1. Letter to the Editor
The meaning and implication of sun protection
factor
DOI: 10.1111/bjd.14015
DEAR EDITOR, The Sun Protection Factor (SPF) was introduced
in 1962 and has become a worldwide standard for measuring
the efficacy of sunscreen products in shielding the sun’s ultra-
violet radiation (UVR) and thereby protecting the skin against
sunburn. We have noticed that among many healthcare pro-
fessionals, organizations and agencies a persistent confusion
prevails on its meaning and implication.
The SPF is defined as the ratio of the UVR dose that induces
the first perceptible erythema on sunscreen-protected skin to
the UVR dose that induces the same erythema on unprotected
skin.1
Sunscreens with SPF 15, 30 and 60 (in Europe labelled
50+) absorb 93Á3%, 96Á7% and 98Á3% of the erythemogenic
UVR, respectively. A popular, but erroneous, inference is that
higher SPF numbers offer only a tiny percentage more attenu-
ation and therefore do not provide substantial extra protection.
In reality, it is not the amount of UVR absorbed by the sun-
screen that is relevant for the determination of the SPF, but
the amount that penetrates into the skin and hence is respon-
sible for sunburn, the subsequent development of skin cancer
and premature skin ageing. As shown in Figure 1, the UVR
dose reaching the skin is halved between SPF 15 and 30
(6Á7% vs. 3Á3%) and again between SPF 30 and 60 (3Á3% vs.
1Á7%). This means that protection doubles between SPF 15
and 30 and again between SPF 30 and 60.2
Even though the extra protection provided by very high
SPF formulations was recently demonstrated in a clinical
trial,3
this reasoning is not valid ad infinitum. For example,
SPF 300+ does not make sense for sunscreens, although
such values can be achieved with tightly woven thick cloth-
ing. With sunscreens there are limitations that lead to the
recommended or regulated cap at SPF 60. SPF assessments
(determined at 2Á0 mg cmÀ2
) are subject to a relatively
wide 95% confidence interval of Æ 17%.1
Variations in the
amount of sunscreen applied have a large negative impact
at higher SPFs. With the typical consumer application of
0Á5–1Á0 mg cmÀ2
, an SPF 60-labelled sunscreen may offer
only an SPF of 15 (the minimum recommended SPF for
skin cancer prevention) or even less.4,5
To cope with this
real-life situation we recommend the application of sun-
screen twice before sun exposure in order to reach the
needed quantities of sunscreen, and possibly to cover areas
that have not been covered after the first application.
In order to curb the skin cancer epidemic, healthcare
professionals should consistently advise people staying out-
doors to use high to very high SPF broad-spectrum sun-
screens along with other sun-protective measures, such as
limiting sun exposure by seeking shade and wearing protec-
tive clothing.
D. RE I N A U
1
U. OS T E R W A L D E R
2
E. ST O C K F L E T H
3
C. SU R B E R
4 , 5
1
Basel Pharmacoepidemiology Unit, Division
of Clinical Pharmacy and Epidemiology,
Department of Pharmaceutical Sciences,
University of Basel, Basel, Switzerland
2
BASF Personal Care and Nutrition GmbH,
Monheim, Germany
3
Department of Dermatology, Klinikum fu¨r
Dermatologie, Venerologie und Allergologie,
St. Josef-Hospital, Ruhr-Universita¨t Bochum,
Bochum, Germany
4
Department of Dermatology, University
Hospital, Basel, Switzerland
5
Department of Dermatology, University
Hospital, Z€urich, Switzerland
Correspondence: Christian Surber.
E-mail: christian.surber@unibas.ch
References
1 International Organization for Standards. ISO 24444:2010. Cos-
metics – sun protection test methods – in vivo determination of
the sun protection factor (SPF). Available at: http://www.
iso.org/iso/catalogue_detail.htm?csnumber=46523 (last accessed 7
October 2015).
12
10
8
6
4
2
0
96
94
98
100
%
1 15 30 60 SPF
2
ErythemogenicUVR(%)
100
90,0
Sun protection factor (SPF)
6·7
3·3
1·7
93·3
96·7
98·3
UV dose [%]
absorbed by
the sunscreen
UV dose [%]
absorbed by
the skin that is
part of the
sunburn dose
Fig 1. Ultraviolet radiation (UVR) doses (red) absorbed by the skin
protected by sunscreens with different sun protection factors (SPFs).
© 2015 British Association of Dermatologists British Journal of Dermatology (2015) 1

2. 2 Surber C, Ulrich C, Hinrichs B, Stockfleth E. Photoprotection in
immunocompetent and immunocompromised people. Br J Dermatol
2012; 167(Suppl. 2):85–93.
3 Russak JE, Chen T, Appa Y, Rigel DS. A comparison of sunburn
protection of high-sun protection factor (SPF) sunscreens: SPF 85
sunscreen is significantly more protective than SPF 50. J Am Acad Der-
matol 2010; 62:348–9.
4 Lademann J, Schanzer S, Richter H et al. Sunscreen application at the
beach. J Cosmet Dermatol 2004; 3:62–8.
5 Faurschou A, Wulf HC. The relation between sun protection factor
and amount of suncreen applied in vivo. Br J Dermatol 2007;
156:716–19.
Funding sources: none.
Conflicts of interest: none declared.
© 2015 British Association of DermatologistsBritish Journal of Dermatology (2015)
2 Letter to the Editor