Reading the label
SPF is measured under ISO 24444 at 2 mg of product per square centimetre of skin. In the EU, the UVA logo in a circle means UVA protection is at least one third of the SPF, following Commission Recommendation 2006/647/EC. In the UK, the Boots star rating shows UVA as a ratio, up to five stars. In the US, "Broad Spectrum" requires a critical wavelength of 370 nm. In Australia, sunscreens are regulated as therapeutic goods by the TGA.
The white cast, honestly
Bigger particles mean more cast. Nano-sized particles reduce it. The EU's Scientific Committee on Consumer Safety considers nano zinc oxide and nano titanium dioxide safe on healthy, intact skin in non-spray products, and advises against sprays and powders that could be inhaled. If cast is your issue, choose a tinted formula in your shade family, not a heavier one.
How much, how often
The label SPF assumes that 2 mg/cm² test dose. For the face alone, that is roughly a quarter teaspoon, and most people apply far less. Reapply every two hours outdoors and after swimming, sweating or towelling. In the US, "water resistant" means tested for 40 or 80 minutes. Mineral films also rub off on hats, collars and fingers.
"Reef safe" is mostly marketing
Hawaii banned the sale of sunscreens containing oxybenzone and octinoxate from 1 January 2021. Key West passed a similar ban, later blocked by Florida state law. Mineral sunscreens avoid those two filters, but "reef safe" has no legal definition, and research on nanoparticles in seawater is ongoing. The most reef-friendly sunscreen is a rash guard.
