Sun light can damage our skin in many ways leading to premature or faster ageing, skin disorders and even skin cancer. The damage can be as small and unnoticeable as reducing the concentration of certain must have molecules in our skin, such as for example vitamins, or extreme causing cancer. So just because you cannot see it, the damage is there! And similarly, just because it is not Mediterranean sunny, the UV-light is there too!
But before we dive into the details, let’s quickly review what the threat actually is. UV light is divided into three categories: UVA (320-400 nm), UVB (290-320 nm) and UVC (200-290 nm).  Thankfully, as long as Earth’s atmosphere is not damaged, UVC is completely absorbed by the stratospheric zone and therefore, we do not have to worry about it.  On the other hand, UVA makes up most of the UV light we might come in contact with, at about 95-99%, followed by 1-5% UVB, exact numbers depending on the latitude, time of the day and time in the year. UVB is fully absorbed by the outer layer of our skin whereas roughly 50% of UVA penetrates Caucasian skin more deeply. [1, 2] However, even if the damage is on the smaller side, such as a simple reduction in vitamin concentration let’s say, it can cause domino effects on our skin as each molecule present takes part in many processes to keep us healthy and good looking. Reduction of vitamin C for example, can reduce collagen and lead to wrinkles!
So what can we do to protect ourselves as much as possible, is there any science behind this and do any of the SPF skin care products work?
Sun creams fall into two different classes in terms of what molecules they contain and how they act: (a) chemical absorbers and (b) physical blockers.  As the name suggests, the chemical absorbers are compounds that absorb UV rays of a particular wavelength (hence why they would be good for only UVA, B or C). As they absorb that specific part of light they become excited and then release the absorbed energy in a range of different ways, including heat. Such agents include para-aminobenzoic acid (PABA) and its ester derivatives, salicylates, cinnamates, benzophenones, butylmethoxydibenzoylmethane (Parsol 1789), drometrizole trisulphonic (Mexoryl XL), terephthalydene dicamphor sulphonic acid (Mexoryl SX), methylene bisbenzotriazol tetramethylbutylphenol (Tinasorb M), anisotriazine (Tinasorb S.) and more.  On the other hand, physical blockers protect the skin by blocking the light, through reflecting and scattering UV rays, mainly because of their large molecular size acting almost like physical shields in the tiny molecular world. Such agents include titanium dioxide and zinc oxide. 
Parsol MCX (octinoxate, octyl methoxycinnamate or OMC) is one of the most commonly used UV filters, especially when combined with other UVB absorbers, to achieve high SPF values.  It is very well tolerated and skin reactions are very rare. 
But have you even wondered what SPF means? SPF stands for Sun Protection Factor and scientifically speaking SPF is defined as the ratio of the dose of UV (290-400 nm) required to turn sunscreen protected skin temporarily red (=erythema dose) after application of 2mg/cm2 of product. Do note however, that the absence of redness does not equal zero UV damage and that SPF is primarily a measurement of UVB protection as it is 1000 times more likely to cause skin irritation (redness) as opposed to UVA.  Additionally, the SPF number does not correlate to its UVA protection ability, as scientists have not been able to agree just yet on a unified protection measurement for this one. As a result, always check the full characteristics of the sun cream you are buying to make sure it includes UVA protection too.
You might think that the higher the SPF value the longer you are able to spend in the sun damage free. Not exactly true! The higher the value the higher the percentage of protection but none are really 100%. To give you an example, SPF 15 gives a 93% UVB protection, SPF 30 a 97% and SPF 50 a 98% protection.  Additionally, SPF values are not accumulative. This means that if you wear one product with SPF 10 and one with SPF 15 (let’s say a moisturiser and foundation make up or moisturiser and a sun cream) they cannot be accumulated so your SPF protection does NOT equal 25. The active SPF protection will be the highest of the products you are using so in this particular example the SPF protection would be 15. Choose wisely and don’t forget that your lips count as skin too! Thankfully, there are plenty of lip moisturisers that have added SPF protection.
Interestingly, there are a few scientific papers that argue that we are not getting the stated SPF protection and they are right. As mentioned earlier you need 2mg/cm2 in order to achieve the SPF factor stated on the bottle. But how much cream is that actually? To cover your face you would need somewhere between 1-1.5mL of product (about a 1/4 of a teaspoon) depending on the size of your face. If you think that’s not much, try squeezing out the amount to get a visual, it will cover your palm! You were thinking a pea size amount weren’t you? Nope! Go crazy with your sunscreen and if you get laughed up for looking like a ghost, remember you are the one being protected from skin disorders, cancer and wrinkles! If you fancy a detailed mathematical blog post on how to calculate how much sunscreen you need see reference. 
But what about water resistance? According to European laws a sunscreen can be classed as “water resistant” if the SPF protection after 40 (=water resistant) or 80 (=extra water resistant) minute water exposures is equal or greater to 50% its activity prior to water exposure.  It is useful to note then that the SPF number on European products equals the pre-water exposure whereas on USA products it is equal to after water exposure, unless stated otherwise.  Bear in mind that since you are losing some protection after exposure to water (this includes sweating) it is a good idea to reapply your sunscreen if you are planning to be exposed to the sun further.
Are sunscreens safe? Interestingly, there is some controversy around the safety of sunscreens. While some studies link the risk of skin cancer (melanoma) [1, 6] with the usage of sunscreens there are others that show that the use of sunscreens decreases the risk of cancer. [1, 7] However, it is very early days on this subject and there is no solid proof at the moment that sunscreens can cause cancer, in fact if nothing else, exposure to UV light without protection definitely does cause cancer and there is no scientific debate on that one!
But did you think that SPF applies to sun creams only? There are some material infused fabrics that can offer enhanced protection, called UPF for clothes. Don’t worry though, even if you didn’t get your hands on technologically advanced clothing, normal clothing offers some protection too. UPF is affected by fabric, colour, thickness, tightness and even the way the material is washed.  As a general rule of thumb, the thicker fabrics such as denim, offer a much higher protection as opposed to the lighter fabrics such as cotton (UPF 1700 vs 5-9 !). [8, 9] Similarly, looser clothing can offer more protection than when tight or wet [8, 10] and of course, the darker the colour the more protection it can offer. Don’t forget your sunglasses too!
Finally, remember that as UV is a natural part of light, it is not only present in the summer. As a result, a product with SPF protection is a must at all times even in rainy winters and especially at snowy places, as snow reflects light (and you are at a higher altitude so closer to the sun). Don’t stress about SPF 50 in the winter but make sure you are using a minimum SPF 15. And if you want to be super clever, look out for creams that have vitamins C & E, either to use prior as part of your daily skin care or contained in your sunscreen, as they can enhance UV-protection. And if you want to go that extra mile further, nowadays there are even creams that absorb in the non-visible IR or IRA range (770-1400nm) which might offer a little extra protection as IR can also contribute to photo ageing.  Your future self will thank you!
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