Sun protection products are unique in cosmetic compositions in that claims for their efficacy are quantified numerically as sun protection factors (SPF) and these must be proven.
What should a sunscreen do?
Final SPF testing is an in vivo procedure whereby 2mg/cm2 of product is applied to the back of a subject, who is then exposed to UV radiation.
SPF is defined as the minimal erythema dose (MED) in sunscreen-protected skin divided by the MED in non-sunscreen-protected skin.
The MED of UV radiation is that required to produce a minimal skin erythema, expressed in joules per square centimetre (J/cm2).
This is a very simplified description and actual testing requires standardised test lamps, multiple test sites, a number of subjects of a particular skin type and standard preparations to act as controls.
There are specialised laboratories that are approved for this testing including AMA Laboratories, BioScreen and CRL Suncare in the US; C.L.A.I.M.S. in Australia; Eurofins and Intertek in the UK; Helioscience in France; and proDERM in Germany.
Unfortunately, there is no global harmonisation, so it is important to commission tests that are accepted in the proposed marketing area.
In Europe, the approved method is ISO 24444; in the US the method is described in FDA 2011 21 CFR Parts 201 and 310; and Australia and New Zealand use AS/NZS 2604:2012.
To formulate sun protection compositions to provide maximum protection it is important that they are photostable, spread readily and provide an even film on the skin
Claims for water-resistance must also be proven by immersing subjects in water at specified temperatures for specified times, then determining the SPF.
As well as measuring MED for SPF determinations, persistent pigment darkening (PPD), which is skin darkening that persists more than