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Photostability of commercial sunscreens upon sun exposure and irradiation by ultraviolet lamps

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Sunscreens are being widely used to reduce exposure to harmful ultraviolet (UV) radiation. The fact that some sunscreens are photounstable has been known for many years. Since the UV-absorbing ingredients of sunscreens may be photounstable, especially in the long wavelength region, it is of great interest to determine their degradation during exposure to UV radiation. Our aim was to investigate the photostability of seven commercial sunscreen products after natural UV exposure (UVnat) and artificial UV exposure (UVart). Seven commercial sunscreens were studied with absorption spectroscopy. Sunscreen product, 0.5 mg/cm2, was placed between plates of silica. The area under the curve (AUC) in the spectrum was calculated for UVA (320-400 nm), UVA1 (340-400 nm), UVA2 (320-340 nm) and UVB (290-320 nm) before (AUCbefore) and after (AUCafter) UVart (980 kJ/m2 UVA and 12 kJ/m2 of UVB) and before and after UVnat. If theAUC Index (AUCI), defined as AUCI = AUCafter/AUCbefore, was > 0.80, the sunscreen was considered photostable. Three sunscreens were unstable after 90 min of UVnat; in the UVA range the AUCI was between 0.41 and 0.76. In the UVB range one of these sunscreens was unstable with an AUCI of 0.75 after 90 min. Three sunscreens were photostable after 120 min of UVnat; in the UVA range the AUCI was between 0.85 and 0.99 and in the UVB range between 0.92 and 1.0. One sunscreen showed in the UVA range an AUCI of 0.87 after UVnat but an AUCI of 0.72 after UVart. Five of the sunscreens were stable in the UVB region. The present study shows that several sunscreens are photounstable in the UVA range after UVnat and UVart. There is a need for a standardized method to measure photostability, and the photostability should be marked on the sunscreen product.
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BioMed Central
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BMC Dermatology
Open Access
Research article
Photostability of commercial sunscreens upon sun exposure and
irradiation by ultraviolet lamps
Helena Gonzalez*
1
, Nils Tarras-Wahlberg
2
, Birgitta Strömdahl
2
,
Asta Juzeniene
3
, Johan Moan
3
, Olle Larkö
1
, Arne Rosén
2
and Ann-
Marie Wennberg
1
Address:
1
Department of Dermatology, Sahlgrenska University Hospital, SE-413 45 Göteborg, Sweden,
2
Department of Physics, Göteborg
University, SE-412 96 Göteborg, Sweden and
3
Department of Radiation Biology, Institute for Cancer Research, The Norwegian Radium Hospital,
Montebello, Oslo, N-0310, Norway
Email: Helena Gonzalez* - helena.gonzalez@vgregion.se; Nils Tarras-Wahlberg - tarras@fy.chalmers.se;
Birgitta Strömdahl - birgitta.stromdahl.922@student.lth.se; Asta Juzeniene - asta.juzeniene@klinmed.uio.no;
Johan Moan - johan.moan@labmed.uio.no; Olle Larkö - olle.larko@derm.gu.se; Arne Rosén - arne.rosen@fy.chalmers.se; Ann-
Marie Wennberg - ann-marie.wennberg@vgregion.se
* Corresponding author
Abstract
Background: Sunscreens are being widely used to reduce exposure to harmful ultraviolet (UV)
radiation. The fact that some sunscreens are photounstable has been known for many years. Since
the UV-absorbing ingredients of sunscreens may be photounstable, especially in the long
wavelength region, it is of great interest to determine their degradation during exposure to UV
radiation. Our aim was to investigate the photostability of seven commercial sunscreen products
after natural UV exposure (UVnat) and artificial UV exposure (UVart).
Methods: Seven commercial sunscreens were studied with absorption spectroscopy. Sunscreen
product, 0.5 mg/cm
2
, was placed between plates of silica. The area under the curve (AUC) in the
spectrum was calculated for UVA (320–400 nm), UVA1 (340–400 nm), UVA2 (320–340 nm) and
UVB (290–320 nm) before (AUC
before
) and after (AUC
after
) UVart (980 kJ/m
2
UVA and 12 kJ/m
2
of
UVB) and before and after UVnat. If theAUC Index (AUCI), defined as AUCI = AUC
after
/AUC
before
,
was > 0.80, the sunscreen was considered photostable.
Results: Three sunscreens were unstable after 90 min of UVnat; in the UVA range the AUCI was
between 0.41 and 0.76. In the UVB range one of these sunscreens was unstable with an AUCI of
0.75 after 90 min. Three sunscreens were photostable after 120 min of UVnat; in the UVA range
the AUCI was between 0.85 and 0.99 and in the UVB range between 0.92 and 1.0. One sunscreen
showed in the UVA range an AUCI of 0.87 after UVnat but an AUCI of 0.72 after UVart. Five of
the sunscreens were stable in the UVB region.
Conclusion: The present study shows that several sunscreens are photounstable in the UVA
range after UVnat and UVart. There is a need for a standardized method to measure photostability,
and the photostability should be marked on the sunscreen product.
Published: 26 February 2007
BMC Dermatology 2007, 7:1 doi:10.1186/1471-5945-7-1
Received: 1 September 2006
Accepted: 26 February 2007
This article is available from: http://www.biomedcentral.com/1471-5945/7/1
© 2007 Gonzalez et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0
),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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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 [1-3]. One expla-
nation for this can be that UVA radiation (320–400 nm)
plays a role for induction of CMM [4] and that it is mainly
in the UVA range the photodegradation of the sunscreen
occurs. In the present work, commercially available sun-
screens, containing organic chemical and/or inorganic
chemical filters, have been exposed to natural UV (UVnat)
as well as to artificial UV (UVart) in order to study their
photostability.
Previous studies have shown that some sunscreens lose
part of their protection when exposed to UV radiation [5-
10]. Several sunscreen producers claim that their products
give good protection against both UVA and UVB radia-
tion; however, the photostability of the product is rarely
declared. This is also important for the consumer to know
when choosing a sunscreen. Since it has been known for
several years that some products may be photounstable,
one would have expected a large improvement in the pho-
tostability of sunscreen products. Up to now, there is no
standard method for determining photostability of a sun-
screen [6,11,12]. Neither is there an international stand-
ard method for measuring UVA protection, and several
different systems are currently in use [13-16].
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 combina-
tion of inorganic and organic chemical filters, and one
sunscreen contained solely inorganic chemical filters. In
Table 1 the photoactive compounds of the sunscreens and
the Sun Protection Factor (SPF) of the product are shown.
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/cm
2
. The absorbance was too high for proper meas-
urements when the recommended amount of 2 mg/cm
2
was applied, causing distortion in the absorption spec-
trum. For this reason a thinner layer was applied. A previ-
ous study has shown that the result were independent
whether an application thickness of 1 or 2 mg/cm
2
was
used [17].
Light sources
For UVA radiation, a UVASUN 2000 (MUTZHAS, Ger-
many) 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/m
2
when measured from a distance
of 25 cm. Twenty minutes' exposure gave a dose of 980 kJ/
m
2
. This corresponds to the UVA dose that reaches the
earth's surface during one sunny summer day in Gothen-
burg [18]. We also measured the spectral distribution of
the UVB lamp. By combining the spectral distribution
with the action spectrum of the probe, the fluence rate of
the UVB radiation was 9.8 W/m
2
. Twenty minutes of
exposure gave a dose of 12 kJ/m
2
UV radiation (including
UVB). This corresponds to 45 Standard Erythema Doses
(SED) when further weighted by the CIE action spectrum
[19]. This is a much higher dose than normal for one sum-
mer day in Gothenburg [18] or what has been reported
from Denmark [20]. In spite of that fact, the majority of
sunscreens showed good stability in the UVB range. In
Table 2 the UV doses reported from the Swedish Metro-
logical and Hydrological Institute (SMHI) are listed.
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 2) with measurements of the
absorption spectra before exposure and after 30 min, 90
min and 120 min of UVnat. SMHI measures the global
irradiance in many places in Sweden and gives the CIE
erythema weighted UV radiation as well (Table 2).
To eliminate the possibility that the degradation of the
photoactive compounds could be caused by a tempera-
ture 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 tem-
perature did not influence the degradation since the
absorption spectra did not change after heating.
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Spectrometer
In all studies the spectra were recorded by a Cary 4 spec-
trophotometer (Varian, USA). It is a two-beam spectro-
photometer 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. There-
fore, 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
(AUC
before
) and after (AUC
after
) UVart (980 kJ/m
2
UVA
Table 2: 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.
Table 1: 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 414101061015
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
TiO
2
titanium dioxide ZnO zinc oxide
SPF Sun Protection Factor
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and 12 kJ/m
2
of UV radiation (UVB included) and before
and after UVnat. If the AUCI (AUCI = AUC
after
/AUC
before
)
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 meas-
ured in steps of 1 nm.
For UVA λ
max
= 400 nm and λ
min
= 320 nm. The same cal-
culation 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 trans-
mission before and after a defined UV exposure, ΔT. A
product was labeled photounstable if the mean photoin-
stability was higher than 5% (1 mg/cm
2
product was
used) [9]. In our study we chose the AUCI instead. Since
we used 0.5 mg/cm
2
we considered the product photosta-
ble if the AUCI was higher than 0.8.
Results
Sunscreens
The photostability of the sunscreens tested varies consid-
erably. 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. Sun-
screen 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. 1a–c).
In Table 3 the AUCI is presented.
This is true for all three samples, after both UVnat and
UVart. Sunscreen 4 is more unstable after UVart than to
UVnat (Fig. 2).
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. 3a–c). In
agreement with findings from other studies, sunscreens
with the UV filter combination ethylhexyl methoxycinna-
mate (EHMC) and butyl methoxydibenzoylmethane
(BMDBM) were unstable [6,9,21,22].
Discussion
In most cases UVnat compared to the UVart gave qualita-
tively similar results. However, UVnat, with a lower flu-
ence 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 protec-
tion 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 heat-
ing plate for 20 min at 50°C showed a similar spectrum
before and after heating.
Four of the seven sunscreens contain TiO
2
. If the particles
are too small they may lose their scattering effect and, con-
sequently, not give as good protection as larger particles.
This may be the case for Sunscreen 5 (Fig. 3a). Several
other studies show that inorganic chemical filters are not
always photostable [6,9-11]. Our study indicates the
opposite, but seven sunscreens are a quite small amount
of material, so this finding should be interpreted with cau-
tion.
When mixed with petrolatum, some sunscreens undergo
degradation during exposure to UV radiation, especially
in the UVA range [5]. This is also the case for one of the
most frequently used UV filters BMDBM. This compound
is included in six of the seven sunscreens studied here
(Table 1). Our results confirm the findings from other
studies that sunscreens containing the combination of
EHMC and BMDBM are photounstable, regardless of
what other UV filters they contain [6,9].
A
λλ
λ
λ
()
Δ
min
max
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UV absorbance spectra of UVA photounstable sunscreens (AUCI <0.80)Figure 1
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.
1.2
1
Absorbance (normalized)
0.8
0.6
before UVnat
0.4
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
0.2
after UVart
UVB UVA2 UVA1
0
290 300 310 320 330 340 350 360
Wavelength (nm)
370 380 390 400
(a)
UVB
1.2
Absorbance (normalized)
1
0.8
0.6
before UVnat
0.4
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
after UVart
0.2
UVA2 UVA1
0
290 300 310 320 330 340 350 360
Wavelenght (nm)
370 380 390 400
(b)
1.2
Absorbance (normalized)
1
0.8
0.6
before UVnat
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
after UVart
0.4
0.2
0
290 300 310 320 330
UVB UVA2 UVA1
340 350 360 370 380 390 400
Wavelength (nm)
(c)
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Some manufacturers of sunscreens claim that commer-
cially 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 pho-
tostability may be obtained, e.g. with nanoparticle encap-
sulation of EHMC [23], liposphere preparation of
BMDBM [24] or a combination with diethylhexyl
syringylidene malonate and BMDBM [25]. These findings
are very interesting and will hopefully lead to an improve-
ment in photostability in commercial available products.
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-absorb-
ing filters present in Sunscreen 2, and the only one in Sun-
screen 1. EHMC dissolved in petrolatum is rather
photounstable [5]. The vehicles of Sunscreens 1 and 2 are
nearly identical. The UVA-absorbing compound benzo-
phenone-3 (BZ-3) is added in Sunscreen 2. The presence
of this compound may stabilize BMDBM, in agreement
with earlier findings [26]. Another stabilizer that may
UV absorbance spectra of Sunscreen 4Figure 2
UV absorbance spectra of Sunscreen 4. Before and after natural UV exposure (UVnat), and before and after artificial UV expo-
sure (UVart). Sunscreen 4 was photostable when exposed to natural UV in the UVA range but not to UVAart.
1.2
UVA2 UVA1
Absorbance (normalized)
1
0.8
0.6
before UVnat
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
after UVart
0.4
0.2
UVB
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Table 3: 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 0.720.460.360.690.380.29 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 0.670.410.410.590.300.34 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 AUC
after
/AUC
before
. The bold numbers show when AUCI is <0.80.
* Sunscreen 7 was exposed to natural UV during 240 min.
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UV absorbance spectra of UVA photostable sunscreens (AUCI >0.80)Figure 3
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.
1.2
1
Absorbance (normalized)
0.8
0.6
before UVnat
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
after UVart
0.4
0.2
UVB UVA2 UVA1
0
290 300 310 320 330 340 350 360
Wavelength (nm)
370 380 390 400
(a)
1.2
Absorbance (normalized)
1
0.8
0.6
before UVnat
after 30 min UVnat
after 90 min UVnat
after 120 min UVnat
before UVart
after UVart
0.4
0.2
UVB UVA2 UVA1
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
(b)
1.2
Absorbance (normalized)
1
0.8
0.6
0.4
before UVnat
after 240 min UVnat
before UVar
t
after UVart
0.2
UVB UVA2 UVA1
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
(c)
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work is anisotrizine (CAS no 187393-00-6)is [27]; how-
ever, that compound was not included in any of the prod-
ucts in this study. Sunscreen 2 also has a higher SPF.
However, a degradation manifesting itself in the UVA1
region should be noted.
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 micro-
structures of the emulsion itself (Fig. 3a). It is interesting
to compare this spectrum with that of Sunscreen 3 (Fig.
1c) which, according to the list of contents, includes TiO
2
particles but does not show the scattering slope. The size
of the particles may be too small (15 nm, according to the
producer) to influence the absorption spectrum in the vis-
ible range. Small particles of TiO
2
are expected to give
maximal scattering in the UVB or UVC region. Larger par-
ticles can cause significant scattering also in the UVA and
visible region. It follows that the small nanoparticles can-
not give good protection in the UVA region in this case.
The peak between 350 and 375 nm in the absorption
spectra of Sunscreens 3, 4 and 5 (Figs. 1c, 2, 3a) can be
attributed to BMDBM. In view of this it should be noted
that the UV exposure makes the products react quite dif-
ferently. In Sunscreen 3 the BMDBM peak almost vanishes
totally after 30 min of UVnat, while the peaks in the other
two sunscreens are more stable. We suggested above why
there could be degradation in the UVA range in Sunscreen
3 despite the presence of TiO
2
particles. The reported sta-
bilizing effect of 4-Methylbenzylidene camphor (MBC)
[26] does not manifest itself in the case of Sunscreen 3.
The photostable Sunscreen 6 contains, in addition to
BMDBM and TiO
2
, a third UVA absorber, terephthalyli-
dene dicamphor sulfonic acid (TLDCSA), which can stabi-
lize BMDBM. It has also been shown that TiO
2
may
stabilize ketoprofen and may be used in protecting phot-
ounstable species [28].
Many commercial sunscreens give, according to the man-
ufacturers, 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 prod-
ucts 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 differ-
ent brands and the photostability is not marked on the
bottle. To know which photoactive compound the sun-
screen contains is not good enough. The stability also
depends on factors like preservatives, oxygen radical scav-
engers, 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 protec-
tion 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 availa-
ble sunscreens are not photo stable. Degradation is clearly
manifested in the absorption region in the UVA range
after solar irradiation. In general, sunscreens with TiO
2
particles seem to be more photostable, with Sunscreens 3
and 5 as exceptions. Special focus should be on the com-
monly used UVA absorber BMDBM. In three out of six
sunscreens in our study this molecule was degraded dur-
ing UV exposure. Stabilizers of BMDBM may work, but
not under all conditions. There is a need for a standard-
ized method to measure photostability and the photosta-
bility should be marked on the sunscreen product.
Competing interests
The author(s) declare that they have no competing inter-
ests.
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 revis-
ing the manuscript. AR also participated in the coordina-
tion of the study.
AJ, JM, OL and A-MW have made substantial contribution
to concept of design and drafting and revising the manu-
script critically.
All authors read and approved the final manuscript.
Acknowledgements
We gratefully acknowledge financial support from the Swedish Research
Council for Engineering Science (TFR, contract no. 98–797) and the
Welander Foundation. We also thank Dr Jerker Mårtensson, at Chalmers
University of Technology for fruitful discussion, and Tomas Landelius and
Weine Josefsson at SMHI.
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Pre-publication history
The pre-publication history for this paper can be accessed
here:
http://www.biomedcentral.com/1471-5945/7/1/prepub
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An hypothesis for melanoma induction is presented: UV radiation absorbed by melanin in melanocytes generates products that may activate the carcinogenic process. Products formed by UV absorption in the upper layers of the epidermis cannot diffuse down as far as to the melanocytes. Thus, melanin in the upper layer of the skin may be protective, while that in melanocytes may be pho-tocarcinogenic. Observations that support this hypothesis include: (1) Africans with dark skin have a reduced risk of getting all types of skin cancer as compared with Caucasians, but the ratio of their incidence rates of cutaneous malignant melanoma to that of squamous cell carcinoma is larger than the corresponding ratio for Caucasians. (2) Albino Africans, as compared with normally pigmented Africans, seem to have a relatively small risk of getting cutaneous malignant melanomas compared to nonmela-nomas. This is probably also true for albino and normally pigmented Caucasians. (3) Among sun-sensitive, poorly tanning persons, frequent UV exposures are associated with increased risk of melanoma, wherease among sun-resistant, well-tanning persons, increased frequency of exposure is associated with decreased melanoma risk. (4) It is likely that UVA, being absorbed by melanin, might have a melanoma-inducing effect. This is in agreement with some epidemiological investigations which indicate that sun-screen lotions may not protect sufficiently against melanoma induction. The relative latitude gradient for UVA is much smaller than that for UVB. The same is true for the relative latitude gradient of cutaneous malignant melanoma as compared with squamous cell carcinoma and basal cell carcinoma. Under the assumption that the average slopes of the curves relating incidence rates with fluences of carcinogenic UV radiation are similar for melanomas and nonmelanomas, these facts are in agreement with the assumption that UVA plays a significant role in the induction of melano.
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The use of sunscreens on the skin can prevent sunburn but whether long-term use can prevent skin cancer is not known. Also, there is evidence that oral betacarotene supplementation lowers skin-cancer rates in animals, but there is limited evidence of its effect in human beings. In a community-based randomised trial with a 2 by 2 factorial design, individuals were assigned to four treatment groups: daily application of a sun protection factor 15-plus sunscreen to the head, neck, arms, and hands, and betacarotene supplementation (30 mg per day); sunscreen plus placebo tablets; betacarotene only; or placebo only. Participants were 1621 residents of Nambour in southeast Queensland, Australia. The endpoints after 4.5 years of follow-up were the incidence of basal-cell and squamous-cell carcinomas both in terms of people treated for newly diagnosed disease and in terms of the numbers of tumours that occurred. Analysis of the effect of sunscreen was based only on skin cancers that developed on sites of daily application. All analyses were by intention to treat. 1383 participants underwent full skin examination by a dermatologist in the follow-up period. 250 of them developed 758 new skin cancers during the follow-up period. There were no significant differences in the incidence of first new skin cancers between groups randomly assigned daily sunscreen and no daily sunscreen (basal-cell carcinoma 2588 vs 2509 per 100,000; rate ratio 1.03 [95% CI 0.73-1.46]; squamous-cell carcinoma 876 vs 996 per 100,000; rate ratio 0.88 [0.50-1.56]). Similarly, there was no significant difference between the betacarotene and placebo groups in incidence of either cancer (basal-cell carcinoma 3954 vs 3806 per 100,000; 1.04 [0.73-1.27]; squamous-cell carcinoma 1508 vs 1146 per 100,000; 1.35 [0.84-2.19]). In terms of the number of tumours, there was no effect on incidence of basal-cell carcinoma by sunscreen use or by betacarotene but the incidence of squamous-cell carcinoma was significantly lower in the sunscreen group than in the no daily sunscreen group (1115 vs 1832 per 100,000; 0.61 [0.46-0.81]). There was no harmful effect of daily use of sunscreen in this medium-term study. Cutaneous squamous-cell carcinoma, but not basal-cell carcinoma seems to be amenable to prevention through the routine use of sunscreen by adults for 4.5 years. There was no beneficial or harmful effect on the rates of either type of skin cancer, as a result of betacarotene supplementation.
Article
The incidence of skin cancer is increasing rapidly and sunscreens have been recommended in order to reduce damage from sunlight. In this investigation we have studied the change in the absorption spectrum of some photoactive organic species in sunscreens after ultraviolet A and ultraviolet B irradiation in a dose normally encountered during a full day in the sun. Samples of a number of photoactive compounds commonly used in sunscreens were irradiated with ultraviolet A and ultraviolet B light. A UVASUN 2000 MUTZHAS sunlamp was used for ultraviolet A irradiation and an Esshå Corona mini, equipped with two Philips TL12 20 W lamps, was used as the ultraviolet B source. The ultraviolet A dose was 100 J per cm2. The ultraviolet B dose corresponded to 20 minimal erythema doses. The absorption spectra of the compounds were recorded before and after irradiation. The absorbance of 2-ethylhexyl 4-methoxycinnamate was reduced significantly, whereas 3-(4-methylbenzyliden)camphor seemed to be rather stable. The benzophenones studied seemed to be relatively stable. In the case of 4-tert. butyl-4'-methoxy-dibenzoylmethane there was a rapid decrease in the ultraviolet A absorption leading to unsatisfactory protection in the ultraviolet A region. 4-Isopropyl-dibenzoylmethane also lost most of its ultraviolet protective capacity after irradiation with ultraviolet A. Ultraviolet B seemed to have a minor effect on all the samples. It is important for the clinician not only to know the initial absorption spectrum in the ultraviolet region for a specific sunscreen substance, but also whether it is altered during irradiation and in what way. This study including gas chromatography and mass spectrometry analysis indicates that some of the photoactive organic species commonly used today in sunscreens are unstable following ultraviolet irradiation.
Article
There are considerable data to suggest that protection from solar ultraviolet (UV) radiation will reduce the risk of acute and chronic skin damage in humans. Whereas the sun protection factor (SPF) provides an index of protection against erythemally effective solar UV, largely confined to the UVB (290-320 nm) and short-wavelength UVA (320-340 nm) region, there is currently no agreed-upon method to measure broad-spectrum protection against long-wavelength UVA (340-400 nm). The objective of these studies was to assess the potential of in vitro UV substrate spectrophotometry and subsequent calculation of the "critical wavelength" value as a measure of broad-spectrum UV protection and as a routine, practical procedure for classification of sunscreen products. The spectral absorption of 59 commercially available sunscreen products and multiple experimental formulas with one or more UV filters was measured. Sunscreen product, 1 mg/cm(2), was applied to a hydrated synthetic collagen substrate, preirradiated with a solar simulator, and then subjected to UV substrate spectrophotometry. Multiple determinations from 5 independent samples per product were used to calculate the critical wavelength value, defined as the wavelength at which the integral of the spectral absorbance curve reached 90% of the integral from 290 to 400 nm. We found that a recognized long-wave UVA active ingredient such as titanium dioxide, zinc oxide, or avobenzone is a necessary but insufficient product requirement for achieving the highest proposed broad-spectrum classification, that is, critical wavelength of 370 nm or more. Although SPF and critical wavelength are largely independent of each other, UVA absorbance must increase commensurate with SPF to maintain the same critical wavelength value. Substrate spectrophotometry and the calculation of critical wavelength can readily account for sunscreen photostability by UV preirradiation. Finally, there is also a strong positive relationship between critical wavelength and a currently available in vivo measure of UVA protection. Determination of critical wavelength by means of UV substrate spectrophotometry provides a rapid, inexpensive, and reliable measure of broad-spectrum protection, which is largely independent of SPF, yet ensures long-wavelength UVA protection commensurate with SPF. The procedure provides a routine, sensitive means of differentiating and classifying sunscreen products and, importantly, obviates the need to subject volunteers to acute exposures of high-dose, nonterrestrial UV, the health risks of which are still poorly understood.