Sunscreen is the single most evidence-backed step in any routine, and also the step people most often get wrong — usually by applying a quarter of the amount the SPF rating assumes.
What SPF actually measures
SPF measures protection against UVB, the wavelength responsible for burning. It says nothing about UVA, which penetrates more deeply. For UVA you want a broad spectrum label, or a PA rating — more plus signs mean more protection.
The amount matters more than the number
SPF is tested at two milligrams per square centimetre, which works out at roughly a quarter teaspoon for the face and neck. Apply half that and you get considerably less than half the protection. A generously applied SPF 30 beats a thin SPF 50.
Mineral or chemical
Mineral filters — zinc oxide, titanium dioxide — sit on the surface and work immediately. Organic filters absorb UV and convert it to heat, and tend to feel lighter with less white cast. Neither is inherently safer or more effective; the better one is whichever you will reapply.
Reapplication
Every two hours of direct sun exposure, and after swimming or heavy sweating. Indoors away from windows, a single morning application is generally fine — the risk is in the hours actually spent outside.
This is general information rather than medical advice. If you have a specific skin condition or a history of skin cancer, your dermatologist’s guidance takes precedence.