Scientific Skincare - Frequently Asked Sunscreen Questions

13 Frequently Asked Sunscreen Questions (FASQs)

Regular sunscreen use is essential for skin health, but why? Do you really need to wear sunscreen every day? How much sunscreen should you apply? How often should you reapply sunscreen? What does SPF mean? Are physical sunscreens better than chemical sunscreens? So many questions! In order to answer some of your burning questions, here are 13 frequently asked sunscreen questions!

Sunscreen FAQs

1. Should You Wear Sunscreen Every Day?

Yes! You should wear sunscreen every day. Not only is sunscreen scientifically proven to reduce the risks of certain types of skin cancer, but up to 80-90% of all facial skin aging is due to UV radiation.

Although most people think sunscreen is only necessary on days when it is hot and sunny and the risk of sunburn is high, sun damage can occur in all weathers. This is due to the fact that UVA rays account for the majority of UV radiation and are relatively constant all year round.

UVA rays have less energy than UVB rays but are able to penetrate deeper into the skin. Although UVA radiation is not directly absorbed by DNA like UVB radiation is, it causes indirect damage via the production of free radicals. These free radicals also cause the break-down of collagen in the skin.

Check out the sunscreens with the highest UVA protection here!


2. How Often Should I Apply Sunscreen?

To achieve and maintain the SPF and UVA-PF/PPD values stated by a specific sunscreen, sunscreen should be applied every 2 hours. This is due to the stability of the UV filters used within the sunscreen. UV filters absorb UVA and/or UVB radiation to reduce the amount of radiation absorbed by DNA or melanin.

The UV filters need to be able to absorb and disperse the UV energy before they can absorb more energy. If they are unable to disperse the energy fast enough then they become ‘de-stabilized’, meaning that they are unable to absorb further UV radiation. This is why sunscreen needs to be reapplied after prolonged exposure to UV radiation.

In this sense, sunscreen needs to be re-applied every 2-hours of sun exposure. This means that if you are wearing sunscreen daily but your only sun exposure is during your lunch break, you are probably ok! (That is unless you’re directly next to a window – for reasons covered below).


3. How Much Sunscreen Should I Apply?

In order to achieve the stated SPF and UVA-PF/PPD values stated on sunscreen, you need to be using 2mg of sunscreen per cm2 (2mg/cm2). This is because this is the standard amount of sunscreen used to calculate the SPF and UVA-PF/PPD values during testing.

2mg/cm2 is pretty hard to visualize, but it is approximately a nickel-sized (or ten-pence-sized) dollop for the face or the equivalent of a shot glass (or two tablespoons) for the exposed areas of the face and body.

Most people don’t apply enough sunscreen which means that they do not receive the stated UV-protection. This is particularly the case when people use ‘moisturizers with added SPF’. In addition, these moisturizers often contain no UVA protection.


4. What Does SPF Mean?

SPF stands for Sun Protection Factor and is a measure of how much protection a sunscreen offers against UVB radiation. The damage associated with UVA wasn’t given much consideration until a couple of decades later.

In order to determine the SPF of a sunscreen, it has to be tested on at least 10 volunteers. These volunteers have their skin (usually a forearm) exposed to an artificial UVB source with and without sunscreen. The SPF is then calculated by timing how long it takes for the skin to begin to burn – officially termed the ‘Minimum Erythema Dose’ or ‘MED’ for short – with and without sunscreen.

Here’s the equation used:

SPF = MED Protected/MED Unprotected


Higher SPF values indicate better protection from UVB radiation.


5. How Is UVA Protection Measured?

As mentioned, SPF only measures the protection a sunscreen offers against UVB protection. We now know that UVA radiation possesses similar health risks but there is currently no internationally recognized method to measure UVA protection.

Instead, there are a number of different methods that are used and you can determine which method has been used by the way the sunscreen has been labeled.

How UVA Sun Protection is Measured

For example:

Broad-Spectrum – If a sunscreen has the words ‘Broad-Spectrum’ on it, it means that it contains both UVA and UVB protection, but the exact level of UVA protection has not been measured. This is determined by ‘the critical wavelength method’ which means that at least 10% of the sunscreen’s protection has to be for wavelengths over a given number. That given number (usually 370nm) is known as ‘the critical wavelength’.

UVA Seal – The UVA seal is a European standard that requires all sunscreens to have a UVA-PF or PPD value of at least one-third of the stated SPF. This means that a sunscreen with SPF30 has to have a UVA-PF or PPD value of at least 10.

PA+ System – The PA+ system grades UVA protection according to the PPD value, but the PPD value does not have to be relative to the SPF value. The PA+ system goes up to PA++++ which requires the PPD value to be 16 or more. You read more about the PA+ system here.

The Boots Star Rating System – The Boots Star Rating System grades UVA protection according to the percentage of UVA protection compared to UVB protection. For example, sunscreen has to have a UVA-PF value that is at least 90% that of the SPF value in order to achieve the highest 5-star rating. You can read more about the Boots Star Rating system here.


6. What’s The Difference Between Chemical & Physical Sunscreens?

Chemical sunscreens (e.g. Avobenzone, Tinosorb S) work by absorbing UV radiation, while physical sunscreens (Zinc Oxide & Titanium Dioxide) work by reflecting and dispersing UV radiation, as well as by absorbing it.


7. Are Physical Sunscreens Better Than Chemical Sunscreens?

Often people consider physical sunscreens to be better because they are ‘natural’. This is largely to do with a wide-spread fear of chemicals. The thing is, everything is chemicals! In fact, zinc oxide and titanium oxide are also chemicals – just naturally occurring ones.

Physical sunscreens do tend to be more stable than chemical sunscreens as they work by reflecting and dispersing UV radiation rather than by only absorbing it. In addition, they are well-suited to those with very sensitive skin or contact allergies.

However, newer chemical filters are a lot more stable than their predecessors and offer superior UVA protection. In fact, if you are looking for a sunscreen with very high UVA protection (e.g. an SPF50+ with a 5-star Boots rating – UVA-PF 54), it is highly likely that you will only be able to find a chemical sunscreen with certain UV filters.


8. Are There Any Sunscreen Ingredients That I Should Avoid?

There are some chemical sunscreens that are better to avoid where possible. This is because there is some research to suggest that they may damage marine ecosystems and alter human estrogen levels. In particular, oxybenzone and octinoxate have the most research supporting their avoidance.

However, there isn’t a huge amount of research and nothing to prove conclusively that these ingredients pose any health threats. In other words, if you are currently using a sunscreen that you like and it contains either of these ingredients, don’t panic! It is probably fine but if you are worried, there are plenty of oxybenzone and octinoxate free sunscreens available.

As mentioned above, the newer chemical UV filters are better at protecting against UVA radiation, which is something to consider if your focus is primarily on preventing premature aging. In addition, these filters are generally considered safe, although they are yet to be approved by the FDA for sale in the US.


9. Is Sunscreen Alone Enough?

Unfortunately, sunscreens can only prevent approximately 55% of the free radicals generated by UV radiation. As free radicals can damage DNA and break-down collagen, this is not ideal. Fortunately, a lot of sunscreens are now formulated with added antioxidants.

Antioxidants are able to neutralize free radicals and offer additional skin benefits. In fact, when antioxidants are added into sunscreens, they can reduce the number of free radicals generated by up to 78%. Antioxidants that are particularly effective at preventing sun-damage include vitamin C (particularly when combined with vitamin E and ferulic acid), green tea, and niacinamide.


10. Should I Wear Sunscreen Indoors?

At first glance, this may seem like a silly question – why would you need to wear sunscreen indoors? However, the reality is that you may indeed need to!

This is due to the longer UVA rays which, unlike UVB rays, are able to penetrate glass. This means that if you are sitting by a window for the majority of your day, you are unknowingly exposing your skin to UVA radiation.

In fact, research has found that those who sit next to a window for long durations, due to occupation or other activities, often experience un-symmetrical aging. In other words, the side of their face that is exposed to the window looks significantly older than the side of their face that is not exposed.

A great example of this phenomenon is the well-known image of the trucker who, after 28-years of long-haul driving, experienced premature aging on the left side of his face (drivers-side) to a much greater degree than on the right side.


11. What SPF Should I Be Wearing Every Day?

The majority of the original research that demonstrated that sunscreen was protective against some forms of skin cancer used a sunscreen with SPF15. For this reason, only sunscreens with an SPF of 15 or more are legally able to claim that sunscreen helps protect against cancer or premature aging.

Because of this, sunscreen should be at least SPF15 but the higher the SPF, the more UVB-protection offered. (This is discussed in more detail below).


12. How Can I Work Out The Percentage Of UV Radiation That Sunscreens Block?

You may have seen people mention that SPF15 blocks 93.4% of UVB rays, SPF30 blocks 96.7%, and SPF50 blocks 98.1%, but where did they get these values?

As it turns out, this is what is called a logarithmic expression, which basically means that we can work out the percentage of UVB blocked with an equation if we know the SPF value.

This is due to the way in which the SPF value is determined. As mentioned above, SPF is worked out by dividing the amount of time taken for a UVB source to cause a sunburn response when the skin is protected by the amount of time taken when the skin is unprotected.

SPF = MED Protected/MED Unprotected

Essentially, this means that the SPF is a measure of how much longer a person can stay in the sun without burning when they are wearing sunscreen. Of course, it is not this simple as the strength of UVB rays varies throughout the day.

In theory, this means that a sunscreen with an SPF of 2 blocks 50% of UVB radiation as it takes twice as long for the skin to burn.

We can arrive at this percentage by using the following calculation:

100% – (100%/SPF) or 100 – (100/2) = 100 – 50 = 50%

(The math is explained beautifully by Dan of DanLovesSkincare)

For example, SPF 50:

100 – (100/50) = 100 – 2 = 98%

Percent of UVB Blocked By SPF

When it comes to measuring UVA protection, the PPD method uses the same method as SPF but with a different endpoint. In this case, it’s the amount of time taken for the pigment to darken when protected with sunscreen divided by the time taken for the pigment to darken when there is no sunscreen protection. For this reason, we can work out the % of UVA blocked by sunscreen in a similar way if we know the PPD value.

Percentage UVA Protection From PPD

As you can see in this chart:

PPD 2 Blocks 50% of UVA

PPD 4 Blocks 75% of UVA

PPD 8 Blocks 87.5% of UVA

PPD 16 Blocks 93.8% of UVA

As the PPD method of determining UVA protection has been shown to be highly correlated with the in vitro EU Colipa method (a.k.a. UVA-PF) and the values are usually roughly similar. So, in theory, this means that we can also work out the highest regulated UVA rating – An SPF50+ sunscreen with 5 Boots stars (a.k.a. UVA-PF 54) as:

100 – (100/54) = 100 – 1.85 = 98.2%


13. Are Higher SPFs Always Better?

Higher SPFs are better to a certain extent. As mentioned above, sunscreen can only claim to be protective against skin cancer and premature aging if it is SPF15 or above.

For this reason, the sunscreen you use should be at least SPF15 but preferably SPF30 or higher. This is because the percentage increase in additional UVB protection is smaller the higher the SPF value. For example, there is a 3.3% difference between the protection offered by SPF15 and SPF30, but only a 1.4% difference between the protection offered by SPF30 and SPF50.

Usually, you will hear that the 1.4% difference between SPF30 and SPF50 is so negligible that there isn’t much additional benefit in choosing an SPF50 over an SPF30. However, if you look at this in terms of how much UVB radiation is still able to penetrate the skin, you may see this in a different way.

This is due to the fact that SPF30 allows 3.3% of UVB radiation to penetrate, while SPF50 allows only 1.9% – That’s nearly double the amount of UVB able to penetrate the skin (1.7x the amount to be specific).

In addition, as the European regulations require that sunscreens contain a UVA-PF or PPD of at least 1/3 of the stated SPF value, a higher SPF will, therefore, have a higher level of UVA protection (if being sold in Europe). The Boots star rating system is also worked out based on the ratio of SPF to UVA-PF.

Additional Questions

Do you have any burning sunscreen questions (see what I did there?) that you would like us to answer? If so, please leave a comment below or contact us via our Contact Page. If we can answer, or find the answer to, your sunscreen query, we will add it to this article!



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