What Is SPF?
SPF — Sun Protection Factor — is a laboratory measure of how much UV radiation (specifically UVB) is required to cause sunburn on protected skin compared to unprotected skin. An SPF 50 means it takes 50× more UVB energy to cause sunburn when the sunscreen is correctly applied, compared to no protection. Critically, SPF measures only UVB protection (the sunburn-causing wavelengths, 280–320nm) — it does not measure UVA protection, which requires separate testing indicated by "broad-spectrum" labelling.
The SPF Numbers
SPF 15 blocks approximately 93% of UVB. SPF 30 blocks approximately 97%. SPF 50 blocks approximately 98%. SPF 100 blocks approximately 99%. The incremental difference between SPF 50 and SPF 100 is small (1%), but the difference between SPF 15 and SPF 50 is clinically meaningful — particularly for photosensitive individuals, those on photosensitising medications, and anyone with hyperpigmentation or history of skin cancer. SPF 50 is the internationally recommended standard for daily use.
Application: Where Most Protection Is Lost
The SPF rating on a product is achieved at a specific application density: 2mg per cm² of skin — approximately 1/4 to 1/3 teaspoon for the face alone. Studies consistently show that average real-world sunscreen application is 25–50% of this amount, reducing effective protection dramatically (SPF 50 applied at half the tested density provides roughly SPF 7 protection). Reapplication every 2 hours in sun exposure is required — UV filters degrade.
The daily SPF case: UVA — which causes photoageing and contributes to skin cancer risk — is present year-round, penetrates cloud cover and glass, and is not blocked by most indoor environments. Daily broad-spectrum SPF application is justified regardless of outdoor plans, season, or weather.
See Sun Filters Guide and UVA & UVB.