Intended for healthcare professionals

Rapid response to:

Education And Debate

Has the sun protection factor had its day?

BMJ 2000; 320 doi: (Published 15 January 2000) Cite this as: BMJ 2000;320:176

Rapid Response:

Two Fingers to Sunscreen? What about One Finger?

EDITOR - The protection offered by a sunscreen - defined by its Sun
Protection Factor (SPF) - is assessed after phototesting in vivo at an
internationally agreed application thickness of 2 mg/cm2. Yet a number of
studies have shown that consumers apply much less than this1 typically
between 0.5 to 1.5 mg/cm2. Application thickness has a significant effect
on protection with most users probably achieving a mean value of between
20-50% of that expected from the product label as a result of common
application thickness2. A useful rule-of-thumb is that the protection most
people get from a sunscreen is numerically equal to about one-third the
SPF. This mismatch between expectation and realisation may be one
contributing factor why sunscreens have been reported to be a risk factor
in melanoma3.

Neither of the Australasian4 nor European5 Standards, nor the United
States' FDA Final Monograph6, for sunscreens attempt to specify the dosage
of sunscreen to be used by consumers although, by inference, it is often
taken to be 2 mg/cm2. It is hardly surprising, therefore, that users do
not know how to use sunscreens effectively. We suggest a simple method
that would allow users to apply a quantity of sunscreen resulting in
closer agreement between the expected and delivered protection. This is
achieved by adopting two well-established formulae used in clinical

The 'Rule of Nines' is a formula to assess the extent of a patient's
burns as a percentage of their total body surface area6. The 'Fingertip
Unit' is a measure used in dermatology to quantify the amount of a product
to be used. It is a strip of cream or ointment squeezed out of the tube
onto the fingertip, from the distal crease of the index finger to the end
of the finger itself. Using a standard 5mm nozzle, such a fingertip
measure (for adults) equates reasonably accurately to 0.5g of product8.

Using the 'Rule of Nines' formula the surface area of the body is
divided up into 11 areas, each comprising approximately 9% of the body
surface area, as follows:

1. Head, neck & face; 2. Left arm; 3. Right arm; 4. Upper back;
5. Lower back; 6. Upper front torso; 7. Lower front torso; 8. Left upper
leg/thigh; 9. Right upper leg/thigh;
10. Left lower leg/foot; 11. Right lower leg/foot

To ensure that an adequate amount of sunscreen is dispensed for each
of the above eleven body areas (for an adult approximately 3g of product
to achieve a surface density of 2 mg/cm2), a modification of the
"Fingertip Unit" can be used. Instead of applying just one fingertip's
worth of sunscreen, two strips of sunscreen should be squeezed out onto
both the index and middle fingers from the crease between the palm and the
fingers themselves to the fingertips. For each person's body size the
application of such "Two Fingers" worth of sunscreen will provide a dosage
of the product that is a reasonable approximation to that used during the
laboratory determination of SPF.

The "Two Fingers" amount of sunscreen should be measured out
individually for each user. Children and babies' fingers are, naturally,
much smaller than adults' but applying two of their fingers' worth of
sunscreen to each of their eleven body areas, as necessary, will also
provide them with the required dosage to achieve the product's stated
level of SPF protection. Such a "dosage guide" is a simple means of
ensuring that users are protected according to their expectations.

Whilst this approach should provide a reasonable means of ensuring an
accurate dosage of sunscreen commensurate with the application thickness
used during SPF testing, experience has shown that it is unlikely that
users will be keen to cover themselves or their families with such a
copious layer of sunscreen and prefer to apply quantities averaging at
around one half this thickness. It is likely, therefore, that this
approach to encourage people to apply the “authorised” quantity of
sunscreen at a single application will fail.

A much less daunting proposition, and the one which we actually
suggest, would be to apply "One Finger's" worth of sunscreen, with the
corollary that the resultant protection would be only about 50% of that
stated on the product.4,5,6 Users would be encouraged to re-apply "One
Finger's" worth within half an hour of the first application in order to
achieve optimal protection9.

An important factor in sunscreen performance is the uneven nature of
the skin surface. If an opaque sunscreen is applied to the skin,
particularly at low surface densities typical of practical usage, the
epidermal markings become visible as the surface grooves, or sulci, are
filled. With further application, the intervening ridges are also covered
and the surface becomes more featureless. The situation is analogous to
painting a wall with a textured surface when two coats of paint are almost
always required for satisfactory coverage. In the same way two “coats of
sunscreen” may be required for adequate protection.

Steve Taylor, general practitioner
Sunset Road Family Doctors, 14/326 Sunset Rd, Mairangi Bay, Auckland, New

Brian Diffey, professor of medical physics
Regional Medical Physics Department, Newcastle General Hospital, Newcastle

1. Diffey BL. Has the sun protection factor had its day? Br Med J
2000; 320: 176-7

2. Stokes RP, Diffey BL. How well are sunscreen users protected?
Photodermatol Photoimmunol Photomed 1997; 13: 186-8

3. IARC Handbooks of Cancer Prevention: volume 5 Sunscreens,
International Agency for Research on Cancer, Lyon, 2001

4. AS/NZS 2604: 1998. Sunscreen products – evaluation and
classification. Standards Australia/ Standards New Zealand

5. COLIPA Sun Protection Factor Method published by the European
Cosmetic Toiletry, and Perfumery Association (COLIPA), Brussels, Belgium,

6. Sunscreen Products For Human Use; Final Monograph, Federal Drug
Administration, Washington, USA, May 1999

7. Lund CC, Browder NC. The estimation of the areas of burns. Surg
Gyn Obs 1944; 79: 352-8

8. Long CC, Finlay AY. The fingertip unit…a new practical measure.
Clin Exp Dermatol 1991; 16: 444-7

9. Diffey BL. When should sunscreen be reapplied? J Am Acad Dermatol
2001; 45: 882-5

Competing interests: No competing interests

25 May 2002
Steve Taylor
General Practitioner
Brian L.Diffey
Mairangi Bay, Auckland, NZ