Most people in Thailand are not buying sun care because they are afraid of burning. They are buying it because of ฝ้า — the patches that creep across the cheeks and forehead and do not wash off. And for that, the number almost nobody reads on the bottle matters more than the big one on the front.
SPF is the wrong number for this problem
SPF measures one thing: how well a product filters UVB, the shorter wavelength that reddens and burns you. That is genuinely useful, and it is most of what the label shouts about. But burning and pigmenting are not the same injury, and they are not driven by the same light.
UVA is the longer wavelength. It makes up the large majority of the UV reaching the ground, it is far more constant through the day and the year than UVB, and it penetrates deeper into the skin. It is the wavelength most associated with photoageing and with pigmentation. A bottle can carry a very high SPF and still be mediocre at UVA — the two are measured separately, because they are separate things.
What PA actually measures
PA stands for Protection Grade of UVA. It was introduced by the Japan Cosmetic Industry Association in 1996 and later extended to four grades. Unlike SPF, which is based on how long skin takes to redden, PA is based on persistent pigment darkening — how long UVA takes to produce a lasting tan on protected versus unprotected skin. The measured ratio is the UVA protection factor, and the plus signs are bands of it.
| Rating | UVA protection factor | What it means in practice |
|---|---|---|
| PA+ | 2 to 4 | Some protection. Incidental exposure only. |
| PA++ | above 4 to 8 | Moderate. A normal day with some time outdoors. |
| PA+++ | above 8 to 16 | High. The band to look for if pigmentation is your concern. |
| PA++++ | above 16 | Very high. Added in the 2013 revision of the scale. |
Note what the test is measuring: darkening. The scale was built around exactly the outcome this article is about. In Europe and the US the equivalent information is carried by the words broad spectrum, or by the UVA-in-a-circle mark, rather than by plus signs — different notation, same question.
Why melasma behaves differently from a burn
Melasma is a pigmentation disorder, not an injury you can point to. It shows up as symmetrical brown or grey-brown patches, most often across the cheeks, forehead, upper lip and jaw. It is far more common in women, and considerably more common during pregnancy — which is where the old name, the mask of pregnancy, comes from. It is also more common in people with more pigment in their skin to begin with, which is most of this country.
Three things make it frustrating in a way sunburn is not. It is chronic and relapsing rather than an event. It is provoked by heat as well as light, which matters in a climate like this one. And it is driven by more of the spectrum than UV alone.
The part no SPF and no PA number covers
Here is the uncomfortable bit, and it is why this article is longer than the SPF one. Visible light — the light you can actually see, especially the high-energy blue-violet end next to UVA — can itself induce pigmentation, and it does so most readily in darker skin types. Neither SPF nor PA says anything about it. Both scales stop at the edge of the ultraviolet.
What has been shown to block visible light in a cosmetic is pigment: iron oxides, the colourants that make a tinted sunscreen tinted. A randomised trial published in the Journal of the American Academy of Dermatology followed melasma patients through the summer and found that a broad-spectrum sunscreen containing iron oxides significantly reduced relapse compared with a UVA/UVB sunscreen alone — while tinted make-up without iron oxides made no difference. Subsequent reviews have reached the same conclusion.
What actually works, in order
Nothing here is a treatment — melasma that is already established is a doctor’s problem, and there are prescription options that work. This is the prevention side, which is the part that is yours.
- Read the PA rating before the SPF number. PA+++ or higher if pigmentation is the concern. A high SPF with no UVA mark is half a product.
- Use enough, and reapply. All the protection numbers assume a layer nobody applies. Half a teaspoon for the face and neck, again every two hours outdoors. The maths is here.
- Add physical shade. A broad brim does more for your face than any bottle, works on visible light and UV alike, and never needs reapplying. It is the WHO’s first line, ahead of sunscreen, for good reason.
- Consider a tinted product if melasma is your actual diagnosis rather than a general worry. That is where the iron-oxide evidence sits.
- Mind the heat, not just the light. Melasma is provoked by heat too, which is why it flares in April here and why a hot kitchen or a long ride can set it off on an overcast day.
- Be patient and be consistent. Relapse is the norm, not a sign you did it wrong. The studies measure success in reduced recurrence, not cure.
A Bangkok footnote
UVB swings hard with season and time of day. UVA does not swing nearly as much — it is there through the cloudy months, through the haze, and through window glass, which UVB largely does not pass. That is the practical reason a pigmentation problem here is a year-round, every-day problem rather than a beach-day problem, and why the people who get on top of it are the ones who made it boring and daily rather than heroic and occasional.