Scientific Skincare - Scientific Sunscreen Guide Part II: How is Sunscreen Tested?

How Is Sunscreen Tested? SPF vs UVA-PF: Scientific Sunscreen Guide Part II.

How do we know how effective a sunscreen is at protecting our skin from UV radiation? How is sunscreen tested? What do SPF and UVA-PF stand for? What’s the difference between the two? So many sunscreen-related questions, so little time! In part II of this three-part scientific sunscreen guide, we are looking at how sunscreen is tested and how to work out which sunscreens will offer better protection (particularly UVA protection). If you missed part I or want to skip ahead to Part III, here are the links you need:

Part I – The Importance of Sunscreen.

Part II – How is Sunscreen Tested? (This article)

Part III – The Best Sunscreens with High UVA Protection.

Here is a quick recap of Part I in case you missed it…

How Is Sunscreen Tested? SPF vs UVAPF

Why Is UV Radiation Harmful To Skin?

While ultraviolet (UV) radiation only accounts for a small percentage of total solar radiation, it is by far the most damaging. This is due to the fact that shorter wavelengths of light contain more energy than longer wavelengths. This also means that UVA rays have less energy than UVB rays. 

This energy is transferred to light-absorbing molecules in the skin called chromophores. Chromophores that absorb light in the UV spectrum include DNA and melanin. When UV radiation is absorbed by DNA it causes damage that can lead to genetic mutations. If this damage is not corrected by the cells natural defenses, mutated cells can multiply rapidly and eventually result in skin cancer.

UVA and UVB rays damage DNA in different ways. UVB rays are directly absorbed by DNA which is why they are more closely associated with skin cancer. UVA rays, on the other hand, cause indirect DNA damage through the generation of free radicals. It is these free radicals that cause the DNA damage rather than the UVA rays themselves.


Why Does UV Radiation Cause ‘Tanning’?

Melanin acts as a natural defense against UV radiation by creating a ‘cap’ above the cell’s nucleus. As melanin absorbs UV radiation, it reduces the transmission of UV rays into the nucleus to be absorbed by DNA. The more melanin contained within skin, the better the skins natural protection. An increase in melanin production in response to UV radiation is called ‘tanning’ and consists of three separate stages:

  • Immediate Pigment Darkening (IPD) is temporary, happens almost immediately, and has no protective effect. It is caused by melanin oxidization and melanosome redistribution.
  • Persistent Pigment Darkening (PPD) follows IPD but lasts at least 3-5 days and is caused by melanin oxidisation. It is particularly activated by UVA radiation.
  • Delayed Pigment Darkening (DPD) is the last phase of tanning, is caused by an increase in melanin production, and lasts a couple of weeks. DPD caused by UVB seems to have a protective effect, unlike DPD caused by UVA.


Why Does UV Radiation Cause Premature Aging?

Finally, UV radiation is responsible for 80-90% of facial skin aging. This is due to the fact that it causes the generation of free radicals that activate enzymes that degrade collagen and elastin. As collagen and elastin are contained within the dermis, and as UVA can penetrate the dermis, the premature aging associated with UV radiation is predominantly due to UVA.


How Do Sunscreens Protect Against UV Radiation?

Sunscreen acts as an exogenous (external) chromophore by absorbing UV radiation instead of the natural chromophores. This drastically reduces the amount of UV radiation able to enter the skin and be absorbed by DNA and melanin. The energy that the sunscreen filter particles absorb is dissipated as heat. However, if the sunscreen molecule cannot dissipate this energy fast enough, it is unable to absorb further UV rays and becomes unstable (photo unstable).


How Is Sunscreen Tested?

The effectiveness of a sunscreen against UV radiation has traditionally focussed on the sun protection factor (SPF) which is accepted as a world-wide standard. However, SPF is only a measure of a sunscreens protection against UVB radiation. Until relatively recently, UVA-induced skin damage has been overlooked and, as a result, there is currently no single standardised measure of UVA protection.



As mentioned, SPF measures a sunscreens ability to protect skin from UVB radiation which, in theory, means that products with higher SPFs should provide more protection than lower SPFs. SPF is measured as a ratio of the amount of UVB radiation required to burn the skin when it is protected with sunscreen divided by the amount required when there is no sunscreen protection. The dose of radiation required to cause the skin to burn is referred to as the minimal erythema dose or MED [1].

SPF = MED of protected skin/MED of unprotected skin

 If all other factors are equal, then this means that a sunscreen with an SPF of 50 will allow skin to be exposed to 50x more UVB radiation than unprotected skin before burning. However, sunscreen needs to be applied in the correct amount (2mg/cm2), reapplied frequently (every 2-hours), and reapplied after swimming or excessive sweating in order to achieve and maintain a stated SPF.

So, the measure of a sunscreens ability to protect the skin from UVB radiation is relatively straight forward – higher SPF = higher UVB protection. However, a sunscreens ability to protect the skin from UVA damage is a little less clear.

Some research suggests that if a sunscreen has an SPF of 11 or above then, mathematically speaking, it must provide some amount of UVA protection [2]. In addition, shorter UVA wavelengths (UVA-2) also cause skin erythema which means that if a sunscreen is able to block erythema-inducing UV radiation then it is also blocking some amount of UVA-2. This means that the protective effects of a sunscreen against UVA radiation may be able to be mathematically predicted from the erythema action spectrum, the emission spectrum of the UV source, and the absorption spectrum of the test product [3].

The SPF method is an example of an in vivo evaluation of sunscreen efficacy, meaning that the test is performed on humans. There are three in vivo methods that have been suggested to measure UVA protection; IPD, PPD, and UVA-PF (UVA Protection Factor).

How UVA Sun Protection is Measured


As mentioned earlier, IPD stands for Immediate Pigment Darkening which is the skins initial response to exposure to UV radiation. It is a result of melanin oxidization and the redistribution of melanosomes. IPD is a temporary darkening of pigment that occurs immediately after UV exposure, doesn’t appear to have any protective effect for the skin, and is particularly activated by UVA radiation.

The IPD method measures the amount of UVA radiation required to produce a darkening of the skin with a clearly defined margin, observed immediately after exposure [4]. The IPD protection factor (IPD-PF) is the ratio of the UVA dose required to produce IPD when skin is protected with the sunscreen to the dose required when the skin is not protected [3].

One benefit of the IPD method is that it is able to use low doses of UVA which means less UVA-induced skin damage for test subjects. However, it is not known how clinically significant these results are and responses are often highly variable. In addition, the IPD-PF method is usually only performed on Fitzpatrick skin types III-IV [1].



Persistent Pigment Darkening (PPD) is a skin response to UV radiation that occurs after IPD, lasts around 3-5 days, is particularly sensitive to the entire UVA spectrum and decreases very slowly with longer wavelengths. The PPD response requires large doses of UVA (>10 J cm-2– the equivalent of 40mins midday summer sunlight) which means that the PPD method can also challenge a sunscreens stability.

The PPD method measures the amount of UVA radiation required to produce the first unambiguous pigmented reaction. The PPD protection factor (PPD-PF) is the ratio of the minimal UVA dose to produce PPD when the skin is protected with the sunscreen (MPDp) to the dose required when the skin is not protected (MPDu) [5].


 The PPD-PF method has demonstrated reliability by proving consistent with other measures of UVA protection [5]. However, it is also considered a questionable biological end-point in that PPD to UVA is not as relevant as erythema is to UVB [6].

The PPD method is the basis for the PA +++ UVA protection rating system used in Japan, Korea, and China. A PPD-PF of 2-4 is given a rating of PA+, a PPD-PF of 4-8 is given a rating of PA++, and a PPD-PF of 8 or more is given a rating of PA+++ [1][7].

In 2013, this rating system was updated to include a PA++++ category, which requires a PPD-PF of 16 or more.



UVA-PF stands for UVA protection factor and is a measure of how much UVA protection is offered by a sunscreen. Both IPD-PF and PPD-IF are UVA-PFs, however, PPD-IF (the PPD method), is generally considered the standard in vivo UVA-PF measure. In fact, it is recommended that UVA-PF should be determined using the in vivo PPD method or any in vitro method able to provide equivalent results [7].

There are three different in vitro methods for assessing UVA-PF; one from the United States (USA; FDA), one from the European Union (EU; COLIPA), and one from the United Kingdom (UK; Boots) [4].

In vitro testing determines the amount of protection a sunscreen offers by spreading the sunscreen onto a quartz, acrylic, or plastic (PMMA) slide and measuring how much UV radiation passes through the slide. The less radiation that passes through the slide, the higher the protection factor.


US FDA Critical Wavelength Method

The FDA uses the critical wavelength method to determine whether a sunscreen can be classed as broad-spectrum. The critical wavelength method measures the breadth of protection of a sunscreen across the UV spectrum (290-400nm). For a sunscreen to be considered broad-spectrum, it must have a critical wavelength of 370nm or more. This means that 10% of the protection that the sunscreen offers has to be for wavelengths above 370nm [4].

However, the critical wavelength method does not offer much information about the amplitude of UVA protection offered by a sunscreen, merely that it offers some UVA protection.

How is sunscreen measured - the critical wavelength


In addition to requiring a critical wavelength of 370nm and above, the EU also requires that the UVA-PF offered by a sunscreen be at least 1/3 of the stated SPF. For example, a sunscreen with an SPF of 60 must have at least a UVA-PF of 20 [7]. UVA-PF ratings from this method appear to correlate well with the in vivoPPD-PF ratings [4].

The 2006 recommendation by The Comission Of The European Communities states that:

Scientific findings show that certain biological damage to the skin can be prevented and reduced if the ratio of the protection factor measured in the persistent pigment darkening test (i.e. addressing mainly UVA radiation) is at least 1/3 of the factor measured by the sun protection factor testing method (i.e. addressing mainly UVB radiation). Moreover, in order to ensure a broad protection, dermatologists recommend a critical wavelength of at least 370 nm.

So it looks like there is some evidence to suggest that the UVA-PF/SPF ratio of 1/3 may reduce skin damage, however, they do not elaborate on specific research findings.

UK Boots Method UVA/UVB Ratio

The UK boots method also calculates the UVA/UVB ratio using the final UVA-PF value and the SPF value (UVA-PF/SPF). However, this method goes a step further and classifies products into categories from 0-5 stars based on the initial mean UVA/UVB ratio and the post-exposure mean UVA/UVB ratio. This means that it can also provide a better idea of how stable the UVA protection is [7]. In order to achieve the highest rating (5 stars), The UVA/UVB ratio is required to be at least 9/10. In other words, a sunscreen would have to offer a UVA-PF of more than 90% of the stated SPF [6]. (80-90% would be 4 stars and 60-80% would be 3 stars) [8].


How Can I Find Out A Sunscreens UVA-PF?

Unfortunately, unless a sunscreen brand advertises its UVA-PF rating, there is no way to tell what the exact UVA-PF is. However, due to the regulations discussed above, we can tell what the minimum UVA-PF required is depending on the rating method.

First of all, we know that any sunscreen labelled as broad-spectrum in the US has to contain both UVA and UVB protection with a minimum critical wavelength of 370nm. However, this alone does not tell us how good the UVA protection is.

If a sunscreen contains the UVA seal (i.e. the letters U, V, and A in a circle) and/or is labelled as broad-spectrum in the EU, then we know that the UVA-PF is at least 1/3 of the SPF value (in addition to the critical wavelength requirements stated above). For example:

  • SPF 15 = UVAPF 5
  • SPF 30 = UVAPF 10
  • SPF 50 = UVAPF 16
  • SPF 50+ (a.k.a SPF 60) = UVAPF 20

What Does The UVA Seal mean

The PA+++ rating system for UVA-PF determined by the in vivoPPD method correlates well with the in vitroCOLIPA method used by the EU (the UVA-PF ratings mentioned above) to the point that PPD response can be predicted by the COLIPA method with relative accuracy [9].

If a sunscreen has a PA+++ rating, then we have a rough idea of its UVA-PF.

  • PA+ = UVAPF between 2 and 4
  • PA++ = UVAPF between 4 and 8
  • PA+++ = UVAPF more than 8
  • PA++++ (added in 2013) = UVAPF more than 16

One downside to the PA System is that there is no way to tell whether a sunscreen with a PA rating of ++++ has a PPD/UVA-PF of 20, 40, or 60 (etc.) unless the brand discloses that information themselves.

What does the PA System for UVA protection measurement mean?

If a sunscreen has a star rating according to the Boots rating system, then we can work out the minimum UVA-PF that it should contain according to its SPF value and star rating. For example:

  • SPF15:
    • 3 stars = UVAPF between 9 and 12
    • 4 stars = UVAPF between 12 and 13.5
    • 5 stars = UVAPF more than 13.5
  • SPF30:
    • 3 stars = UVAPF between 18 and 24
    • 4 stars = UVAPF between 24 and 27
    • 5 stars = UVAPF more than 27
  • SPF50:
    • 3 stars = UVAPF between 30 and 40
    • 4 stars = UVAPF between 40 and 45
    • 5 stars = UVAPF more than 45
  • SPF50+ (SPF 60):
    • 3 stars = UVAPF between 36 and 48
    • 4 stars = UVAPF between 48 and 54
    • 5 stars = UVAPF more than 54

Boots Star Rating System for Testing Sunscreen Protection

The highest four and five star categories according to the Boots star system are considered to be very close to the ideal sunscreen due to their near even protection across both the UVA and UVB spectrums [6].



Unlike SPF, there is no internationally universal method for measuring a sunscreens protection against UVA radiation. However, we know that if a sunscreen is labelled as broad-spectrum it has to offer both UVA and UVB protection. Some measurement methods help us to determine a sunscreens UVA protection better than others.

The PA system of measuring UVA protection through a PPD response is consistent, in that any sunscreen with a rating of PA++++ is going to have a PPD-PF (UVA-PF) of at least 16.

The UVA seal and star rating systems of measuring UVA protection are relative to the sunscreens SPF and, as such, require a little more understanding. However, sunscreens with SPF50 and a five star UVA-PF rating are considered to be the closest to the ideal sunscreen currently available.



  1. Latha, M., Martis, J., Shobha, V., Shinde, R. et al. (2013). ‘Sunscreening Agents: A review’, J Clin Aesthet Dermatol., 6(1), 16-26. Available at:
  2. Sayre, R., Dowdy, J., Lott, D. & Marlowe, E. (2008). ‘Commentary on ‘UVB-SPF’: the SPF labels of sunscreen products convey more than just UVB protection’, Photodermatol Photoimmunol Photomed., 24(4), 218-220. Available at:
  3. Kaidbey, K. & Barnes, A. (1991). ‘Determination of UVA protection factors by means of immediate pigment darkening in normal skin’, J Am Acad Dermatol., 25(2 Pt 1), 262-266. Available at:
  4. Pelizzo, M., Zattra, E., Nicolosi, P. et al. (2012). ‘In vitro evaluation of sunscreens: An update for the clinicians’, ISRN Dermatology, Article ID 352135, 4 pages. Available at:
  5. Moyal, D., Chardon, A. & Kollias, N. (2002). ‘Determination of UVA protection factors using the persistent pigment darkening (PPD) as the end point’, Phtodermatol Photoimmunol Photomed., 16(6), 245-249. Available at:
  6. Osterwalder, U. & Herzog, B. (2010). ‘The long way towards the ideal sunscreen – where we stand and what still needs to be done’, Photochemical and Photobiological Sci., 9(4), 470-481. Available at:
  7. Moyal, D. (2010). ‘UVA protection labelling and in vitro testing methods’, Photochem & Photobiol Sci., 9, 516-523. Available at:!divAbstract
  8. Polonini, H., Lopes, R., Beatriz, A. et al. (2014). ‘Synthesis and evaluation of octocrylene-inspired compounds for UV-filter activity’, Quim Nova., 37(6), 1004-1009. Available at:
  9. Matts, P., Alard, V., Brown, M. et al. (2010). ‘The COLIPA in vitro UVA method: a standard and reproducible measure of sunscreen UVA protection’, Int J Cosmet Sci., 32(1), 35-46. Available at:

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  1. I can’t find the actual article/post where I read about the idea but if I recall correctly it discusses how the Boots certification has become a marketing strategy since it requires going through Boots (which isn’t an independent sunscreen certification) in order to demonstrate better UVA protection than the UVA circle. What are your thoughts on this statement?

    Thank you so much for your posts – I discovered you recently and really enjoy how technical you get.

    1. Thank you so much for reading and taking the time to comment.

      From what I gather, brands have to pay Boots a licensing fee in order to display the UVA star rating symbol. So you are right that it’s highly likely a marketing strategy by some of the brands. It also gives Boots a certain element of power and I believe they came under scrutiny at one point for refusing to let some competitors use the star rating system. Some brands get around this by using the UVA circle but listing the exact UVA-PF/PPD on their website (La Roche Possay is one example), but this information is less obvious unless someone is specifically looking for it.

      Hopefully, one of these days, they will finally standardize UVA ratings!

        1. Because the boots star ratio is the UVA/UVB absorption ratio, not the protection factor ratio UVAPF/SPF. It is a logarithmic relation and UVAPF is therefore not equal to Boots x SPF.

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