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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 SPF actually is#

SPF is the ratio between the UV dose needed to produce erythema on protected skin and the dose needed on unprotected skin, measured 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 not an in-vitro method, and this page called it one. It is a hybrid: UVA is measured non-invasively by diffuse reflectance on sunscreen-treated human skin in vivo, then merged with an in-vitro UVB spectrum. What it avoids is inducing a biological erythema response — not human participation.

That is a precise, useful, reproducible thing to measure. Almost every problem with SPF in practice comes from translating it into something it was never measuring — time, total safety, or UVA.

One disclosure on our own sourcing: the four ISO standards are paywalled at CHF 181–204 each, so only the catalogue records were verifiable here. Where this entry states a protocol detail, it is sourced from the European Commission recommendation rather than from the standard itself.

SPF is not a timer#

The most persistent error in sun care is “SPF 30 means you can stay out thirty times longer”. SPF is not a guaranteed multiplier of safe time in natural sunlight.[1, 3, 4, 8]i

Why it fails: the test uses a fixed film of product and a controlled UV source. 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.

It is worth knowing how mainstream this error is. The Boots page this entry cites for the star system also tells readers that “applying an SPF 15 sunscreen will protect you for 15 times longer... up to 150 minutes” and recommends a minimum of SPF 15, against the NHS minimum of SPF 30. We cite that page for what the star rating is, and we disclose the rest of it rather than quietly leaning on it.

The amount problem#

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. Applying less, or leaving gaps, lowers achieved protection.[3, 6, 7, 8]

One precision note: that density belongs to the conventionalroute. ISO 23675 and ISO 23698 run their own procedures, so “SPF is tested at 2 mg/cm²” is true of the classic method rather than of every method now in use.

The tempting next step is a conversion rule: apply half, get a quarter, and so on. Be careful in both directions here.

What the evidence does say

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]

What it does not say

That same study found the relationship diverged at higher SPFs, and differed across the four formulations. So there is no single curve that converts any under-applied sunscreen to a new real-world number.[6, 7]i

The honest position sits between the two claims you will usually hear. It is not true that under-application is harmless, and it is not true that a clean mathematical rule exists. Under-application clearly reduces protection, by an amount that depends on the product.

Which makes the practical advice straightforward: teach application volume and coverage, not arithmetic.

UVA is a separate question#

SPF is an erythemaendpoint, and erythema is predominantly a UVB effect. So SPF alone tells you nothing about a product’s numerical UVA protection or its spectral breadth.[1, 2, 3, 12, 14]

UVA is measured separately — as UVA-PF, and as critical wavelength, the concept introduced in Diffey’s 1994 laboratory paper. A high SPF with poor UVA protection is entirely possible, and historically was common.

Stars, circles and what they mean#

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 critical wavelength is at least 370 nm.[3, 15]

Note what that means, and what it does not. 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.

But the circle does not disclose eitherproduct’s actual measured UVA-PF, and an SPF 15 product can clear its minimum by a wide margin. So the logo alone cannot tell you that a circled SPF 50 has more absolute UVA protection than a circled SPF 15 — which is what this page previously claimed. It tells you both cleared a threshold, and the thresholds differ.

The star rating

The Boots star system is proprietary, not regulatory. It expresses UVA absorption relative to that same product’s UVB absorption.[18]

The consequence is counter-intuitive and worth being precise about: 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.

We are stating that as our own reading of how a ratio works. Both the NHS and Boots pages say “the higher the star rating, the better”, and neither draws the distinction — so we are not going to attribute it to them.

The practical version for a consultation: look for SPF 30 or above and a UVA indicator, and read the star rating alongside the SPF rather than instead of it.[4]

What sunscreen is actually shown to do#

This is where a reference work has to be more careful than a campaign.

  • Photoageing. A community randomised trial found less progression of measured hand photoageing with a daily sunscreen regimen over four and a half years.[9]
  • Melanoma.Follow-up of that trial found 11 new primary melanomas versus 22, HR 0.50 (95% CI 0.24–1.02, p = 0.051). Suggestive — but the interval crosses no effect and the event count is small.[10]
  • Basal and squamous cell carcinoma. Cochrane found only one eligible randomised trial and low-certainty evidence that did not demonstrate a difference between daily and discretionary sunscreen use.[16]

None of that is an argument against sunscreen. The mechanistic case is strong, the photoageing result is real, and the melanoma signal points the right way. It is an argument for accuracy: the randomised long-term cancer evidence is thin, and a practitioner who states it as settled will be caught out by anyone who checks.

Visible light and pigmentation#

For pigmentation, UV protection alone may not be enough — 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 UV-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.

The UK regulatory picture#

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.[5, 17]

Great Britain and Northern Ireland are often described as having different claims standards. They do not — the Article 20 substantiation duty is 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.

On oral supplements, the substantive point stands and the legal framing was wrong. An oral product cannot inherit or alter a topical sunscreen’s labelled SPF — that number belongs to the tested finished sunscreen. But a food supplement is not governed by Article 20 cosmetics rules at all. 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

In professional practice#

  • Teach quantity and coverage, not maths. The single highest-yield intervention is getting enough product on, everywhere.
  • Never say “this gives you X hours”. Reapplication restores coverage lost to water, sweat and rubbing — it is not a countdown.[3, 4, 8]i
  • Check the UVA indicator separately from the SPF number, and read stars in the context of the SPF they sit on.
  • For pigmentation clients, visible-light protection is a specific formulation question, not a box the word “tinted” ticks.
  • Avoid “sunblock”. Two of the sources here say the term should not be used, because nothing blocks all UV.

What remains uncertain#

  • Whether regular sunscreen use reduces keratinocyte cancers — one trial, low certainty, no demonstrated difference.
  • The true real-world protection factor at typical application densities, which differs by formulation.
  • Which tinted formulations deliver meaningful visible-light protection, and how much iron oxide is needed.
  • What the 2026 amendment to the in-vitro UVA standard changed — the public listing does not disclose it.

Common misconceptions#

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

SPF is a laboratory dose ratio, not a measure of time.[1, 3, 4, 8]i

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

The 97% figure for SPF 30 is stated by the European Commission. The 98% figure is arithmetic, and no source reviewed here states it. More usefully: the comparison ignores that people under-apply, and a higher starting number leaves more margin when they do.[3]

“Five stars is better than four, whatever the SPF.”

Stars are a ratio to the product’s own UVB absorption. A five-star SPF 15 can offer less absolute UVA protection than a four-star SPF 50.[18]

“Broad spectrum means it’s covered.”

The phrase alone does not tell you which UVA test was used or what was achieved.[1, 2, 3, 12, 14]

“Sunscreen is proven to prevent skin cancer.”

For melanoma the signal points that way but the interval crosses no effect. For basal and squamous cell carcinoma, Cochrane found no demonstrated difference on low-certainty evidence.[16]

Frequently asked questions#

How much should a client actually apply?

Enough to reach the tested film. Teach it by volume and coverage — roughly two fingers’ length for the face and neck is the usual practical translation — rather than by trying to convert a percentage.[3, 6, 7, 8]

Is SPF 50 worth it over SPF 30?

The marginal difference in absorbed UVB is small on paper. The practical argument is margin: since almost everyone under-applies, 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. Treat it 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 UV-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 boost SPF?

No. It is a separate intervention and cannot inherit or raise a topical product’s tested SPF — and claiming otherwise would not meet the UK substantiation duty.[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.

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