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Case study 1: John
John, 27, is going on a hiking holiday in the mountainous regions of Kenya. He has read that using SPF15 products means he can spend 15 times longer in the sun before burning. Since he normally burns after about 30 minutes in the sun, he believes that this would be suitable for him and asks to purchase an SPF15 sunscreen. What would you advise?
Explanation
Many people interpret SPF in relation to the amount of extra time that can be spent in the sun before burning. But this is simply not the case.
SPF is in fact a ratio defined as: “The dose of ultraviolet B (UVB) radiation required to produce one minimal erythema dose (MED) on protected skin after the application of 2mg/cm2 of product, divided by the UVB to produce one MED on unprotected skin”.
The term ‘one minimal erythema dose’ represents the amount (dose) of UVB radiation that gives rise to perceptible erythema 24 hours after application of a sunscreen. This is determined based on an application of 2mg/cm2 of product, which is roughly equivalent to one 5ml spoonful spread over the face.
If this amount of sunscreen is applied, it will have an SPF of 15. The misunderstanding around the interpretation of the SPF arises because the term relates to the amount of solar radiation rather than the time period of exposure.
While spending more time in the sun increases the amount of solar energy that reaches the skin, there are additional factors at play here. One of the most important is the time of day, which is best explained with the following example:
Imagine sitting in the UK sun during the summer months at two different times: 9am and 12 noon. Sunburn is far less likely to occur at 9am than at 12 noon because earlier in the morning, the sun is at a lower angle in the sky. Since the sunlight has to pass through much more atmosphere, there is a greater degree of absorption of UVB radiation.
In contrast, at 12 noon, the sun is at its peak and the UV radiation has a much shorter, more direct path to Earth. As a result, it will retain more energy, so a person will experience sunburn in less time. In other words, to obtain the same amount of solar energy, they would have to spend much longer in the sun at 9am than at 12 noon.
However, the amount of solar energy also depends on several other factors:
- Latitude: solar energy is higher closer to the equator
- Altitude: for every 1,000m increase in altitude, UV levels increase by roughly 10 per cent
- Season: UV levels are highest during the summer months
- Surface reflection: sand, water, grass and snow can all reflect UV radiation.
While it is clear from Table 1 (page 29)that there are only small differences in the amount of UVB radiation absorbed by products with a higher SPF, it is important to recognise that in reality, few people apply sunscreens in sufficient amounts to obtain the stated SPF.
In fact, research suggests that consumers apply variable amounts of a sunscreen, which can be anywhere between 0.39 and 1mg/cm2 – and often unevenly. So, while the UVB absorption differences are small, it makes sense to recommend a higher SPF product, and this is backed up by research.
For instance, in one study conducted among skiers, participants applied an SPF100 product to one arm and an SPF50 product to the other arm. It was found that while the amount applied was roughly half the recommended amount (i.e. 1mg/cm2), participants experienced more sunburn on the arm to which the SPF50 was applied.
Other work has shown that applying a product with an SPF70 or 100 at 0.5mg/cm2 gives rise to an actual SPF of 19 and 27 respectively.
These observations suggest that because in practice patients invariably apply less than adequate amounts of a sunscreen, recommending a higher SPF product means that an individual is more likely to achieve the minimum recommended level (i.e. SPF15) for photoprotection against sunburn.
Advice for John
Since SPF only relates to protection against UVB radiation, it would be sensible to recommend that John uses a high SPF, broad spectrum sunscreen, which also protects against UVA. In addition, he should be advised that the UV index (see page 29) will be much higher in Africa and the amount of UV radiation will also be greater in mountainous regions, increasing his risk of sunburn. He should seek shade during the hottest parts of the day (11am-3pm) and ensure that he has sufficient supplies of water to maintain hydration.
Case study 2: Jodie
Jodie, 23, has been out in the sun all day and comes to the pharmacy seeking advice for her sunburn.
She recently purchased diclofenac gel (1.16%) for a sprained ankle. She knows that diclofenac is an anti-inflammatory, so asks if it would be okay to use the gel on her sunburn since her skin is inflamed. How would you respond?
Explanation
Interestingly, there is some evidence that topical diclofenac gel 0.1% is effective for the relief of pain and erythema associated with sunburn. In addition, one article also concluded that diclofenac provided analgesic relief for up to 48 hours after UV irradiation.
However, OTC use of topical diclofenac is unlicensed for the treatment of sunburn. A 3% diclofenac gel that is commercially available for the treatment of actinic keratosis is likely to be effective through an anti-inflammatory action.
Hydrocortisone 1% is known to reduce the inflammatory response induced by the sun. Although it can be prescribed and is recommended by the British Skin Foundation for short-term use, this is not a licensed OTC use and evidence for the use of topical steroids for sunburn in limited.
One randomised trial of moderate and high potency topical steroids for the management of acute sunburn found that neither agent reduced the acute sunburn reaction when applied six or 23 hours after UVB exposure.
Aloe vera gel does appear to be effective for sunburn and one study suggests that it is more effective that 1% hydrocortisone cream.
Advice for Jodie
Based on the extent of her sunburn and the absence of skin blistering, perhaps the best treatment for Jodie is to leave it alone as mild sunburn is self-limiting. She should be advised to maintain adequate hydration and can use emollients to soothe her skin or take oral analgesics such as paracetamol or ibuprofen
if her skin is painful.
Case study 3: Jane
Jane, 45, is worried about getting skin cancer because she is moving to live in southern Spain, which is much sunnier than the UK. She wants to know if using a sunscreen will prevent the development of skin cancer. What would you say to her?
Explanation
Since getting sunburn increases the risk of skin cancer, it would seem logical that in the longer term, sunscreens should also reduce the risk of developing skin cancer. But surprisingly, there is little supportive evidence for this.
A Cochrane systematic review from 2016 concluded that the available evidence did not show whether sunscreen use prevented the non-melanoma skin cancers, basal cell or squamous cell carcinoma.
Nevertheless, data from a follow-up trial that began in 1992, looking at the impact of sunscreen use on the incidence of squamous cell carcinoma, found that when participants were reassessed in 2006, there was a 50 per cent lower incidence of primary melanomas in those who applied a sunscreen every day. There was also a 73 per cent reduced risk of invasive melanomas. Other data suggests limited benefits.
A meta-analysis from 2018 that included 29 studies with 313,000 participants concluded that there was no association between both melanoma and non-melanoma skin cancer and sunscreen use. A further analysis found weak and heterogenous evidence for an association between sunscreen use and melanoma.
Some evidence from a study of sunscreen use in young adults and the risk of melanoma before 40 years of age did provide evidence that regular sunscreen use did lower the risk of melanoma.
Advice for Jane
While the evidence remains ambiguous, it would seem sensible to advise Jane to use a sunscreen regularly when she is outdoors, together with wearing sun-protective clothing and avoiding direct sunlight between 11am and 3pm. It is also worth mentioning how the UV index in Spain will be higher than it is in the UK during the summer months, which increases the risk of sunburn.
Which first: sunscreen or insect repellent?
Research suggests that the protective effects of sunscreen are reduced by roughly a third when a DEET-containing insect repellent is applied to the skin. What’s more, sunscreen can enhance the absorption of DEET into the skin, potentially increasing toxicity, which could be a problem in children.
It seems likely that DEET-based insect repellents thin or disrupt sunscreen that has been applied to the skin. It is therefore recommended that they are applied after sunscreen.
The researcher who uncovered how DEET products reduce sunscreen efficacy also suggested that when using the two products, it is best to avoid direct sunlight during peak hours (11am-3 pm) and to make use of protective clothing and headgear at all times.
Do sunscreens interfere with vitamin D?
One action of sunlight on the skin is the formation of vitamin D. The UVB portion of the solar spectrum ranges from 280 to 315nm and this not only induces sunburn but also enables the synthesis of vitamin D. In theory, therefore, the use of sunscreens would also prevent vitamin D synthesis.
Several studies have looked into this and the overarching conclusion seems to be that they do not. In a 2019 analysis, for instance, it was found that even when an SPF15 product was optimally applied, sufficient vitamin D was still produced in the skin.
Despite this, no studies have been undertaken in products with a high SPF. But given how people tend to suboptimally use sunscreen, it seems likely that vitamin D synthesis might still occur. Whether this holds true when a high SPF is optimally applied is unclear.
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