WOMEN’S HEALTH
THE SCIENCE OF SPF
With summer’s arrival, Dr Ginni Mansberg explains why now is a good opportunity to reinforce sun protection with your patients.
Ultraviolet (UV) radiation reaching the earth’s surface consists primarily of UVA (320– 400 nm) and UVB (280–320 nm), which differ in biological effects and clinical relevance. UVA penetrates deeply into the dermis and is strongly linked to photoageing, pigmentation and cumulative structural damage¹, ². UVB is more energetic, largely absorbed in the epidermis, and is the main cause of erythema and a key contributor to photocarcinogenesis. Both induce DNA damage, oxidative stress and immunomodulation, driving skin ageing and malignancy¹, ². Although DNA absorbs four times less UVA than UVB radiation, which leads to less formation of DNA photodamage and subsequent mutations.3
Sun Protection Factor (SPF) is widely used to measure sunscreen efficacy but is often misunderstood. It reflects protection against UVB-induced erythema, defined as the ratio of UV exposure required to cause minimal erythema on protected versus unprotected skin⁴. However, SPF does not measure UVA protection. Given UVA’s role in photoageing and pigmentation, broad-spectrum sunscreens are essential for effective photoprotection.
BENEFITS OF SPF
The clinical value of sunscreen is supported by strong evidence. A landmark randomised controlled trial in Australia showed that daily sunscreen use reduced skin ageing progression by 24% over 4.5 years compared with discretionary use, while ß-carotene supplementation showed no benefit⁵. This remains key evidence that sunscreen is both protective against cancer and an effective anti-ageing intervention. Long-term use is also associated with reduced incidence of squamous cell carcinoma and melanoma, reinforcing its role in primary prevention⁶.
SPF PRODUCT FORMULATION
Modern sunscreens combine organic (chemical) and inorganic (mineral) filters. Organic filters such as avobenzone and octocrylene absorb UV radiation, while inorganic filters including zinc oxide and titanium dioxide reflect and scatter UV, with some absorption⁷. Combining these filters improves photostability, broadens coverage and enhances cosmetic acceptability.
Formulations often include antioxidants and anti-inflammatory agents to reduce UV-induced oxidative stress and inflammation linked to photoageing and carcinogenesis. Although concerns exist that these ingredients may inflate SPF by suppressing erythema, in vivo studies show they do not alter measured SPF, confirming labelled SPF reflects true photoprotection.
SPF LIMITATIONS
Despite strong evidence of efficacy, real-world sunscreen effectiveness is heavily influenced by user behaviour. Adherence is often limited by factors such as texture, greasiness, irritation, white cast and cost, alongside safety misconceptions amplified on social media.
Evidence shows that routine sunscreen use has minimal impact on 25(OH)D levels, meaning concerns about vitamin D should not outweigh skin cancer prevention⁸. This highlights the importance of clear, evidence-based patient education.
CURRENT GUIDELINES
UK guidance recommends SPF 30 or higher for daily use, increasing to SPF 50 for prolonged or high-intensity exposure, with adequate UVA protection⁶. Achieving labelled SPF requires correct application: 2 mg/cm² (approximately 6–8 teaspoons for full body coverage), applied 15–20 minutes before exposure and reapplied every two hours or after swimming, sweating or drying⁸.
Emerging evidence shows that repeated low-dose UVA1 exposure can cause collagen degradation without visible sunburn, reinforcing that a tan does not provide meaningful protection¹.
SUNSCREENS IN SKIN OF COLOUR
Sunscreen guidance must consider skin of colour. Although higher melanin offers some natural photoprotection, individuals with darker skin tones remain at risk, with skin cancers often diagnosed later and linked to poorer outcomes¹⁰.
UV exposure also contributes to pigmentary disorders such as melasma and post-inflammatory hyperpigmentation, making sunscreen essential for all skin types. Tinted formulations provide additional protection against visible light¹¹.
In conclusion, SPF is a validated measure of UVB protection, but optimal photoprotection requires a broader approach. Effective sun protection depends on broad-spectrum coverage, correct quantity, timing and consistent use. When applied properly, sunscreen remains one of the most evidence-based interventions for reducing skin cancer, photoageing and pigmentary disorders.
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DR GINNI MANSBERG
Dr Ginni Mansberg is a GP, TV presenter, podcaster, author and columnist. She is a physician specialising in women’s health, menopause and all things skin. She is also the co-founder and medical director of science-based cosmeceutical skincare brand, ESK.