Sunscreen labelling is genuinely confusing, and the confusion causes people to under-protect. Here is what the numbers mean in plain terms.
SPF measures one thing only
SPF is a UVB rating. UVB is the burning, tanning wavelength — it is what causes visible sunburn and most skin cancers. SPF 30 filters roughly 97% of it, SPF 50 about 98%.
That one percentage point sounds trivial, and in a laboratory it nearly is. In real life it is not, because nobody applies the laboratory amount. Most people apply between a quarter and half of what SPF testing assumes. Starting from 50+ means your real-world protection lands somewhere useful even when your application is imperfect.
PA is the rating that matters for ageing
PA measures UVA protection. UVA penetrates deeper, does not burn you, and is responsible for most photo-ageing — the pigmentation, the loss of firmness, the uneven tone. It also passes straight through cloud cover and window glass.
PA++++ is the highest rating on the scale. In a country with high year-round UV index, it should be the default, not the premium option.
How much to actually apply
The tested amount is 2mg per square centimetre. Translated to a face and neck, that is roughly two finger-lengths — squeeze a line along your index finger and another along your middle finger. It will look like too much. It is the correct amount.
Reapplication
Every three hours if you are outdoors, sweating, or near water. If you are indoors away from windows all day, your morning application will hold.
The white cast problem
The reason so many people skip sunscreen on Indian skin tones is the grey-white cast that older mineral filters leave. This is a formulation problem, not an unavoidable one. Modern filters in a gel base absorb clear. If your sunscreen makes you look ashy, the answer is a different sunscreen, not less sunscreen.
The cloudy-day excuse
UVA passes through cloud largely unaffected. A grey monsoon morning is not a day off.