In order to quantify the skin irritation potential of the test formulations,
each study included 20% SDS as both a positive irritant control and a benchmark chemical.
Because of the inherent variability in the human skin irritation response, it is important to calibrate each study population by their collective response to a standard irritant. Twenty percent SDS serves this purpose, but also provides a point of comparison for irritation classification since it is the minimum concentration classified as R38 (‘‘irritating to skin’’) under the Dangerous Substances Directive of the European Communities (EEC, 1992). Hence, by definition, any chemical (or formulation) showing equal or greater skin irritation than 20% SDS (by statistical analysis), should be classified as irritating to skin under the acute exposure conditions used. Using the convenient calculation of TR50 (time of exposure required for 50% of subjects to show positive skin irritation response), it has been reported that the range of response for 20% SDS across multiple studies is approximately 1.5–3 h (Robinson et al., 1998). Testing under