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  • Ivan Bristow

Sun protection and the SPF Factor

As the long summer spell continues in the UK protecting the skin against the harmful effects of UV radiation is key to preventing sunburn and skin cancer. The key sun protection messages are:

1. Spend time in the shade during the sunniest part of the day when the sun is at its strongest, which is usually between 11am and 3pm in the summer months.

2. Avoid direct sun exposure for babies and very young children.

3. When it is not possible to limit your time in the sun, keeping yourself well covered, with a hat, T-shirt, and sunglasses, can give you additional protection.

4. Apply sunscreen liberally to exposed areas of skin. Re-apply every two hours and straight after swimming, sweating or towelling to maintain protection.

(B.A.D Website [2018])

In the advice, you’ll notice what comes first – staying out of the sun when UV is at its strongest. Secondly, cover up with additional protection and finally, if you must go out in the sunshine - use a sunscreen on all sun exposed skin. Sunscreens are effectively a last resort.

Sunscreens primarily work by reducing the amount of UVB radiation reaching the skin causing the harmful erythema which leads to sunburn and harmful cellular damage. This protection is achieved by organic absorbing agents and physical blocker chemicals like titanium dioxide and zinc oxide which collectively can reflect or absorb harmful radiation preventing skin damage. Studies have demonstrated their ability in reducing skin cancers (1-3).

The ability of the sunscreen to prevent skin erythema and subsequent burning is measured by the SPF (Sun Protection Factor). The scale itself is not linear and can be confusing.

Put simply, if your skin turns red, say in 20 minutes of summer sunshine, applying an SPF 15 product will mean that the cream prevents reddening 15 times longer (for about 5 hours). Another way to think about it is like this:

A SPF 15 product will block out 93% of the suns UVB radiation to the skin

A SPF 30 product will block out 97% of the suns UVB radiation to the skin

A SPF 50 product will block out 98% of the suns UVB radiation to the skin

Only Half the Story - UVA

It is important to remember though sunshine contains two harmful forms of UV light – UVA and UVB. If you think about UVA (“A” for ageing) and UVB (“B” for burning) you will begin to understand the effects on the skin of both. UVA causes premature skin ageing (“photoageing”) - wrinkling, loss of elasticity and uneven brown pigmentation. UVA radiation can travel through window glass more readily then UVB and penetrates more deeply into human skin.

Sunscreens sold in the EU which contain UVA protection are rated by a star system (1 to 5 stars). 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.

Ultimately, the advice is to select a product with a high SPF and 4-5 star protection level. These may be sold as “broad spectrum sunscreens” in some shops. In the EU, products with UVA coverage of a third of more of its UVB coverage are marked with a UVA in a circle to indicate a recommended level of protection.

The Pitfalls of Sunscreens

Despite the good research showing that these products can effectively reduce the amount of solar radiation reaching the skin, there are several issues which can seriously affect their effectiveness. This mostly comes down to how they are applied and the users subsequent behaviour. The main issues identified in studies which reduce sunscreen effectiveness include:

1. Failure to apply the sunscreen prior to sun exposure.

It is suggested that they should be applied at least 20 minutes before any sun exposure. One study identified that subjects many receive 13% of UV radiation prior to sunscreen application as they only apply it as they arrive on the beach/at the poolside (4).

2. Failure to apply the correct amount to all sun exposed areas.

Difficult to reach areas often are missed or have a reduced coverage. Also, studies have highlighted consistently that people underestimate the dosages required. The recommended amount to be applied is 2mg/cm2 of sunscreen product. A reduced dose will effectively reduce the SPF. The data below highlights how under-application can result in significant reduction in effectiveness. As the table below shows, under dosing significantly affects the sunscreen. For example, even if a SPF 50 is used at a dose of half the recommended amount (1mg/cm2) its SPF in reality becomes equivalent to just SPF 7! (5)

Source: Wulf et al. (1997)

Its suggested an average adult in beachwear would require around 35mls of sunscreen at first application with regular top ups.

3. Failure to re-apply regularly (every two hours).

Studies again have shown that some products become less effective after just two hours after application, even if the correct dose is used. Therefore regular re-application at this interval is recommended. Other activities which may reduce effectiveness: sweating, swimming towelling etc., can all remove sunscreen and therefore decrease protection.

The rise of the convenient “Once a day application” sunscreen products although attractive to the consumer are generally not recommended on the basis of the failings given above and should be applied frequently, particularly after excessive sweating, bathing or towelling.

Many moisturising products now contain an amount of SPF within in its formulation which is on the face of it, is a welcome addition. However, research conducted on these products has highlighted the shortcomings of these – demonstrating sunscreen formulations offered greater protection even with equal SPF ratings (6).

Recent Research - A Higher SPF?

If you read up on which is the SPF to choose, it is generally accepted by the British Association of Dermatologists that SPF 30 and above is a suitable choice. In the UK, SPF in most products goes up to 50+. The mathematical difference between an SPF 30 and SPF 50 product is around 1% of additional UVB blocking protection (in theory). Which as a message for the general public, may actually in practice be mis-leading.

A recently published study in the Journal of the American Academy of Dermatology (JAAD) compared the difference between SPF 50+ and SPF 100+ in a randomised controlled trial in 200 healthy, active individuals (7). They covered half their face in SPF 50 and half in SPF 100 and after a day’s activity measured the amount of sun induced erythema (a measure of sun exposure towards sunburn) on each side of their face. The results were surprising. In the SPF 50 protected side of the face 41% showed increased erythema versus just 14% in the SPF 100 protected side.

What this study suggests, despite the theoretical 1% extra protection between SPF 50 and SPF 100 being mathematical, in real day-to-day use SPF 100+ provides much safer protection against harmful UVB. There will be further studies in this direction no doubt soon but for the moment the advice still should be use a high SPF product regularly (every two hours) and ensure it has good UVA coverage (5 star) as well to prevent premature skin ageing, the hidden side of skin exposure to the skin.

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1. Green AC, Williams GM, Logan V, Strutton GM. Reduced melanoma after regular sunscreen use: randomized trial follow-up. J Clin Oncol. 2011;29(3):257-63.

2. Olsen CM, Wilson LF, Green AC, Bain CJ, Fritschi L, Neale RE, et al. Cancers in Australia attributable to exposure to solar ultraviolet radiation and prevented by regular sunscreen use. Aust N Z J Public Health. 2015;39(5):471-6.

3. Lazovich D, Choi K, Vogel RI. Time to get serious about skin cancer prevention. Cancer Epidemiol Biomarkers Prev. 2012;21(11):1893-901.

4. 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.

5. Wulf H-C, Stender I-M, Lock-Andersen J. Sunscreens used at the beach do not protect against erythema: A new definition of SPF is proposed. Photodermatol Photoimmunol Photomed. 1997;13(4):129-32.

6. Lourenco E, Pratt H, Hamill K, Czanner G, Zheng Y, McCormick A. Application of SPF moisturizers is inferior to sunscreens in terms of percentage coverage of facial and eyelid area. British Association of Dermatologist Annual Conference; Edinburgh2018.

7. 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.


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