Safety Factor Factors used in the calculation of acceptable humans or environmental exposures.  They are applied to data from laboratory experiments or epidemiology studies.  Factors of 10 are normally used to account for such uncertainties in the data on which risk assessments are made.  Similar to uncertainty factors. Selection Bias Systematic error that may be introduced in sampling by selecting one population over another. Selective Toxicity Selective toxicity refers to species differences in toxicity between two species simultaneously exposed. Sensitization An immune capability developed following an individual's exposure to a specific antigen. Subsequent exposure results in an immune reaction. Sensitizer A substance that causes an allergic immune response. Sister Chromatid Exchange Assay (SCE) A mutation test in which bone marrow cells or lymphocytes of exposed individuals are microscopically examined for complete chromosome breakage and errors in rejoining of chromatid fragments.  Errors are detected by demonstrating that there has been an exchange in the sister chromatids during the rejoining process. Slope of the Dose-Response Curve Rate of buildup of toxic effects with increasing doses. Solubility Ability of a substance to be dissolved in a solvent.  The solubility is expressed according to the solvent, e.g., water solubility, solubility in acetone, etc. Somatic Cell A body cell other than germ cell. Standard Deviation The statistical calculation denoting the variability of responses to an exposure.  One standard deviation incorporates 68% of the responses while two standard deviations incorporates 95% of the responses. Standardized Mortality Ratio (SMR) A statistical calculation in an epidemiology study which compares the relative risk of death of an exposed group to a non-exposed group.  It is the most commonly used measure for reporting the results of cohort studies. Steatosis Lipid accumulation in hepatocytes. Structure-Activity Relationship (SAR) The process by which the toxicity of a substance can be predicted based on its similarity in structure to that of other chemicals for which the toxicity is known. Subchronic Toxicity The adverse effects of a substance resulting from repeated exposure to a toxic agent over a period of several weeks or months. Systemic toxin A toxin that affects the entire body or many organs.