Background
Sunscreens give good protection against sunburn, actinic keratosis and squamous cell carcinoma. The results for preventing cutaneous malignant melanoma (CMM) and basal cell carcinoma are less conclusive
Previous studies have shown that some sunscreens lose part of their protection when exposed to UV radiation
The aim of this study was to investigate the photostability of commercial sunscreen products after UVnat and after UVart.
Methods
Sunscreens
Seven commercial sunscreens were included, all available on the Swedish market. Three sunscreens contained only organic chemical filters, three sunscreens had a combination of inorganic and organic chemical filters, and one sunscreen contained solely inorganic chemical filters. In Table
The photoactive compounds in the investigated sunscreens, CAS no and SPF of the product.
Photoactive compound
CAS no
Mainly protection against
Active ingredients in the seven investigated sunscreen products
UVA
UVB
1
2
3
4
5
6
7
EHMC
5466-77-3
x
x
x
x
MBC
36861-47-9
x
x
x
x
EHT
88122-99-0
x
x
OC
6197-30-4
x
x
BMDBM
70356-09-1
x
x
x
x
x
x
x
BZ-3
131-57-7
x
x
x
TLDCSA
90457-82-2
x
x
TiO 2
13463-67-7
x
x
x
x
x
ZnO
1314-13-2
x
x
SPF
4
14
10
10
6
10
15
CAS Chemical Abstracts Service
EHMC ethylhexyl methoxycinnamate MBC 4-methylbenzylidene camphor
EHT ethylhexyl triazone OC octocrylene BMDBM butyl methoxydibenzoylmethane
BZ-3 benzophenone-3 TLDCSA terephthalylidene dicamphor sulfonic acid
TiO2titanium dioxide ZnO zinc oxide
SPF Sun Protection Factor
The sunscreen was weighed and placed between two plates of polished fused silica (quartz) with diameter 25 mm and thickness 5 mm. The amount applied was 0.5 mg/cm2. The absorbance was too high for proper measurements when the recommended amount of 2 mg/cm2 was applied, causing distortion in the absorption spectrum. For this reason a thinner layer was applied. A previous study has shown that the result were independent whether an application thickness of 1 or 2 mg/cm2 was used
Light sources
For UVA radiation, a UVASUN 2000 (MUTZHAS, Germany) was used. The output is mainly between 340 and 400 nm.
For UV radiation (including UVB), an Esshå Corona Mini (Sweden), equipped with two fluorescent tubes, Philips TL 12 (20 W), was used. This is a broadband radiation source from 280 to 380 nm with a major peak at 313 nm. There are strong mercury peaks at 313 nm and 365 nm.
The irradiance at the exposure plane was measured with an International Light IL 1350 Radiometer/Photometer using a probe named SED 240 for UVA and a probe named SED 015 for UVB radiation. The fluence rate of the UVA lamp was 820 W/m2 when measured from a distance of 25 cm. Twenty minutes' exposure gave a dose of 980 kJ/m2. This corresponds to the UVA dose that reaches the earth's surface during one sunny summer day in Gothenburg
The dose of natural UV radiation the investigated sunscreens received
Sunscreen
Exposure time (min)
UVA radiation (kJ/m 2 )
Erythemal effective radiation (J/m 2 )
SED
1
30
54
**
90
180
**
120
260
**
2
30
54
100
1
90
180
410
4.1
120
260
610
6.1
3
30
65
140
1.4
90
210
530
5.3
120
280
730
7.3
4
30
65
**
90
210
**
120
280
**
5
30
**
**
90
**
**
120
210
**
6
30
47
140
1.4
90
140
300
3.0
120
210
370
3.7
7
30
65
150
1.5
90
130
320
3.2
240*
570
1500
15
Data from the Swedish Metrological and Hydrological Institute (SMHI)
* The exposure time for Sunscreen 7 it was 30 min, 90 min and 240 min due to technical obstacles
** Data not available due to technical obstacles at SMHI.
For UVnat, samples were placed horizontally outdoors when the weather was sunny. This was done in early July in Gothenburg (latitude: 57° 42' N). The total exposure time was 120 min (Table
To eliminate the possibility that the degradation of the photoactive compounds could be caused by a temperature increase, control samples of sunscreen between silica plates were placed on a heating plate for 20 minutes. The temperature was kept at 50°C ± 2°C, which was the same as that measured during exposure to the UVA lamp. This is about 15°C higher than the temperature of the skin. Spectra were recorded prior to and after heating. The temperature did not influence the degradation since the absorption spectra did not change after heating.
Spectrometer
In all studies the spectra were recorded by a Cary 4 spectrophotometer (Varian, USA). It is a two-beam spectrophotometer without integrating sphere, which measures the transmission by scanning over the wavelength range of interest. Without integrating sphere the measured absorbance includes also some scattered radiation. Therefore, the spectra of samples with inorganic filters, which scatter light, may show a too high absorbance.
Area under the curve index (AUCI)
The AUC for UVA, UVA1 (340–400 nm), UVA2 (320–340 nm) and UVB was calculated for each spectrum before (AUCbefore) and after (AUCafter) UVart (980 kJ/m2 UVA and 12 kJ/m2 of UV radiation (UVB included) and before and after UVnat. If the AUCI (AUCI = AUCafter/AUCbefore) was >0.80, the sunscreen was considered photostable.
The AUC was calculated with the following equation:
where A is absorption and λ is wavelength. It was measured in steps of 1 nm.
For UVA λmax = 400 nm and λmin = 320 nm. The same calculation was done for each UV range respectively, before and after UVart and before and after UVnat.
Maier et al. used the difference between the spectral transmission before and after a defined UV exposure, ΔT. A product was labeled photounstable if the mean photoinstability was higher than 5% (1 mg/cm2 product was used)
Results
Sunscreens
The photostability of the sunscreens tested varies considerably. The photounstable sunscreens start to degrade rather rapidly when exposed to the sun. After 30 min of UVnat, Sunscreens 1 and 3 are unstable (AUCI <0.80). Sunscreens containing inorganic chemical filters are more photostable in our study than sunscreens with organic chemical filters with the exception of Sunscreens 3 and 5
Sunscreens 5, 6 and 7 are photostable after UVnat; in the UVA range the AUCI was between 0.85 and 0.99 after 120 min and between 0.92 and 1.0 in the UVB range. Sunscreen 4 shows in the UVA range an AUCI of 0.87 after UVnat but 0.72 after UVart.
Sunscreens 1, 2 and 3 are unstable. They show after 90 min UVnat an AUCI between 0.41 and 0.76 in the UVA range and between 0.30 and 0.69 in the UVA1 range.
Sunscreens 2, 4, 5, 6 and 7 are stable in the UVB region whereas Sunscreens 1 and 3 are not. During exposure, absorption ranges of Sunscreens 1, 2 and 3 are shifted towards shorter wavelengths (Fig.
UV absorbance spectra of UVA photounstable sunscreens (AUCI <0.80)
UV absorbance spectra of UVA photounstable sunscreens (AUCI <0.80). Before and after natural UV exposure (UVnat), and before and after artificial UV exposure (UVart). (a) Sunscreen 1 (b) Sunscreen 2 (c) Sunscreen 3.
In Table
Summary of the AUCI values for the investigated sunscreens
After natural UV exposure
After artificial UV exposure
Sunscreen
UVA
UVA1
UVA2
UVB
UVA
UVA1
UVA2
UVB
30 min
90 min
120 min
30 min
90 min
120 min
30 min
90 min
120 min
30 min
90 min
120 min
1
0.72
0.46
0.36
0.69
0.38
0.29
0.83
0.65
0.54
0.91
0.87
0.81
0.36
0.32
0.45
0.69
2
0.84
0.76
0.75
0.83
0.69
0.67
0.86
0.92
0.97
0.86
0.92
0.97
0.63
0.53
0.88
0.89
3
0.67
0.41
0.41
0.59
0.30
0.34
0.81
0.58
0.52
0.92
0.75
0.63
0.40
0.31
0.58
0.73
4
0.92
0.86
0.87
0.91
0.85
0.83
0.94
0.91
0.91
0.95
0.93
0.95
0.72
0.69
0.81
0.83
5
0.96
0.89
0.85
0.94
0.87
0.83
0.99
0.95
0.93
0.99
0.99
0.98
0.90
0.88
0.97
0.97
6
0.98
0.94
0.94
0.97
0.93
0.93
0.98
0.96
0.97
0.99
0.99
1.00
0.85
0.82
0.92
1.00
7
0.99*
1.00*
0.96*
0.92*
0.99
0.99
1.00
0.99
The AUCI is defined as AUCafter/AUCbefore. The bold numbers show when AUCI is <0.80.
* Sunscreen 7 was exposed to natural UV during 240 min.
This is true for all three samples, after both UVnat and UVart. Sunscreen 4 is more unstable after UVart than to UVnat (Fig.
UV absorbance spectra of Sunscreen 4
UV absorbance spectra of Sunscreen 4. Before and after natural UV exposure (UVnat), and before and after artificial UV exposure (UVart). Sunscreen 4 was photostable when exposed to natural UV in the UVA range but not to UVAart.
The spectra were normalized by dividing the maximum value of the spectrum before irradiation by itself, so that the peak value of the spectrum before irradiation was set to 1.
The temperature was higher, during exposure to the UVart than during exposure to UVnat, but the temperature did not influence the absorption. Sunscreens 5, 6 and 7 were stable both after UVart and after UVnat. Sunscreens 6 and 7 were very little influenced by UV exposure (Fig.
UV absorbance spectra of UVA photostable sunscreens (AUCI >0.80)
UV absorbance spectra of UVA photostable sunscreens (AUCI >0.80). Before and after natural UV exposure (UVnat), and before and after artificial UV exposure (UVart). (a) Sunscreen 5 (b) Sunscreen 6 (c) Sunscreen 7 was exposed to 240 min of UVnat.
Discussion
In most cases UVnat compared to the UVart gave qualitatively similar results. However, UVnat, with a lower fluence rate than UVart, gave similar yields of degradation. In addition, the fluence rate of the UVAart was higher than that of the UVAnat, which could be expected to degrade the sunscreens faster. But this is not the case, except for Sunscreen 4. Since the dose of UVAart was higher than UVAnat, Sunscreen 4 probably provides sufficient protection for the consumer.
Commercial sunscreens generally have low viscosity in order to be easy to apply. The temperature increase of the samples during UV exposure, especially after UVart, may lower the viscosity further. This may result in reductions of the optical path lengths of the samples. However, this was not the case in our study since samples kept on a heating plate for 20 min at 50°C showed a similar spectrum before and after heating.
Four of the seven sunscreens contain TiO2. If the particles are too small they may lose their scattering effect and, consequently, not give as good protection as larger particles. This may be the case for Sunscreen 5 (Fig.
When mixed with petrolatum, some sunscreens undergo degradation during exposure to UV radiation, especially in the UVA range
Some manufacturers of sunscreens claim that commercially available sunscreens are photostable because the photoactive species are in a vehicle that stabilizes them. This claim does not seem to be correct in several cases. There are several studies about how improvement of photostability may be obtained, e.g. with nanoparticle encapsulation of EHMC
When sunscreens without metallic oxide particles are compared, Sunscreen 1 seems to be more rapidly degraded than BMDBM dissolved in petrolatum. Not only does the UVA protection decline after exposure, but also the UVB protection. EHMC is one of the two UVB-absorbing filters present in Sunscreen 2, and the only one in Sunscreen 1. EHMC dissolved in petrolatum is rather photounstable
Sunscreen 5 is photostable but does not contain any metallic oxide particles. This may be due to a vehicle that successfully prevents degradation and/or due to microstructures of the emulsion itself (Fig.
The peak between 350 and 375 nm in the absorption spectra of Sunscreens 3, 4 and 5 (Figs.
The photostable Sunscreen 6 contains, in addition to BMDBM and TiO2, a third UVA absorber, terephthalylidene dicamphor sulfonic acid (TLDCSA), which can stabilize BMDBM. It has also been shown that TiO2 may stabilize ketoprofen and may be used in protecting photounstable species
Many commercial sunscreens give, according to the manufacturers, good UVA and UVB protection. However, the photostability of the sunscreen in the UVA range is not always adequate. Most sunscreens offer good protection against UVB while the UVA photostability of some products decreases substantially during UV exposure. The potential toxicity of the photoproducts also needs to be investigated further.
For the consumer it is very difficult to know what product to choose, since the photostability varies between different brands and the photostability is not marked on the bottle. To know which photoactive compound the sunscreen contains is not good enough. The stability also depends on factors like preservatives, oxygen radical scavengers, and base formulation. It is not reasonable that the ordinary consumer should have knowledge of this. If the product claims to give broadband protection, this protection should remain also after sun exposure. The fact that sunscreens are photounstable has been known for many years. Our study clearly shows that there are still many photounstable products on the market. When buying a sunscreen, the consumer should automatically receive a photostable product.
Conclusion
The present study shows that several commercially available sunscreens are not photo stable. Degradation is clearly manifested in the absorption region in the UVA range after solar irradiation. In general, sunscreens with TiO2 particles seem to be more photostable, with Sunscreens 3 and 5 as exceptions. Special focus should be on the commonly used UVA absorber BMDBM. In three out of six sunscreens in our study this molecule was degraded during UV exposure. Stabilizers of BMDBM may work, but not under all conditions. There is a need for a standardized method to measure photostability and the photostability should be marked on the sunscreen product.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
HG and NT-W have made contributions to conception of design and interpretation of the data. They carried out the experimental set-up during the absorption studies and wrote the main part of the manuscript.
BS carried out some of the absorption studies and drafted the manuscript.
AR have made substantial contributions to conception of design, interpretation of the data and drafting and revising the manuscript. AR also participated in the coordination of the study.
AJ, JM, OL and A-MW have made substantial contribution to concept of design and drafting and revising the manuscript critically.
All authors read and approved the final manuscript.