The Science of Sunscreen — UVA, UVB, and Beyond

Last updated: 5 March 2026

Sunscreen is widely recognised as the single most important anti-ageing product in any skincare routine, yet misconceptions about how it works and which types offer the best protection remain surprisingly common. At Axiom Aesthetics, where we see the cumulative effects of UV damage daily, our practitioners are passionate about evidence-based sun protection education. Understanding the science empowers you to make informed choices that protect both your skin health and your aesthetic investment.

Understanding Ultraviolet Radiation

The sun emits a spectrum of electromagnetic radiation, of which ultraviolet (UV) radiation is the portion most relevant to skin health. UV radiation is classified by wavelength:

• UVA (320-400nm) — the “ageing” rays, comprising approximately 95% of UV radiation reaching the Earth’s surface. UVA penetrates deep into the dermis, damaging collagen and elastin, generating free radicals, and contributing to hyperpigmentation. UVA intensity remains relatively constant throughout daylight hours and can penetrate cloud cover and window glass.
• UVB (280-320nm) — the “burning” rays, responsible for sunburn, direct DNA damage, and the majority of skin cancer risk. UVB intensity varies with time of day, season, and latitude, peaking between 10am and 4pm.
• UVC (100-280nm) — the most energetic UV radiation, but almost entirely absorbed by the Earth’s ozone layer before reaching the surface.

A landmark study published in the New England Journal of Medicine (2012) dramatically illustrated the effects of chronic UVA exposure through the iconic photograph of a truck driver whose window-facing side showed dramatically accelerated ageing compared to his shielded side — despite never having experienced significant sunburn.

How Sunscreen Works

Sunscreen ingredients protect the skin through two primary mechanisms:

Mineral (Physical) Filters

Zinc oxide and titanium dioxide are inorganic particles that sit on the skin surface and primarily reflect and scatter UV radiation. Modern research in Photochemical and Photobiological Sciences (2021) has clarified that mineral filters also absorb UV energy and convert it to heat — they do not solely work by reflection, as was previously believed.

Advantages of mineral filters include immediate protection upon application, broad-spectrum coverage (particularly zinc oxide, which covers both UVA and UVB), and excellent tolerability for sensitive skin. The traditional drawback of a white cast has been largely overcome by micronised and nano-particle formulations.

Chemical (Organic) Filters

Chemical UV filters are organic molecules that absorb UV radiation and convert it to heat energy. Modern sunscreens typically combine multiple chemical filters to achieve broad-spectrum coverage. Common ingredients include:

• Avobenzone — provides excellent UVA1 protection but requires stabilisation
• Octocrylene — UVB filter that also stabilises avobenzone
• Tinosorb S and M — broad-spectrum photostable filters widely used in European formulations
• Mexoryl SX and XL — patented UVA filters developed by L’Oreal
• Uvinul A Plus — highly photostable UVA filter

Understanding SPF

Sun Protection Factor (SPF) measures protection against UVB radiation only. It indicates how much longer it takes for UVB rays to produce erythema (redness) on protected skin compared to unprotected skin. Common misconceptions include:

• SPF 30 blocks 97% of UVB, SPF 50 blocks 98% — the incremental benefit above SPF 30 is marginal
• SPF is not linear — SPF 30 does not provide double the protection of SPF 15
• SPF does not measure UVA protection — a high-SPF product may still offer inadequate UVA coverage
• SPF testing uses 2mg/cm² — most people apply only 0.5-1mg/cm², dramatically reducing actual protection

Beyond SPF: UVA Protection Ratings

In the UK and EU, UVA protection is indicated by the UVA star rating system (developed by Boots) and the UVA circle logo. The star rating ranges from 0 to 5 and measures the ratio of UVA to UVB protection. We recommend a minimum of 4-star UVA rating alongside SPF 30.

The PA system, used in Asian sunscreens, grades UVA protection from PA+ to PA++++, with PA++++ indicating the highest level of UVA protection. This system has gained popularity among skincare enthusiasts who prioritise comprehensive photoprotection.

Emerging Threats: Blue Light and Infrared

Research is increasingly focusing on visible light (particularly high-energy visible or “blue” light at 400-500nm) and infrared radiation as contributors to skin damage. A 2020 study in the Journal of Investigative Dermatology demonstrated that visible light can induce hyperpigmentation, particularly in darker skin types (Fitzpatrick IV-VI), and contribute to photoageing through oxidative stress pathways.

Iron oxide-tinted sunscreens have been shown to provide significant protection against visible light — an additional benefit beyond their cosmetic advantage of eliminating white cast.

Application Guidelines

Evidence-based sunscreen application recommendations from our practitioners:

1. Apply generously — use approximately half a teaspoon for the face and neck (the “two-finger rule”)
2. Apply 15-20 minutes before sun exposure for chemical sunscreens; mineral sunscreens work immediately
3. Reapply every two hours during continuous sun exposure, and immediately after swimming or sweating
4. Use daily — UV radiation is present year-round in the UK, even on cloudy days (up to 80% of UV penetrates cloud cover)
5. Cover often-forgotten areas — ears, back of neck, hands, and the part line of the scalp
6. Choose broad-spectrum — ensure both UVA and UVB protection

Sunscreen and Aesthetic Treatments

Rigorous sun protection is non-negotiable for patients undergoing aesthetic treatments. Many procedures — including chemical peels, laser treatments, microneedling, and retinoid therapy — increase the skin’s photosensitivity. Inadequate sun protection after these treatments can lead to post-inflammatory hyperpigmentation, suboptimal healing, and diminished results.

Frequently Asked Questions

Do I need sunscreen if I work indoors?
Yes. UVA radiation penetrates window glass, and cumulative indoor UV exposure has been shown to contribute to photoageing. Additionally, short periods of incidental outdoor exposure (commuting, walking to lunch) accumulate significantly over time. We recommend daily SPF 30+ application regardless of your planned activities.

Can sunscreen prevent skin cancer?
Strong evidence supports this. A landmark Australian study published in the Journal of Clinical Oncology demonstrated that regular sunscreen use reduced melanoma incidence by 50% and squamous cell carcinoma by 40% over a ten-year period.

Is mineral or chemical sunscreen better?
Neither is inherently superior — both provide effective protection when properly formulated and applied. Mineral sunscreens are preferred for sensitive skin, post-procedure use, and pregnancy. Chemical sunscreens often offer more elegant textures and are preferred for daily cosmetic use. The best sunscreen is one you will use consistently and apply generously.

Does sunscreen block vitamin D production?
Whilst sunscreen theoretically reduces vitamin D synthesis, real-world studies show that sunscreen users do not have significantly lower vitamin D levels, likely because application is rarely perfect and incidental exposure is sufficient. The NHS recommends vitamin D supplementation (10mcg daily) for all UK adults during autumn and winter regardless of sun protection habits.