Determination of concentration-time-mortality relationships versus LC50s according to OECD guideline 403

The effect of decreasing the number of animals on the accuracy of estimated concentration-time-mortality relationships and the LC50 values derived from these was investigated by means of simulation methods. Studies of NH3, Cl2, H2S, and COCl2 were made using 5 rats per sex per group. Mortality rates were analysed in 500 new data sets obtained by randomly removing 4 rats/sex/group from the original data sets. It was concluded that the concentration-time-mortality relationship and the LC50 values can be determined over a 5-10-fold time range using one rat/sex/group. The resulting 5 and 95 percentiles compare favourably with the 90% confidence limits when determining a LC50 according to OECD guideline 403. When using concentration-time-mortality relationships, additional information can be obtained which can be used in inhalation hazard risk assessment.