Institute for Occupational Health. Laboratory of Industrial Toxicology Kiev, Ukraine

There is some deficiency in theoretical elaboration of the problem of the biological norm and in the principles of the evaluation of significance of its disturbances on exposure to environmental chemicals. The differentiated selection of the critical level of significance of changes depending on rigidity and plasticity of indices and on a range of other factors, such as the specificity of indices selected regarding the effect invoved, the goal of the study, the degree of importance of the task in each specific case is advisable. When designing toxicological studies the variation coefficient for comparing different indices in one aggregation or one index in different aggregations should be used. To reveal latent disturbances it is advisable to apply load tests - physiological and extreme.