and UV-B is 'Burning'. You need to look for suncream labelled broad-spectrum which covers both.

What does the star or PA+ rating on sun cream mean?

UVA protection is shown by a star rating. You will see a star rating of between zero and five stars on UK sunscreens. UVA protection is measured as a ratio of the UVB protection. "A five-star rating means that UVA protection is equivalent to the UVB protection." The higher the star rating, the better. Four or five stars is what you need.

UVA in a circle means it meets EU standards and has at least one-third of the protection offered by the UVB rating, according to .

Your bottle might also have PA+ (or even PA++++) on the label which is a rating system from Japan indicating how much UVA protection is offered (the more + the better). However, this is not considered the gold standard as 'broad-spectrum SPF', according to Paula's Choice, because it only measures the protection against UVA turning the skin brown (known as PPD, persistent pigment darkening) and not everyone's skin reacts the same way.

How often should you apply sun cream?

“Top up your sunblock once every four hours or so, but it is important to read the label on your sunblock to follow the directions of use, as this differs from brand to brand and SPF," says Dr Patel. "You should apply sunblock every morning, even if you are not tanning on the beach. It helps to protect your skin from harmful UVA and UVB rays, as well as acts as an anti-ageing protocol. It is much better to be safe than sorry.”

How much sun cream should I use?

More than you think! The recommended amount equates to a teaspoon on each arm and one for the face and neck in order to get the protection it says on the pack. SPF factor is predicated on you using quite a lot. Dr Birnie recommends using enough cream that it looks white to start with and applying it in the direction of hair growth so it fully sinks in.

Does SPF makeup or an SPF moisturiser provide enough protection?

No. “BB creams and foundations which claim to have an SPF are not substitutes for a good, protective sunblock. They rarely contain a high enough SPF to really protect the skin from sun damage, and should only really be applied in conjunction with a traditional sunblock," says Dr Patel.

Bear in mind you'd have to slather on a lot of SPF makeup or foundation every four hours (a whole teaspoon!) to get the same level of protection as a dedicated SPF.

An SPF moisturiser is another matter as you are more likely to put it on thickly enough and it's better than nothing, however skincare expert Caroline Hirons, author of Skin Care, £9.99  advises against relying on it. Firstly, SPF in your active-laden cream could make the ingredients in your moisturiser ineffective, meaning you might be wasting your money, she writes. "SPF is a chemically dominating ingredient and if you load a skin cream with this, that's the sole benefit you'll get from that cream." Secondly, it gives a false sense of security that you're protected all day. "I know no one who takes their makeup off halfway through the day to reapply sunscreen." Thirdly, SPF moisturisers aren't always broad spectrum - they may only protect against UVB and lastly, they are less likely to be rub, sweat or water-resistant. "If you then apply your makeup with your fingers or a brush, or sweat, it's gone."

The bottom line? Use a dedicated SPF.

How effective is water-resistant sun cream?

If you've ever slathered yourself in water-resistant sunblock, hopped in and out of the pool and somehow found yourself burning you'll know that water-resistant suncream has its limits. Water-resistant simply means the product has been tested under very specific lab conditions which include drip-drying after a hot tub session twice and not towelling down, the equivalent of taking two dips. "They then test how effective that sunscreen is, and as long as your SPF is at least 50 per cent of the labelled factor you can label it water-resistant," says Dr Birnie. On top of that, many of us towel-dry which compromises the effectiveness of your suncream.  "I would always advocate reapplying sunscreen after going in the water."

Are cheaper sunscreens less effective?

While there are many high-end formulas on the market, it doesn't necessarily make them a better protectant against UVA and UVB. An inexpensive product will have been through the same tests as a luxury cream. The only difference will be that they might not feel as nice when you apply them. Altruist Sunscreen developed as a social enterprise by Dr Birnie, is sold at the lowest price possible (£8 for two tubes) in utilitarian packaging in order for it to be accessible for everyone to minimise the risk of skin cancer.

Do I need reef-safe suncream?

Increasingly we're seeing 'reef safe' on bottles SPF and this is because high concentrations of some ingredients oxybenzone and oxynocate can cause coral bleaching (reef-safe sun creams don't contain them). Dr Birnie feels it's a moot point for most of us unless we are swimming in coral-rich waters with no tide, such as in a lagoon, where the bleaching ingredients can be more concentrated.

"Reef safe is a bit of a marketing tool," he says. "There are some studies that suggest that high concentrations of certain sunscreen in the water may cause some coral bleaching. "My advice to anyone who would be [swimming near coral] is that wearing UV protective clothing would be a far better option because if you're spending time snorkelling the suncream could potentially wash off anyway so you'll get better quality protection in UV clothing."

Do I need to wear SPF inside or when it's cloudy?

In a word, yes - and when driving. Fifty per cent of UVA rays penetrate glass (depending on the glass, your car windscreen will likely have a filter on it but the side windows probably don't). Find out more on why you need to wear SPF indoors here.

As for wearing sunscreen when it's cloudy, this too is a wise precaution as the effects of UV damage over time can build up even if we never burn and only receive small doses. It's best to protect the areas of the skin such as the face that receive the most exposure, as Dr Birnie explains below.

So in the end, which SPF is best?

Whether you want reef-safe, mineral or chemical is personal preference, but from a skin damage perspective Dr Birnie advises, "SPF factor 30 or 50 and something that's broad-spectrum (UVA/UVB) with the four or five stars [UVA rating] or the UVA in a circle."