Sunscreen guide: SPF, PA ratings, chemical vs. mineral, and how to choose

Sunscreen is the single most evidence-supported intervention in preventive dermatology — the only topical intervention with RCT-level evidence preventing not just cancer but photoaging. It is also one of the most misunderstood categories in skincare, with significant confusion around SPF numbers, UVA protection, filter types, and application. Here is the complete guide.

How SPF works and what the number means

The SPF calculation

SPF (Sun Protection Factor) is not a percentage — it is a ratio. The SPF rating is determined by:

SPF = MED with sunscreen ÷ MED without sunscreen

Where MED = Minimum Erythemal Dose (the minimum UV dose to produce visible redness on unprotected skin).

In practice:
SPF 15 blocks ~93% of UVB rays (transmits 1/15 = 6.7%)
SPF 30 blocks ~97% of UVB rays (transmits 1/30 = 3.3%)
SPF 50 blocks ~98% of UVB rays (transmits 1/50 = 2%)
SPF 100 blocks ~99% of UVB rays (transmits 1/100 = 1%)

The critical observation: Above SPF 30, the marginal protection increase per SPF unit diminishes sharply. SPF 30 blocks 97%; SPF 50 blocks 98% — a 1% absolute difference. This is why dermatologists recommend SPF 30 as the minimum for daily use rather than demanding SPF 100 for everyone.

However: These figures assume correct application quantity. Most people apply 25–50% of the tested amount (2 mg/cm² is the test standard). At half the quantity, SPF 50 performs like SPF 15. The argument for higher SPF is the real-world underapplication buffer.

SPF only measures UVB — not UVA

SPF is a UVB-only metric. It measures protection against the wavelengths (290–320 nm) that cause sunburn and contribute to squamous cell carcinoma. It says nothing about UVA protection.

UVA (320–400 nm) causes:
Tanning (immediate and delayed pigmentation)
Photoaging (collagen/elastin degradation, the primary driver of wrinkles)
UVA-driven DNA damage and melanoma risk
Penetrates to the dermis (deeper than UVB)

A sunscreen with high SPF but poor UVA coverage protects against burning but not against photoaging or melanoma.

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UVA protection: PA ratings and broad-spectrum

The PA system (Japan/Korea/Asia)

The PA rating (Protection Grade of UVA) is based on the PPD test (Persistent Pigment Darkening) — a direct measure of UVA protection:

| PA Rating | PPD Value | UVA Multiplier |
|---|---|---|
| PA+ | PPD 2–3 | At least 2–3× |
| PA++ | PPD 4–7 | At least 4× |
| PA+++ | PPD 8–15 | At least 8× |
| PA++++ | PPD ≥ 16 | At least 16× |

Asian sunscreens typically carry PA ratings. For daily photoaging prevention, PA+++ or PA++++ is the goal.

US broad-spectrum test

The FDA broad-spectrum designation requires a sunscreen to pass the CW (Critical Wavelength) test — UVA protection extending to at least 370 nm. This is a pass/fail test that indicates some UVA coverage but does not quantify how much. A sunscreen can pass broad-spectrum with relatively modest UVA protection.

Practical guidance: "Broad-spectrum" in the US is a minimum bar, not an optimal target. For meaningful UVA protection, look for sunscreens that:
Contain zinc oxide (broad UVA1+UVA2 coverage)
Contain avobenzone at ≥ 3% (stabilized) for UVA1 coverage
Contain Tinosorb S or M (European filters — outstanding UVA1 coverage; not FDA-approved but legal in imported products)

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Chemical vs. mineral sunscreens

Mineral filters (physical blockers)

Zinc oxide (ZnO) and titanium dioxide (TiO₂) are the only FDA-approved mineral UV filters.

Zinc oxide:
Covers UVB + UVA2 + UVA1 (full broad-spectrum coverage in a single filter)
The preferred mineral filter for complete UVA coverage
Works by absorbing and scattering UV radiation (the "physical reflector" description is outdated — at nanoparticle size, zinc oxide primarily absorbs)
Can appear white/chalky on skin, especially at higher concentrations (> 15%) and in non-nano formulations
Modern zinc oxide formulations using micronized/nano-sized particles are largely transparent on skin

Titanium dioxide:
Covers UVB + UVA2 but has limited UVA1 coverage — not a complete UVA filter
Less white than zinc oxide at equivalent concentrations
Often combined with zinc oxide for complete spectrum coverage

Mineral advantages: Stable on shelf and on skin (no photodegradation). Good for sensitive, rosacea-prone skin. Safe in pregnancy.

Chemical (organic) filters

Chemical filters absorb UV energy and release it as heat. Each filter targets specific wavelengths:

| Filter | Wavelength Coverage | Notes |
|---|---|---|
| Avobenzone | UVA1 (340–400 nm) | Must be stabilized (by octocrylene or Helioplex) — photounstable alone |
| Octinoxate | UVB | Most common UVB filter; photostable; some endocrine concern debate |
| Oxybenzone | UVB + UVA2 | Most controversial re: endocrine disruption; highest absorption |
| Tinosorb S (bis-ethylhexyloxyphenol methoxyphenyl triazine) | UVB+UVA2+UVA1 | Excellent broad-spectrum; EU/Asia; not FDA-approved |
| Mexoryl SX/XL | UVA2+UVA1 | L'Oréal proprietary; FDA-approved (SX); excellent stability |

Chemical advantages: Elegant, lightweight formulations; cosmetically appealing; often no white cast.

Disadvantages: Some filters degrade on UV exposure (avobenzone without stabilizer); some require 20 minutes before sun exposure to bind to skin; some evidence of skin absorption with uncertain significance.

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The evidence for sunscreen preventing photoaging

Green et al. — Nambour cohort (2013)

*Hughes MC, Williams GM, Baker P, Green AC. (2013). Sunscreen and prevention of skin aging: a randomized trial. Annals of Internal Medicine, 158(11), 781–790.*

This is the definitive RCT demonstrating sunscreen prevents photoaging in humans:
903 adults in subtropical Australia randomized to daily SPF 15+ sunscreen vs. discretionary sunscreen use
4.5 years of daily application
Outcome: microtopography analysis of skin aging

Results: The daily sunscreen group showed 24% less photoaging than the discretionary-use group — measured as skin aging (texture, elasticity, pigmentation). This is the only large RCT with a non-cancer aging endpoint confirming what was mechanistically expected.

Combined with extensive evidence linking UV exposure to squamous cell carcinoma and melanoma, daily sunscreen has the strongest preventive evidence of any skincare product.

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Daily minimums and application

Minimum SPF by exposure level

| Daily Sun Exposure | Minimum SPF | Notes |
|---|---|---|
| Indoor, minimal window exposure | SPF 15–30 | UVA penetrates glass — SPF still relevant |
| Walking to/from car, lunch outside | SPF 30 | Minimum recommended by AAD |
| Extended outdoor time | SPF 50 | Higher buffer for underapplication |
| Outdoor sports, beach, mountain | SPF 50+ | Reapply every 2 hours |

The correct application amount

¼ teaspoon (1.25 mL) for the face alone is the standard recommendation for achieving the labeled SPF. Most users apply significantly less. A "pea-sized" amount falls short of the tested dose.

Application routine: After moisturizer, before makeup. Allow 60–120 seconds to settle. Mineral sunscreens can be applied immediately before going outside. Chemical sunscreens require 15–20 minutes before UV exposure to bind to skin.

Reapplication: Every 2 hours during continuous outdoor exposure. "Water-resistant 80 minutes" means the SPF holds for 80 minutes in water — after that, reapply. No sunscreen is waterproof.

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Common formulation types

Fluid/lotion: Most consumer sunscreens. Appropriate for daily use under makeup.

Serum SPF: Lightweight, often contains additional actives (antioxidants, niacinamide). Sits under moisturizer or can replace it. Popular for AM skincare routines.

Tinted mineral: Zinc oxide + iron oxides — tints provide additional coverage of visible light (HEV/blue light) and better coverage of melasma-type pigmentation vs. untinted sunscreens.

Sunscreen sticks: High concentration, occlusive, convenient for reapplication over makeup. Useful for targeted reapplication (ears, nose, hands) but less uniform than lotion for full-face initial application.

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