What do SPF and PA ratings on a sunscreen label mean?

Two sunscreens sitting side by side on a shelf can carry completely different information on the front of the bottle, and neither one is telling the whole story. A tube bought in Seoul or Tokyo prints an SPF number and a row of plus signs after the letters PA. A tube bought in Los Angeles prints an SPF number and the words "broad spectrum". Both are describing protection against two different parts of the ultraviolet spectrum, using two different measurement systems, and both figures are generated in a laboratory at an application thickness that almost nobody reproduces at a bathroom mirror. The number on the bottle is a ceiling rather than a promise, and how close anyone gets to it is decided by quantity and reapplication rather than by which product was chosen.

What does the SPF number on a sunscreen actually measure?#

SPF, or sun protection factor, is a ratio: it compares the amount of ultraviolet energy needed to produce visible redness on protected skin with the amount needed to produce the same redness on unprotected skin. Because redness is driven mainly by ultraviolet B, the shorter-wavelength band that damages the outer layers of skin, the SPF figure is essentially a measure of UVB protection. A product rated SPF 30 required thirty times the ultraviolet dose to redden test skin under laboratory conditions.

That ratio is often translated into time — thirty times longer before burning — and the translation is where the number starts to mislead. Ultraviolet intensity changes with the hour, the season, the latitude, the altitude and the reflective surface underfoot, so the same tube protects for very different lengths of time on a July beach and a February pavement. Sweat, water, towels and the ordinary friction of a face touching a phone screen all remove product. Treating SPF as a timer produces a schedule; treating it as a laboratory ratio produces a habit, and the habit is what holds up outdoors.

The flattening curve is the reason SPF 50 is not close to being twice as protective as SPF 25, and the reason a jump from 50 to 100 buys roughly one percentage point. It is also the reason that headroom is worth something anyway. Because everyday application is thinner than the tested density, a high labelled figure leaves more protection standing once real-world use has eroded it, which is a different argument from the one the number appears to make on its own.

What does PA+++ mean, and how does it differ from "broad spectrum"?#

PA is a grade for ultraviolet A protection, written as PA+ through PA++++, and it exists because SPF says almost nothing about UVA. Ultraviolet A has a longer wavelength than UVB, penetrates deeper into the dermis, and drives pigmentation and photoageing rather than sunburn. Its intensity stays relatively steady across the day and across the seasons, and it passes through ordinary window glass, which is why a face can accumulate UVA exposure during a commute or a day at a desk without a hint of redness to signal it.

The PA scale comes from the Japan Cosmetic Industry Association and is built on a measurement called persistent pigment darkening, abbreviated PPD, which tests how much UVA it takes to produce a lasting tan response on protected versus unprotected skin. The grades map onto PPD values directly: PA+ corresponds to a PPD of 2 to under 4, PA++ to 4 to under 8, PA+++ to 8 to under 16, and PA++++ to 16 or above, the fourth grade having been added in 2013. A row of plus signs is therefore a band rather than a precise figure, and the step from PA+++ to PA++++ is a doubling of the measured value rather than an increment.

The practical difference is granularity. A United States label answers a yes-or-no question, confirming that a product protects across a broad enough slice of the spectrum to earn the phrase, while a Korean or Japanese label answers a how-much question and puts the answer in four visible tiers. Neither system is wrong, and a broad-spectrum product from a US shelf can perform as well as a PA++++ product from a Korean one. The distinction matters when the goal is pigment control rather than sunburn prevention, because that is the situation in which the size of the UVA figure, and not merely its existence, is the variable being managed.

How much sunscreen do you actually need to apply?#

Every SPF and PA figure printed on a bottle was produced at a standardised density of two milligrams of product per square centimetre of skin, specified by ISO 24444:2019 for in vivo SPF testing and mirrored in the equivalent UVA methods. Translated into kitchen terms, that is roughly a quarter to a third of a teaspoon for the face and neck of an adult, and something close to a shot glass, around 30 millilitres, to cover an adult body in swimwear. Almost nobody applies that much, and the gap between the tested density and the applied density is the single largest reason real protection falls short of the label.

Application studies consistently report everyday use well under the tested density, in the region of half to two thirds of it, and laboratory work indicates that protection falls away steeply rather than in proportion as the layer thins. The consequence is uncomfortable arithmetic: a thin layer of an SPF 50 product does not deliver something close to SPF 50, and it does not deliver SPF 25 either. It delivers considerably less, which is why the amount squeezed out of the tube deserves more attention than the choice between one labelled figure and another.

Measuring by volume is more reliable than measuring by eye, and two informal methods have become standard because they work. The two-finger rule asks for a line of product squeezed along the full length of the index and middle fingers, which is then divided between face and neck. The quarter-teaspoon method uses an actual measuring spoon for the face alone, with a separate portion for the neck and ears. Both produce a layer that feels heavier than most people are used to, and that sensation is the point rather than a sign of overapplication.

How often should sunscreen be reapplied, and does it matter indoors?#

The standard interval outdoors is every two hours, and immediately after swimming, towel drying or heavy sweating regardless of how recently the last layer went on. Sunscreen is not consumed by ultraviolet light so much as removed by physical contact and diluted by sweat, with some filters also degrading as they absorb energy. A layer that has survived two hours of a shaded café table is in a different condition from one that has survived two hours of a beach, and the two-hour rule is a floor for the second case rather than a ceiling for the first.

Water resistance claims are narrower than they sound, and the wording is regulated. The FDA's 2011 sunscreen labelling final rule permits only two water resistance claims, 40 minutes or 80 minutes, and prohibits the terms "waterproof" and "sweatproof" entirely, because no sunscreen survives water indefinitely. A product marked water resistant for 80 minutes has been tested to retain its labelled SPF through that period of immersion, after which reapplication is expected. Towel drying removes product mechanically and resets the clock whatever the label says.

Reapplying over makeup is the practical obstacle that stops most routines, and the workable answer is to change the format rather than the schedule. Powder sunscreens, sticks and cushion compacts all deposit less product than a lotion does, which makes them a partial top-up rather than a full second application, and they are still meaningfully better than skipping the step. A generous morning layer followed by an honest partial top-up outperforms a thin morning layer followed by nothing, and it is easier to sustain than a plan that requires removing makeup at lunchtime.

Does any of this change after a cosmetic procedure or in deeper skin tones?#

Both situations raise the stakes attached to the UVA side of the label rather than the SPF side. Skin that has recently been treated with a laser, a peel or microneedling is healing from controlled inflammation, and melanocytes activated by that inflammation respond more strongly to ultraviolet exposure than resting ones do. Skin that produces melanin readily, broadly Fitzpatrick types III to VI, starts from a more reactive baseline. In both cases the exposure that matters is the one that produces pigment rather than the one that produces redness.

Visible light adds a third variable that no SPF or PA figure captures. The higher-energy blue end of visible light contributes to pigmentation in melanin-rich skin, and it passes straight through a clear sunscreen of any rating. Tinted mineral formulations are recommended in pigment-focused care because the iron oxides that provide the tint absorb part of that visible light, which a transparent filter does not. This is the reason a clinician treating melasma often specifies a tinted product rather than simply a high number, and the pigment conditions where that distinction carries most weight are set out in our guide to melasma treatment in Korea.

The consequences of getting this wrong after a procedure are specific rather than general, and they are the reason photoprotection is treated as part of the treatment rather than as an accessory to it. Pigment appearing at a treated site weeks after the appointment is the most common complication in deeper skin tones, and the mechanisms and prevention are covered in our guide to preventing post-inflammatory hyperpigmentation. Where a full aftercare sequence is the question rather than sun exposure alone, the wider routine is set out in our guide to skincare after a procedure in Korea. Anyone with deeper skin planning laser work will find the device and settings side of the same problem in our guide to laser treatment and Fitzpatrick skin type.

✨ In short#

SPF and PA describe two different halves of the ultraviolet problem: SPF measures protection against the ultraviolet B that causes burning, and PA measures protection against the ultraviolet A that drives pigmentation and photoageing. A Korean or Japanese label reports UVA protection as a graded band built on persistent pigment darkening values, while a United States label reports it as a pass on the critical wavelength test and a European label as a UVA circle set at one third of the SPF. All of those figures were generated at two milligrams of product per square centimetre, roughly double what everyday application delivers, which makes quantity and reapplication the variables that decide how much of the printed number reaches the skin. For anyone managing pigment, healing after a procedure, or working with melanin-rich skin, the UVA side of the label and the addition of a tinted mineral formulation carry more weight than the pursuit of a higher SPF figure.