Given that, as yet, there is no equivalent in vitro method capable of providing reliable, compatible results, the surest method for testing the true effectiveness of a sunscreen product in terms of its ability to protect is in vivo. However, we cannot ignore the ethical issues associated with this method, since severe sunburns can cause semi-permanent changes in skin pigmentation, and information on the long-term risks is lacking.

COLIPA, well aware of the problems this creates in terms of volunteer recruitment, has already produced an in vitro test, validated against in vivo tests, for determining the UVA factor. The next step will surely be to investigate a valid alternative to in vivo testing for determining sun protection factor (SPF) as well.
A procedure that would make it possible to obtain an optimal in vitro/in vivo correlation for determining SPF has not yet been established. Indeed, we should not forget that sun protection factor (SPF), as determined based on in vitro transmittance measurements, may be found to be quantitatively uncorrelated with the protection factor that can be obtained in vivo.

The substrate that simulates the skin does not, in fact, have the same properties as the epidermis in terms of polarity components and dispersion of surface free energy. An emulsion may therefore have a high affinity for one substrate and none at all for another.