The inhalation acute toxic class method: test procedures and biometric evaluations

Abstract

A method of inhalation acute toxic (ATC) classification is presented with the use of significantly fewer animals in comparison with the classical LC₅₀ test. The principle of the inhalation ATC method is based on the oral ATC method, which has been adopted in 1996 as an official test guideline of the OECD and the European Union. The inhalation ATC method, like the oral ATC method, is a stepwise procedure; three animals of each sex are used simultaneously for each tested concentration, and not, as in the oral ATC method, three animals of each sex separately for each dose. The method was developed for three starting concentrations and two reference systems (based on ppm and mg/l). Depending on the LC₅₀, slope, classification system and starting concentration, on average 50 to 80% fewer animals will be used in comparison to at least 30 animals with the classical LC₅₀ test. The method was biometrically evaluated with the use of the probit model for dose-response relationships. At present, there are 12 different international classification systems based on LC₅₀ values: 6 systems referring to mg/litre and 6 systems based on ppm values, and exposure time varying from 1 to 4 h. The test procedures and the calculations of the classification probabilities demonstrate that the inhalation ATC method is a reliable alternative to the classical LC₅₀ test with the use of significantly fewer animals. Classification probabilities are presented for all classification systems currently in use, and expected numbers of experimental and of moribund/dead animals are demonstrated for one system of each reference system and for all three starting concentrations. The conclusion is justified that there is no need to validate the inhalation ATC method with the use of experimental animals.