Patch tests versus use tests in skin irritation risk assessment

In the risk assessment of the ability of a substance or preparation to caus e skin irritation, a patch test involving human volunteers is often seen as providing definitive information. However, the skin exposure conditions fo und in an occluded patch may be far removed from those in a particular use situation, not least for such reasons as the high dose per unit area, the d uration of exposure and the maceration of the skin. Whilst these factors ma y enhance sensitivity in the identification of intrinsic hazard, they may d o little to ensure accurate safety evaluation for real use. In this paper, we report data from a series of studies with an unmarketed facial skin cosm etic product. Whilst the product was unexpectedly highly irritating in a st andard patch test in 30 volunteers, subsequent use tests, including a 6x da ily open application to the elbow for 3 weeks and 2x daily application in a half-face test lasting 3 to 4 weeks and involving 52 volunteers failed to show any evidence of skin irritation. It is concluded that the most meaning ful results for skin irritation risk assessment are likely to come from stu dies which involve relevant patterns of exposure.