Median life-span

Rat. Male Sprague-Dawley rats, 9-10 weeks of age, were exposed by whole-body inhalation to 50 ppm [225 mg/m methyl methancsulfonate (purity, > 95%) for 6 h per day on five days per week for six weeks and were observed for life. Nasal tumours were found in 47/80 animals versus 0/98 air controls. The median life spans for the control and exposed groups were 613 days and 495 days, respectively. The median time of nasal tumour appearance was 513 days (range, 256-775) (Snyder et al., 1986 Sellakumar et al., 1987). 

Several experiments were performed to establish the feasibility of this method on large amounts of data. These experiments required a few searches of the Drug Research and Development biological file to find compounds that met various criteria as a result of P388 testing. These criteria are usually stated in terms of the T/C ratio, which is the median life-span of the treated animals divided by that of the controls. 

See Section 5.3.3 for chronic toxicity study design. The TD50 is the median toxic dose, which is defined as the dose of a test chemical, administered repeatedly, usually daily, that results in the manifestation of the specified toxic endpoint in half of the test animals within the time frame of the study, e.g., the adult life span of the test animal. As is the case with acute toxicity, the route of exposure and the test species also have a significant effect on the dose of a chemical causing chronic toxicity, and they are reported with the TDjq. 

The 24 and 48 h median tolerance limits of Pb(C2H5)4 for fresh-water fish are 2.0 and 1.4 ppm (as Pb), respectively, and the safe concentration in water is 0.20 ppm [129]. Fish kept in water with varying concentrations of Pb(C2H5)4 contained virtually identical amounts of lead at the time of their death the life span depended on the concentration [155]. 

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