Sunscreens: Exploring the Spectrum

Written by Dr Rebecca Hellen who is a final year Dermatology Specialist Registrar working in St. Vincent’s University Hospital, Dublin

Sunscreens: Exploring the Spectrum

Exposure to solar radiation is hazardous for human health and causes skin cancer, photodermatoses and eye disorders. The main health benefit of ultraviolet radiation (UVR) is vitamin D synthesis, however oral supplementation is a safer alternative. Solar radiation reaching the earth’s surface contains UVB (280-315nm), UVA (315-400nm), visible light (VL: 400-780nm), and infrared radiation (780nm-1mm). UVR accounts for almost 8% of solar radiation (5% UVB, 95% UVA). The light dose received is influenced by altitude, latitude, time of day, weather, season and reflection from surrounding surfaces.

UVR is a class I carcinogen and UV-signature mutations are found in basal cell carcinoma, squamous cell carcinoma and melanoma. 1 Immunosuppression significantly increases the risk of cutaneous malignancy. This affects a broad range of patients including those with solid organ transplants, iatrogenic immunosuppression for autoimmune diseases, haematological malignancies and HIV. As such, it is incumbent on all clinicians treating these patients to have an understanding of photoprotection and counsel them appropriately.

Ultraviolet light

UVB consists of higher energy, shorter wavelengths which penetrate the epidermis where they induce sunburn and direct DNA damage. 2, 3 Delayed tanning is usually due to UVB exposure. UVB experiences greater atmospheric filtering than UVA due to its shorter wavelengths and varies with the solar zenith angle. In Ireland, UVB levels are negligible during the Winter months. Sun protection factor (SPF) describes a ratio whereby a sunscreen increases the time to burning during sun exposure. As this is a ratio, the difference between SPF 30 and SPF 50 is minimal (97% versus 98% UVB protection).

UVA penetrates to the dermis and causes indirect DNA damage through oxidative stress. 2 Ambient UVA does not cause sunburn, although high doses from sunbeds or phototherapy can induce burning. UVA is implicated in the pathogenesis of photoageing and photosensitive dermatoses including PLE and cutaneous lupus. Compared to UVB, UVA exhibits less variation with latitude and is present year round. There are a number of methods to measure the UVA filtering ability of sunscreens. The Boots star rating system measures UVA protection relative to UVB protection, i.e. 5 stars denotes a 90% UVA-filtering efficacy relative to the SPF. The UVA seal denotes UVA protection that is one third that of the SPF.

Visible light

Approximately half of sunlight is in the visible range. While not considered carcinogenic, VL induces oxidative stress and upregulates metalloproteinases. 4 VL causes hyperpigmentation, especially in darker phototypes.

Sunscreens

Organic (chemical) filters absorb UVR and appear transparent, but can cause irritancy. The inorganic (physical) filters zinc oxide and titanium dioxide absorb, reflect and scatter UVR, however can leave a white cast due to the reflection of visible light. Nanosized particles are more cosmetically acceptable, however they are less effective against longer wavelengths. Sunscreens are regulated as cosmetics by the European Commission, whereas they are regulated as over-thecounter drugs by the FDA. As a result, Europe has access to a broader range of UV filters. Phenylene bis-diphenyltriazine (TriAsorB) is a recently approved broad-spectrum filter which is effective across UVB, UVA and blue light wavelengths. 5

Tinted sunscreens protect against VL and contain mixtures of iron oxides in different shades to match a variety of skin tones. 6 They have been shown to improve melasma when combined with topical hydroquinone. 7, 8

Endogenous antioxidants scavenge free radicals induced by oxidative stress. There is evidence that topical vitamin C and E increase the efficacy of broad-spectrum sunscreen, however there is no standardized way to measure their biological effects. 9 Polypodium leucotomos extract has been reported to reduce susceptibility to sunburn when taken orally. 10 There is currently insufficient evidence to recommend oral antioxidants in photoprotection.

Photoprotection advice

Sunscreen should be used as a last resort after the following measures. All patients should be counselled to wear photoprotective clothing with a tight weave, wear a broadbrimmed hat, seek shade and avoid sun exposure 2 hours either side of solar noon. Sunglasses with UV-protective lenses should also be worn.

An appropriate broad-spectrum sunscreen with SPF ≥ 30 and a high level of UVA protection should be chosen. Children should use paediatric sunscreens with inorganic filters and these can be used from 6 months of age. Sunscreen should be applied to all exposed skin. It should be re-applied every 2 hours or after heavy sweating or swimming. One ounce of sunscreen (approximately a shot glass) is required to cover the whole body. A teaspoon is required to cover the head and neck.

Personalized photoprotection advice should take into account Fitzpatrick skin type (FST), geography, lifestyle, atopic dermatitis, acne-prone skin, rosacea, hyperpigmentation, and photosensitivity. 11

Challenges

Despite public health initiatives to improve awareness of sunscreens and sun-safe behaviours, there remains a significant amount of misinformation. A recent survey of over 1000 people in the US found that 62% rated themselves as good or excellent at sun protection, despite 63% reporting a tan and 33% reporting sunburn. 12 An Irish survey of 178 immunosuppressed patients with inflammatory bowel disease reported that only 70% wore sunscreen and many did not reapply, wear a hat or seek shade. 13

Men are less likely than women to wear sunscreen. Factors that may influence this are that the signs of ageing occur later in men, the increased numbers of sebaceous glands make skin more oily and shaving predisposes facial skin to irritation. 14 Photoprotection advice and sunscreen recommendations should be tailored to men with these considerations in mind.

While claims about photoprotection are regulated, sunscreens often contain potential allergens and irritants such as fragrances and preservatives. A negative experience can deter patients from wearing sunscreen. Navigating the ingredient list effectively can help identify products unsuitable for sensitive skin, however this requires familiarity with the international nomenclature of cosmetic ingredients (INCI).

In conclusion, our daily interactions with sunlight can have lasting effects in relation to skin cancer, photoageing and dyspigmentation. Reliable information from healthcare professionals is key, especially for the immunosuppressed population. Sunscreen is an imperfect defence against solar radiation and should be used appropriately after implementing other more effective photoprotective measures.

References available on request

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