Only a handful of places left for our September 2026 cohort. Applications close soonApply now →

MSTA
Your future
starts here.

Sunscreen and SPF

Also known as: SPF, sunscreen, sun protection factor, sun cream

Sunscreen is a finished topical formulation that reduces ultraviolet exposure at the skin. SPF is the ratio of erythema-producing UV doses on protected and unprotected skin under a standard test; it does not state hours of safe exposure, or by itself quantify UVA protection.

Evidence status

Strong

Strong for standardised SPF and UVA testing and for current public-health guidance. Achieved protection remains formulation-, amount-, coverage- and exposure-dependent; visible-light evidence is formulation-specific, and randomised long-term cancer outcomes are sparse or imprecise.

What sunscreen and SPF are, and the measures in use#

A sunscreen is a finished topical formulation. SPF is one measurement made on it: the ratio between the ultraviolet dose needed to produce erythema on protected skin and the dose needed on unprotected skin, determined under a standardised test. ISO 24444 defines the in-vivo method and ISO 23675 an in-vitro one.[1, 3, 13, 14]

ISO 23698 is a hybrid rather than a third in-vitro route. UVA is measured non-invasively by diffuse reflectance on sunscreen-treated human skin in vivo, then merged with an in-vitro UVB spectrum. What the method avoids is inducing a biological erythema response, rather than human participation.

SPF is an erythemaendpoint, and erythema is predominantly a UVB effect. Ultraviolet A protection is therefore a separate measurement with its own parameters — UVA-PF, and critical wavelength as a measure of spectral breadth — and the SPF number gives neither. A high SPF with poor UVA protection is entirely possible, and historically was common.[1, 2, 3, 12, 14]

Both are precise, reproducible things to measure. Almost every difficulty with SPF in practice comes from translating the number into something it was never measuring — time, total safety, or UVA.

Use of sunscreen in aesthetic practice#

Sunscreen is a cosmetic product throughout the United Kingdom rather than a medicine, so recommending it, applying it and retailing it all sit within your remit; what is regulated is what may be claimed for it.

Claims and their substantiation

A cosmetic sunscreen sold anywhere in the UK needs a Responsible Person, and its claims must be substantiated and must not imply functions the product does not have. Great Britain and Northern Ireland are often described as holding different claims standards; the Article 20 substantiation duty is in fact the same in both. The divergence is in where the Responsible Person must be established and how products are notified, with Northern Ireland following EU cosmetics rules under the Windsor Framework.[5, 17]

One term is settled in the same documents: two of the sources here state that “sunblock” should not be used, because no sunscreen blocks all ultraviolet.

Oral supplements and sun-protection claims

An oral product cannot inherit or alter a topical sunscreen’s labelled SPF — that number belongs to the tested finished sunscreen. A food supplement is also outside the Article 20 cosmetics claims rules altogether. Its claims fall under separate nutrition and health-claims law, with the Great Britain regime in England, Scotland and Wales and EU-aligned rules in Northern Ireland, and they need applicable authorised wording and evidence.[1, 5, 13, 14]i

Contraindications and cautions#

None of the sources reviewed here identifies a contraindication to sunscreen, or a population in which sun protection is withheld. This is a cosmetic category, and the disqualifier check in it concerns the individual product and the client’s presentation rather than sun protection itself.

Where the presentation is a condition already under clinical management, the boundary is a matter of remit rather than of the product, and it is set out under referral below.

Clinical uses and the evidence behind them#

The evidence sits at different strengths for different outcomes, and the denominators travel with them.

Photoageing

This is the strongest randomised outcome. A community randomised trial found less progression of measured hand photoageing with a daily sunscreen regimen over four and a half years.[9] It is a measured difference at one anatomical site in one population, which makes photoageing the outcome that can be stated from randomised evidence rather than from mechanism alone.

Melanoma

Follow-up of the same trial found 11 new primary melanomas against 22, HR 0.50 (95% CI 0.24–1.02, p = 0.051). The direction is favourable; the interval crosses no effect and the event count is small.[10]

Basal and squamous cell carcinoma

A Cochrane review found only one eligible randomised trial, and low-certainty evidence that did not demonstrate a difference between daily and discretionary sunscreen use.[16]

Melasma and visible light

Where pigmentation is the indication, ultraviolet protection alone may not be sufficient, because visible light contributes in melanocompetent skin. The trial usually cited randomised 68 people already using 4% hydroquinone, of whom 61 completed, over eight weeks. It supports a specific ultraviolet-plus-visible-light formulation as an adjunct in melasma.[11]

It does not establish that every tinted sunscreen delivers the same benefit. Iron-oxide content, shade, film formation and how much is actually applied all differ between products, so visible-light protection is a property of a tested formulation rather than of the word “tinted”.

The photoageing outcome rests on randomised evidence. The long-term cancer outcomes do not: a favourable but imprecise melanoma signal, and a single low-certainty trial in keratinocyte cancers.

Selecting a sunscreen#

Current UK advice is a sunscreen of at least SPF 30 carrying at least four-star UVA protection or the UVA circle, used alongside shade, clothing, an adequate amount and reapplication.[4]

Both halves of that carry weight, because the SPF number and the UVA indicator are separate measurements, and the indicator is read alongside the SPF rather than instead of it.

The UVA circle

The circled UVA logo is a voluntary industry indication that UVA-PF is at least one third of the labelled SPF and that critical wavelength is at least 370 nm.[3, 15]

It is a proportional minimum: the qualifying UVA-PF scales with the labelled SPF, roughly 16.7 for an SPF 50 against 5 for an SPF 15. It discloses neither product’s actual measured UVA-PF, and an SPF 15 product can clear its own minimum by a wide margin. The logo therefore records that two products each passed a threshold, and that the thresholds differ; it does not rank their absolute UVA protection against one another.

The star rating

The Boots star system is proprietary rather than regulatory. It expresses UVA absorption relative to that same product’s UVB absorption.[18]

Because it is a ratio, a five-star SPF 15 does not necessarily give more absolute UVA protection than a four-star SPF 50. More stars on a low SPF can mean less UVA protection in absolute terms, which is why the rating is read in the context of the SPF it sits on.

That is a reading of how a ratio behaves rather than a position either source takes. Both the NHS and Boots pages state that a higher star rating is better, and neither draws the distinction.

Application quantity and coverage

The conventional in-vivo SPF benchmark and the European consumer recommendation both use an application density of 2 mg/cm². That is a genuinely thick film, substantially more than most people apply, and applying less or leaving gaps lowers achieved protection.[3, 6, 7, 8] The density belongs to the conventional route: ISO 23675 and ISO 23698 run their own procedures, so it describes the classic method rather than every method now in use.

On how much is lost, the European Commission offers a rule of thumb — if the quantity applied is halved, protection may fall by as much as two-thirds— and one controlled study across 120 participants found an approximately linear dose-response for the SPF 4 and SPF 15 products it tested.[3, 7]

The same study found the relationship diverged at higher SPFs, and differed across the four formulations, so no single curve converts an under-applied sunscreen to a new real-world number.[6, 7]i

The honest position sits between the two accounts usually offered. Under-application is not harmless, and no clean arithmetic rule exists: it clearly reduces protection, by an amount that depends on the product. The modifiable variable is application volume and coverage rather than the calculation, and it is the highest-yield thing to teach in a consultation.

Reapplication

Reapplication restores coverage lost to water, sweat, towelling, rubbing and missed application.[3, 4, 8]i A clock interval does not establish that every filter stops working at the same minute, which makes reapplication a matter of maintaining the film rather than of running down a countdown.

Adverse effects and their management#

Expected and transient effects

No source reviewed here characterises the tolerability of a sunscreen, so no expected local effect, and no rate for one, can be stated. The measurement standards, public-health guidance and outcome trials reviewed here establish how protection is determined and what a daily regimen achieved over years; none of them was designed to record how a product felt to wear.

What is applied is a finished formulation rather than a single ingredient, so a reaction identifies the product a client used rather than sun protection as a category.

Reducing, stopping and referring

No source reviewed here sets a threshold at which a sunscreen is reduced or stopped, so the instructions for the specific product in use govern. Where a product is not tolerated, what falls away is that product rather than photoprotection: shade and clothing sit alongside the SPF number in the same UK advice, and they remain available while another is found.[4]

Referral and scope boundaries#

Recommending, applying and retailing a cosmetic sunscreen sit inside the remit, so the boundary in this category is drawn around what the client presents with rather than around the product.

Pigmentation is the clearest instance. Where a pigmentary condition is being managed clinically — as it was in the population the visible-light evidence was drawn from — the photoprotection conversation runs alongside the treating clinician rather than in place of one.

Nothing reviewed here establishes a threshold at which sunscreen use itself prompts a referral. What sits squarely in the remit is the part of the result that depends on how the product is used: the quantity applied, the areas covered and reapplication account for a substantial part of the protection achieved, and they require no prescription.

Mechanism of action#

A sunscreen reduces the ultraviolet dose that reaches the skin. Erythema, the endpoint SPF is built on, is predominantly a UVB effect, which is why SPF describes protection weighted towards that part of the spectrum and carries no numerical information about UVA. UVA is characterised instead by UVA-PF and by critical wavelength, the measure of spectral breadth introduced in Diffey’s 1994 laboratory paper.[1, 2, 3, 12, 14]

Visible light sits outside both measurements and contributes to pigmentation in melanocompetent skin, which is why ultraviolet protection alone may not be sufficient where pigmentation is the concern.

Commonly misstated claims#

Four statements circulate widely in sun care. Each is set out below with what the primary literature was found to support.

“SPF 30 means thirty times longer in the sun.”

SPF is a laboratory dose ratio, and nothing in it describes time. The test uses a fixed film of product and a controlled ultraviolet source, while real exposure varies by latitude, season, time of day, cloud, reflection and altitude — and the film degrades through sweat, water, towelling and rubbing. A number derived from a constant cannot govern a variable.[1, 3, 4, 8]i

The error is mainstream. The Boots page cited here for the star system also tells readers that an SPF 15 sunscreen protects for fifteen times longer, up to 150 minutes, and recommends a minimum of SPF 15 against the NHS minimum of SPF 30.

Supported statement: SPF is a ratio of ultraviolet doses, and no multiple of safe time follows from it.

“SPF 50 blocks 98%, so SPF 30 is much the same.”

The European Commission states the 97% figure for SPF 30 as fact. The 98% figure paired with it is arithmetic — one divided by the SPF — rather than a measurement, and no source reviewed here states it.[3] The comparison also assumes label conditions, and almost everyone applies less than the tested film.

Supported statement: the Commission states that SPF 30 absorbs 97% of UVB; the 98% commonly paired with it is a calculation rather than a measured value.

“Broad spectrum means the UVA question is settled.”

The phrase alone does not disclose which UVA test was used or what was achieved on it. UVA-PF and critical wavelength are separate measurements, and an SPF number implies neither.[1, 2, 3, 12, 14]

Supported statement:UVA protection is a separate measurement, and “broad spectrum” on its own states neither which test produced it nor what it reached.

“Sunscreen is proven to prevent skin cancer.”

For melanoma the randomised follow-up points that way — 11 new primaries against 22 — on an interval that crosses no effect and a small number of events.[10] For basal and squamous cell carcinoma, Cochrane found one eligible trial and low-certainty evidence that did not demonstrate a difference.[16]

Supported statement: the photoageing evidence is randomised and positive, the melanoma signal is favourable and imprecise, and the keratinocyte-cancer evidence is a single trial of low certainty.

Areas of remaining uncertainty#

  • Whether regular sunscreen use reduces keratinocyte cancers. One trial, low certainty and no demonstrated difference, which leaves photoageing as the outcome that can be stated from randomised evidence.
  • The true real-world protection factor at typical application densities, which differs by formulation and leaves application volume as the variable worth working on.
  • Which tinted formulations deliver meaningful visible-light protection, and how much iron oxide is needed. Until that is settled, the evidence belongs to a tested product rather than to a category.
  • What the 2026 amendment to the in-vitro UVA standard changed. The public listing does not disclose it, so the current text cannot be described from the catalogue record.
  • The internal protocol detail of the ISO sun-protection standards. Every protocol figure stated above is sourced from the European Commission recommendation rather than from the standard itself.

Frequently asked questions#

How much sunscreen should a client apply?

Enough to reach the tested film. In practice that is taught by volume and coverage — roughly two fingers’ length for the face and neck is the usual translation — rather than by converting a percentage.[3, 6, 7, 8]

Is SPF 50 worth choosing over SPF 30?

The marginal difference in absorbed UVB is small on paper. The practical argument is margin: since almost everyone applies less than the tested film, a higher starting number leaves more protection when they do.[3, 7]

Does sunscreen need reapplying every two hours?

Reapplication restores coverage lost to water, sweat, towelling and rubbing. It is best understood as maintaining the film rather than as a filter expiry clock.[3, 4, 8]i

Is a tinted sunscreen enough for melasma?

The evidence is for one specific ultraviolet-plus-visible-light formulation used alongside hydroquinone, in 61 people who completed an eight-week trial. Not every tint qualifies.[11]

Can an oral supplement raise a sunscreen’s SPF?

No. It is a separate intervention and cannot inherit or raise a topical product’s tested SPF, and its own claims sit under nutrition and health-claims law rather than under the cosmetics rules.[1, 5, 13, 14]i

References#

Each source is graded by evidence tier. Tier 4 material (manufacturer documents, expert consensus, practitioner experience) is useful for protocol and context, and is never presented as equivalent to independent clinical evidence.

  1. International Organization for Standardization. ISO 24444:2019 Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF). Edition 2; Amendment 1:2022. Reviewed and confirmed 2026.Tier 1Supports: The current international in-vivo SPF method, based on protection against solar-simulated UV-induced erythema. NOTE: the standard itself is paywalled (CHF 181–204), so only the ISO catalogue record was verifiable — no internal protocol detail, including the 2 mg/cm² application density, is confirmable from this source. That figure is stated in source 3.
  2. International Organization for Standardization. ISO 24443:2021 Cosmetics — Determination of sunscreen UVA photoprotection in vitro. Edition 2; corrected February 2022; Amendment 1:2026.Tier 1Supports: The current in-vitro UVA method and the distinct parameters UVA-PF, critical wavelength and UVA absorbance proportionality. Paywalled; catalogue record only.
  3. European Commission. Commission Recommendation 2006/647/EC of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto. Official Journal of the European Union. 2006;L265:39–43.Tier 1Supports: SPF and UVA-PF definitions, the 2 mg/cm² test quantity, balanced UVA recommendations, usage warnings and reapplication language. States that SPF 30 absorbs 97% of UVB as fact, and offers the rule of thumb that if the quantity applied is halved, protection may fall by as much as two-thirds. Says the term 'sunblock' should not be used. This is a recommendation, not a UK Act.
  4. NHS. Sunscreen and sun safety. Page reviewed 24 March 2026; next review due 24 March 2029.Tier 1Supports: Current UK advice: at least SPF 30, at least four-star UVA protection or the UVA circle, adequate amount, reapplication, shade and clothing. NOTE: the NHS page states 'the higher the star rating, the better', so it does NOT support an argument that star ratings are purely relative.
  5. Office for Product Safety and Standards and Department for Business and Trade. Regulation 1223/2009 and the Cosmetic Products Enforcement Regulations 2013: Great Britain. Updated 29 June 2026.Tier 1Supports: The GB framework for England, Scotland and Wales, including the Responsible Person and the Article 20 requirement that cosmetic claims be supported and not imply functions a product does not have.
  6. Schalka S, dos Reis VMS, Cucé LC. The influence of the amount of sunscreen applied and its sun protection factor (SPF). Photodermatology, Photoimmunology & Photomedicine. 2009;25(4):175–180.Tier 3Supports: Controlled 40-volunteer experiment showing application-density-dependent SPF in two specific formulations.
  7. Liu W, Wang X, Lai W, et al. Sunburn protection as a function of sunscreen application thickness differs between high and low SPFs. Photodermatology, Photoimmunology & Photomedicine. 2012;28(3):120–126.Tier 3Supports: 120 participants across three Chinese regions, four formulations. Found NO single thickness-to-SPF curve across products — but did find an approximately LINEAR dose-response for the SPF 4 and SPF 15 products, with the divergence appearing at higher SPFs.Funding / interest: Corresponding author Mary S. Matsui was affiliated with The Estée Lauder Companies (independently confirmed via OpenAlex). The accessible record did not state the study's funding source.
  8. Petersen B, Wulf HC. Application of sunscreen — theory and reality. Photodermatology, Photoimmunology & Photomedicine. 2014;30(2–3):96–101.Tier 4Supports: Narrative — not systematic — review of under-application, missed areas, film uniformity and practical application teaching.
  9. Hughes MCB, Williams GM, Baker P, Green AC. Sunscreen and prevention of skin aging: a randomized trial. Annals of Internal Medicine. 2013;158(11):781–790.Tier 2Supports: Four-and-a-half-year photoageing outcome. Of 1,621 randomised adults, 903 were age-eligible, 886 supplied at least one valid skin-surface replica and 604 supplied paired baseline and end replicas.
  10. Green AC, Williams GM, Logan V, Strutton GM. Reduced melanoma after regular sunscreen use: randomized trial follow-up. Journal of Clinical Oncology. 2011;29(3):257–263.Tier 2Supports: Long-term Nambour follow-up: 11 versus 22 new primary melanomas, HR 0.50 (95% CI 0.24–1.02, p = 0.051). The confidence interval crosses 1 and event counts were small.
  11. Castanedo-Cazares JP, Hernandez-Blanco D, Carlos-Ortega B, Fuentes-Ahumada C, Torres-Álvarez B. Near-visible light and UV photoprotection in the treatment of melasma: a double-blind randomized trial. Photodermatology, Photoimmunology & Photomedicine. 2014;30(1):35–42.Tier 2Supports: Eight-week comparison of two specific SPF 50+ sunscreens in an intense-solar-exposure setting: 68 participants using hydroquinone 4% randomised, 61 completed. Not class-wide evidence for every tinted product.
  12. Diffey BL. A method for broad spectrum classification of sunscreens. International Journal of Cosmetic Science. 1994;16(2):47–52.In vitroTier 3Supports: The primary laboratory paper proposing critical wavelength as a measure of spectral breadth. Not a clinical efficacy study, and not the proprietary Boots star method.
  13. International Organization for Standardization. ISO 23675:2024 Cosmetics — Sun protection test methods — In vitro determination of sun protection factor (SPF). Edition 1.Tier 1Supports: A standardised in-vitro static-SPF method for eligible emulsion and alcoholic one-phase products. Powder, stick and water-resistance claims are outside its scope. Paywalled; catalogue record only.
  14. International Organization for Standardization. ISO 23698:2024 Cosmetics — Measurement of the sunscreen efficacy by diffuse reflectance spectroscopy. Edition 1.Tier 1Supports: A standardised diffuse-reflectance method for characterising SPF, UVA-PF and critical wavelength without a biological response. Not evaluated for powder-form sunscreens. Paywalled; catalogue record only.
  15. Cosmetics Europe (formerly COLIPA). Recommendation No. 23: Important Usage and Labelling Instructions for Sun Protection Products. Approved 24 February 2009.Tier 4Supports: Trade-association recommendation for usage wording and the voluntary circled-UVA logo, awarded when UVA-PF is at least one third of labelled SPF and critical wavelength is at least 370 nm. Also states that 'sunblock' should not be used. Not legislation.
  16. Sánchez G, Nova J, Rodriguez-Hernandez AE, et al. Sun protection for preventing basal cell and squamous cell skin cancers. Cochrane Database of Systematic Reviews. 2016;(7):CD011161.Tier 1Supports: One eligible 1,621-person trial and low-certainty evidence that did not demonstrate a difference in basal-cell (RR 1.03, 95% CI 0.74–1.43) or cutaneous squamous-cell carcinoma (RR 0.88, 95% CI 0.50–1.54) between daily and discretionary sunscreen groups.
  17. Office for Product Safety and Standards and Department for Business and Trade. Regulation 1223/2009 and the Cosmetic Products Enforcement Regulations 2013: Northern Ireland. Updated 29 June 2026.Tier 1Supports: The separate Northern Ireland framework, aligned with EU cosmetics rules under the Windsor Framework. NOTE: the claims-substantiation duty under Article 20 is the SAME as in GB — the divergence is in Responsible Person location and notification route, not in the standard claims must meet.
  18. Boots. UVA, UVB & SPF — what you need to know. Updated 31 March 2026.Tier 4Supports: Explanation of the proprietary zero-to-five-star system as a rating of UVA absorption RELATIVE to UVB absorption — not an absolute UVA-PF or a regulatory scale.Funding / interest: Published by Boots, which developed and commercially uses the star system it describes. Read with care: the same page states that 'applying an SPF 15 sunscreen will protect you for 15 times longer... up to 150 minutes' — precisely the SPF-as-a-timer error this entry exists to correct — and recommends a minimum of SPF 15, against the NHS minimum of SPF 30.

Continue learning with MSTA#

Skinipedia is written by the educators at MSTA, the medic-led skincare training academy in Liverpool.