There are many ways in which the dose can be expressed in inhalation toxicology studies. This can lead to confusion when comparing results from studies performed in different laboratories. A working party of the Association of Inhalation Toxicologists has reviewed this subject in detail and has collected data from 10 inhalation laboratories and used these data to determine a new algorithm for the calculation of Respiratory Minute Volume (RMV), one of the most important factors in the calculation of delivered dose. The recommendations of the working party for regulatory inhalation toxicology studies with pharmaceuticals are as follows:1. The dose should be reported as the delivered dose calculated according to the formula:DD=C x RMV x D(xIF)/BWwhere DD = delivered dose (mg/Kg); C = concentration of substance in air (mg/L); RMV = respiratory minute volume or the volume of air inhaled in one minute (L/min); D = duration of exposure (min); IF = proportion by weight of particles that are inhalable by the test species, the inhalable fraction (inclusion of this parameter is not essential provided that the aerosol has reasonable respirability for the intended species. If it is included, the way in which it is determined should be clearly stated); BW = bodyweight (Kg).2. The RMV for mice, rats, dogs and cynomolgus monkeys should be calculated according to the formula:RMV(L/min) = 0.608 x BW(Kg)(0.852)3. If deposited dose or the amount of material actually retained in the respiratory tract is presented as supplementary information, the way in which it is calculated should be clearly stated.4. Dose should always be presented in mg/Kg but may also be presented in other ways, such as mg/unit body surface area, as supplementary information.

Groups of 70 male and 70 female Charles River CD (Sprague–Dawley-derived) rats were exposed whole body to styrene vapor at 0, 50, 200, 500, or 1000 ppm 6 h/day 5 days/week for 104 weeks. The rats were observed daily, body weights and food and water consumption were measured periodically, and a battery of hematologic and clinical pathology examinations was conducted at weeks 13, 26, 52, 78, and 104. Nine or 10 rats per sex per group were necropsied after 52 weeks of exposure and the remaining survivors were necropsied after 104 weeks. Control and high-exposure rats received a complete histopathologic examination, while target organs, gross lesions, and all masses were examined in the lower exposure groups. Styrene had no effect on survival in males, but females exposed to 500 or 1000 ppm had a dose-related increase in survival. Levels of styrene in the blood at the end of a 6-h exposure during week 95 were proportional to exposure concentration. Levels of styrene oxide in the blood of rats exposed to 200 ppm or greater styrene were proportional to styrene exposure concentration. There were no changes of toxicologic significance in hematology, clinical chemistry, urinalysis, or organ weights. Males exposed to 500 or 1000 ppm gained less weight than the controls during the first year and maintained the difference during the second year. Females exposed to 200, 500, or 1000 ppm gained less weight during the first year; those exposed to 500 or 1000 ppm continued to gain less during months 13–18. Styrene-related nonneoplastic histopathologic changes were confined to the olfactory epithelium of the nasal mucosa. There was no evidence that styrene exposure caused treatment-related increases of any tumor type in males or females or in the number of tumor-bearing rats in the exposed groups compared to controls. In females, there were treatment-related decreases in pituitary adenomas and mammary adenocarcinomas. Based on an overall evaluation of eight oncogenicity studies, there is clear evidence that styrene does not induce cancer in rats.

Data from conventional rat toxicology studies from 2 centres are used to estimate the coefficients of variation of organ weights, haematology and blood chemistry characteristics commonly measured in such studies. Charts, based on simple statistical formulae, are provided which enable this information to be used to indicate the probability of failing to detect a treatment effect of a given magnitude as statistically significant. The results, for a study with 20 rats/treatment group, when the treatment causes a change of 10% in the characteristic, indicate wide variability in the false negative error rate. With some characteristics (e.g. haemoglobin, total protein, brain weight, red blood cell count) the false negative rate is less than 0.1%, but for others (e.g. white blood cell count, glutamicpyruvic transaminase, pituitary weight, cholesterol) it is in excess of 70%.