SPF, PA, PPD, UVA, Stars…What does it all mean?
Sun Protection Factor (SPF)
The SPF number is a ratio of how much UV exposure your skin can be exposed to with the sunscreen on compared to without. The SPF test uses erythema or sunburn as a marker, which is mostly caused by UVB.
Hypothetically, an SPF 30 would mean you can be exposed to 30 times more sunburn causing UV compared to without the sunscreen on.
SPF 30 does not necessarily mean that you can stay out 30 times longer in the sun before sunburn. That is because the amount of UV coming from the sun is not always the same.
Broad spectrum is a statement on how a sunscreen absorbs different wavelengths of UV light.
While the label requirements differ between regions, they all must meet a minimum critical wavelength of 370 nanometers.
That means when measured, at least 90% of the UV light the sunscreen absorbs is below the wavelength of 370 nanometers.
Broad spectrum is a signal that the sunscreen offers better UVA protection than a comparable sunscreen without the label.
The UVA Circle symbol means that the sunscreen’s UVA protection is at least a third of the SPF.
If a sunscreen with the UVA Circle symbol has an SPF 30, you can expect a UVA 10 or higher.
The UVA Circle symbol does not tell you the exact UVA protection, just the minimum you might expect.
UVA Protection Factor
The UVA number (sometimes UVAPF number) is a description of how protective a sunscreen is against UVA radiation.
The UVA number is a ratio, like SPF. A UVA 30 means hypothetically you can be exposed to about 30 times more UVA with proper application of the sunscreen than without.
The UVA number can be determined in vitro (non-human testing) by measuring and comparing the amount of UVA light that passes through the sunscreen on textured plastic.
The UVA number can also be determined in vivo (human testing) with persistent pigment darkening or PPD.
Persistent Pigment Darkening (PPD)
The PPD number is a ratio of how much UV the skin can be exposed to before developing a long-lasting tan with the sunscreen on compared to without. Long lasting tans are mostly caused by UVA.
PPD is tested for on people with Fitzpatrick Skin Types 2 to 4 and mostly measures the effects of UVA.
Hypothetically, a PPD 10 would mean you could be exposed to 10 times the UVA with the sunscreen on, compared to without.
PPD results are sometimes labelled as UVA or UVAP. To find out, contact a brand’s customer service and ask which method they used to determine their UVA or UVAPF number.
The PA system ranges from + to ++++ and is based on values from PPD. It was developed in Japan and used in countries like South Korea.
PA+ = PPD 2 to 4
PA++ = PPD 4 to 8
PA+++ = PPD 8 to 16
PA++++ = PPD 16 or greater
The PA system runs into issues when comparing sunscreens with higher PPD numbers. For example, a PPD 20 and PPD 25 sunscreen would both be labelled as PA++++.
Boots Star Rating
The Boots Star Rating is a UVA rating system used by the British retailer Boots.
It describes the ratio of the UVA to UVB absorption of a sunscreen and is tested in vitro (not on people.
The test compares the ratio before and after UV exposure, which accounts for the sunscreen’s photostability.
|PRE 0.6 to 0.79||PRE 0.8 to 0.89||PRE 0.9 and over|
|POST 0.57 to 0.75||* * *||* * *||* * *|
|POST 0.76 to 0.85||* * *||* * * *||* * * *|
|POST 0.86 and over||* * *||* * * *||* * * * *|
Getting what’s on the label
Most people don’t apply enough sunscreen when unprompted. To get close to the protection label on the sunscreen, the recommended amount is 2 milligrams per square centimeter of skin (2 mg / cm2).
There’s many different techniques to increase the amount of sunscreen you use. The one I prefer and do most often is to apply it twice, with an aim to reapply at least once during an outdoor day.
We don’t need to stress out too much about the exact amount we’re applying…more is generally better and reapplication can make up for a first application.
The best sunscreen is the one you enjoy using consistently, often, and liberally.