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  • Writer's pictureIvan Bristow

SPF 100 sunscreens - are they worth it?

Run a search on Google asking if SPF 100 sunscreen is any good and you may come across a few older posts saying SPF 100 “should not be used”, but is the most recent evidence changing the opinion?

With summer temperatures continuing to break records and the enduring rise in skin cancers, if we are to spend time outdoors then sun protection, particularly in the summer months, should high SPF sunscreen become everyday practice? Current sun awareness advice advocates staying out of the sun during the middle of the day, wearing UV protective clothing and the regular application of an effective sunscreen.

Sunscreens have been developed to reduce the amount of ultraviolet light (UV) reaching the skin, particularly in the wavelength (280-315nm) known as UVB which can cause sunburn and in the long-term lead to the development of skin cancer. When buying a sunscreen, they are available in a range of SPF ratings typically SPF 15, SPF 30, SPF 50 and SPF 50+. In the last few years, newer SPF 100 products have appeared on the market.

What is the SPF rating?

When selecting a sunscreen, protection against burning (UVB) is given by an SPF (sun protection factor) rating. Mathematically, the SPF is calculated by measuring something called the minimal erythema dosage. That is, the minimum amount of UV radiation required to cause reddening of the skin:

Sun Protection Factor = Minimal Erythema Dosage with sunscreen ­­­­­­­­­­­­­­­ _____________________________________ Minimal Erythema dosage without sunscreen

(This is based calculated on the basis of 2mg/cm2 is applied to the skin under testing).

Simplifying this calculation, if your skin burns after 20 minutes in direct sun, SPF15 means you should be able to stay in the sun 15 times longer before burning (5 hours). The NHS and National Institute for Health and Clinical Excellence recommend at least an SPF of 30. This product will block out 97% of the suns UVB radiation to the skin whereas a SPF 50 product will block out 98% of the suns UVB radiation reaching the skin, if applied appropriately.

A woman n the beach in the sun
Is SPF100 any good?

Raising the SPF to 100

In the last few years there have been papers published examining the effectiveness of SPF 100 formulations. One study compared the effectiveness of SPF 50 against SPF 100 in a five-day consecutive trial of beach goers (each using the two formulations on different sides of the body). Researchers found afterwards, 56% of SPF 50 protected areas of skin had evidence of sunburn versus just on 7% of SPF 100 protected skin (1). Similar results have been obtained on a split face study comparing the SPF 50 / SPF 100 in the USA with the SPF 50 side showing sunburn in 40% of participants and just 13% of the SPF 100 side (2). So, does this mean the higher SPF 100 factor is more effective and should be recommended? This is an area for debate, but products are appearing in the market within the SPF ranges 50 - 100.

In the real world, this may be of benefit but the difference in protection between SPF 50 and SPF 100 is only 1% (97% versus 98%) which may not seem very much, but is there another dimension to this? One of the most common user errors when using sunscreen is under dosing or put simply not putting enough on or losing sunscreen through sweating, swimming or towelling. Recall the SPF testing dose relies on 2mg/cm2. Studies have shown that consumers typically apply only 20 - 50% of this amount (3). On this basis, it can be a major reduction in the amount of protection. For example, an SPF 50 applied at half the dosage (1mg/cm2) effectively renders the SPF factor down to just SPF7 (4). On that basis (theoretically) an SPF100 product at 50% dosage would still give higher protection in the SPF range.

What about UVA protection?

UV light consists of two components UVA and UVB. UVA rays (315 - 400nm range) can penetrate further into the skin and are responsible for photoageing such as wrinkles and blemishes, but excessive amounts are also known to contribute to skin cancer. The SPF rating on sunscreen only refers to UVB protection – a fact only 5% of the UK public were aware of in a 2015 survey (5). In Europe, a star system on the packaging denotes the products UVA blocking capabilities.

The star system indicates the percentage of UVA radiation absorbed by the sunscreen in comparison to UVB, in other words, the ratio between the level of protection afforded by the UVA protection and the UVB protection. Consequently, a low SPF product may still have a high level of stars, not because it is providing lots of UVA protection, but because the ratio between the UVA and UVB protection is about the same. This is confusing for most on any level and calls have been made to simplify the message so the public can understand how effective a product is against all UV radiation.

So, when selecting any SPF product, a product with a high UVA filter should also be sought. Based on the above, one would expect the UVA protection to be high as the SPF is also high. An example is Eucerin® Actinic Control MD SPF 100 is an SPF 100 product which also has a high UVA protection.

Avoiding common mistakes when using sunscreen

Occasionally, you hear about people who purported to have used sunscreen but still succumb to sunburn. There may be several reasons for this. Firstly, it should be applied as per the instructions given on the packaging. Sunscreens work best when applied about 20 minutes before going into direct sun. Applying on arrival at the pool or beach risks unnecessary UV exposure. Up to 13% of UV exposure can occurs before the sunscreen is applied according to one study (3).

Additionally, sunscreens need regular application. Most suggest every 2 hours or after bathing, sweating or towelling as much is removed. Research has suggested that reapplication should be carried out on an hourly basis to ensure adequate dosing on the skin.

Ensuring all areas are covered. When applying sunscreen, its very easy to miss an area and consequently, sunburn is always a possibility. It’s also an interesting fact that when people are using sunscreens, sunburn is a more common occurrence. This suggests that our behaviour changes believing we have protection we stay out in the sun longer but of course, no sunscreen is 100% effective, particularly for many of the reasons outlined here and so sunburn will always be a risk.

For a sunscreen to work effectively, enough of it must be applied and as discussed below, a dosage of 2mg/cm2 is the calculated dosage. In practice, studies have shown (6, 7) that in real life applied doses are much lower between 0.39 – 0.79mg/cm2 which as noted below can seriously lower the SPF. One study of beachgoers discovered that most used an SPF 15 applied at a dose of just 0.79mg/cm2 . This effectively, lowered the product performance to an SPF of just 3 (3).

It has been noted that the specific formulation and delivery method (cream, spray, lotion etc.,) can have an effect on the dosing as some products are easier to apply and spread ensure adequate coverage and dosing (4).

Like all cosmetic products, sunscreens can degrade with time and consequently, they generally come with a shelf life of typically three years (unopened). Once opened, they can be used for up to 12 months provided they have been stored properly. Sunscreens should be stored in a dark place, away from excessive heat and moisture which can reduce their effectiveness with time (8).


Ultimately, if a sun protection product is to be effective, the correct application procedure needs to be followed to prevent sunburn but there are many variables that can affect the performance of a product. Products offering SPF up to 100 are now available, and research suggests tangible benefits when used correctly.

The author declares no conflict of interest in this article or with any products mentioned.


1. Kohli I, Nicholson CL, Williams JD, Lyons AB, Seo I, Maitra P, et al. Greater efficacy of SPF 100+ sunscreen compared to SPF 50+ in sunburn prevention during five consecutive days of sunlight exposure: A Randomized, Double-Blind Clinical Trial. J Am Acad Dermatol. 2019.

2. Williams JD, Maitra P, Atillasoy E, Wu M-M, Farberg AS, Rigel DS. SPF 100+ sunscreen is more protective against sunburn than SPF 50+ in actual use: Results of a randomized, double-blind, split-face, natural sunlight exposure clinical trial. J Am Acad Dermatol. 2018;78(5):902-10.e2.

3. Petersen B, Datta P, Philipsen PA, Wulf HC. Sunscreen use and failures--on site observations on a sun-holiday. Photochem Photobiol Sci. 2013;12(1):190-6.

4. Petersen B, Wulf HC. Application of sunscreen − theory and reality. Photodermatol Photoimmunol Photomed. 2014;30(2-3):96-101.

5. Anon. Public confused by sunscreen labelling, say experts: BBC News; 2015 [cited 2023 July]. Available from: Public confused by sunscreen labelling, say experts.

6. Autier P, Boniol M, Severi G, Doré JF, For The European Organization For R, Treatment Of Cancer Melanoma Co-Operative G. Quantity of sunscreen used by European students. Br J Dermatol. 2001;144(2):288-91.

7. Neale R, Williams G, Green A. Application patterns among participants randomized to daily sunscreen use in a skin cancer prevention trial. Arch Dermatol. 2002;138(10):1319-25.

8. Anon. How long do sunscreens last? : BDF, Hamburg; 2023 [Available from:

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