SPF and sunscreen: the most-evidence-backed skincare step

Sunscreen prevents UV radiation from damaging skin cells. UVA wavelengths (320-400nm) penetrate deeper into the dermis and are primarily responsible for photoageing (fine lines, pigmentation, loss of elasticity). UVB wavelengths (280-320nm) cause surface sunburn and are the primary driver of skin cancer risk. Broad-spectrum sunscreens block both ranges.

The SPF number measures UVB protection only. SPF 30 blocks ~97% of UVB; SPF 50 blocks ~98%; SPF 100 blocks ~99%. The diminishing returns above SPF 30 are real, but the higher SPF still adds meaningful protection for prolonged outdoor exposure. The PA system (PA+, PA++, PA+++, PA++++) measures UVA protection and is the more useful number for photoageing prevention.

Mineral (physical) sunscreens contain zinc oxide and/or titanium dioxide. They sit on top of the skin and reflect UV radiation. Advantages: instant protection on application, low irritation risk, well-tolerated by sensitive and rosacea-prone skin, reef-safe. Disadvantages: can leave a white cast on darker skin tones, can feel thicker, less aesthetically refined.

Chemical (organic) sunscreens contain UV-absorbing compounds like avobenzone, octisalate, octocrylene, homosalate, and (in non-US markets) Tinosorb S/M and Uvinul A/T. They absorb UV radiation and dissipate it as heat. Advantages: cosmetically elegant, easily layered under makeup, no white cast. Disadvantages: some absorption into the bloodstream (the long-term significance of which is still being studied), some ingredients have reef-impact concerns, irritation more common.

Asian (Korean, Japanese) sunscreens generally outperform American ones because the Asian filters (Tinosorb, Mexoryl) provide better UVA protection and feel more elegant. American sunscreen filter approval has lagged decades behind Asian and European markets due to FDA regulatory differences.

The standard guidance is about a quarter-teaspoon (~1.25ml) for the face alone, or two finger-lengths of cream / gel. This is significantly more than most people apply. Under-applying is the most-common reason that sunscreen 'does not work'; you get the SPF on the label only if you apply at the tested concentration of 2mg per square centimetre of skin.

Reapply every two hours of outdoor exposure, or after sweating or swimming. For indoor-mostly days, one morning application is generally sufficient; for outdoor activity, reapplication is non-negotiable. Powder mineral sunscreens (like Colorescience or Brush On Block) provide a way to reapply over makeup without removing it.

The misconception that darker skin does not need sunscreen because of natural melanin is partially true but dangerously oversimplified. Higher melanin levels do provide some natural UV protection (roughly SPF 3-13 depending on the depth of pigmentation), but this is nowhere near enough to prevent photoageing or skin cancer entirely.

Darker skin tones are also significantly more prone to post-inflammatory hyperpigmentation, where any sun exposure to an already-irritated area (acne spot, scratched skin, sunburn) produces persistent dark patches that take months to fade. SPF is the primary prevention strategy here, more important on darker skin tones than fairer ones in some ways.

For darker skin tones, look specifically for chemical or hybrid sunscreens that do not leave a white cast (Black Girl Sunscreen, Supergoop, Beauty of Joseon's Korean filters, Bioderma Photoderm). Mineral sunscreens have improved significantly in 2020+ with new formulations like Eucerin and ISDIN Eryfotona that minimise the white cast.